the relationship between elements and atoms the simple model of an atom ions B the development of the Periodic Table groups in the Periodic Table compounds and mixtures how we distinguish among elements, compounds and mixtures QTE history of the use of metals how we extract metals properties and uses of metals alloys useful materials from crude oil including fuels, solvents and plastics environmental problems associated with the use of materialseee Wood and stone have been used to construct buildings for thousands of years but they have some disadvantages—wood is flammable and stone is heavy. They are also not strong enough to support large structures on their own. A breakthrough in arc technology came with the use of cast iron in the 19th century. Cast iron is an alloy consisting of iron and carbon. Buildings made of cast iron are strong and light. The Eiffel Tower in Paris is made of cast iron. Today, steel (another alloy of iron) and plastics are used widely as building materials. Can you name some famous buildings which are made of steel and plastics? © The Eiffel Tower in Paris, builtin @ HS8CMain Building in Hong Kong The Eden Projectin the United Kingdom 11897, is made mainly of cast iron. is constructed mainly of steel in 1985. is constructed mainly of plasticsin 1998. BD 1. Why has there been a change in the choice of building materials? D 2. What are alloys? What are the advantages of using, alloys over metals? (® 3. How are plastics made? What are the different kinds of plastics and their uses? conton it see) ay 8 2 Eifel Tower S38TE 13.1) Atoms and elements The models shown on the right are made up of toy bricks. There are only a few kinds of toy bricks but they can be arranged in different ways to form many different models. Similarly, all matter is made up of small particles called atoms. There are just over one hundred different kinds of atoms. However, they combine in many different ways to form millions of substances in the world. Atoms are the smallest units of matter. Now we have some ideas about atoms. Let us next study the relationship between atoms and elements. better understand atoms byan analogy with toy brick models. e Elements What are elements? An element is a substance that is made up of only one kind of atom. For example, gold is an element. It is made up of gold atoms only. Iron, another element, is made up of iron atoms only. ° gold atom ® iron atom 1999999) Fig. 13.2 _(@) Goldis made up of gold atoms only while (b) ron is made up of iron atoms only, atom BE element 2%@ TE © Allanswers 13 ees @ Some elements are made up of small particles known as molecules. A molecule of an element consists of atoms of the same kind. For example, hydrogen is made up of hydrogen molecules. Each hydrogen molecule consists of two hydrogen atoms. Sulphur, another element, is made up of sulphur molecules. Each sulphur molecule consists of eight sulphur atoms. sulphur atom hydrogen atom - hydrogen molecule a hydrogen sulphur ‘sulphur molecule 13.3 (@) Hydrogen is made up of hydrogen molecules while (0) sulphur Is made up of sulphur molecules. (The atoms ‘and molecules in the above diagrams are represented by mode's) @ A total of 118 elements have been identified by 2017. Among these elements, 92 of them occur naturally and the rest have been made by scientists in laboratories. * An element is a substance that is made up of only one kind of atom. @ = A molecule of an element consists of same kind. of the 9 in Checkpoint 13.1 oo Write ‘T’ for a true statement and ‘F' for a false statement. (a) All matter is made up of atoms. a (b) The atoms of an element ate the same as those of another element. 3 (c) Each hydrogen molecule consists of one hydrogen atom. oa inlecle 5: sulphur@ TE Cher al symbols and chemical formulae Chemical symbols Look at the label of the gas jar shown on the left. Do you know what 2 =e substance is stored insidez | die) In order to facilitate communications among, people from different parts of the world, scientists have given each element a specific symbol, which is called its chemical symbol. @ The chemical symbol of an element usually consists of one or two letters. These letters mostly come from the element's English name or Latin name. Typically, the first letter in the full English name (or 13.4 Asubstance (928)'S the Latin name in some cases) of an element is used as its chemical storedin this gasjar. The name of this substance is written in. symbol, and is written in UPPER CASE. If more than one element Japanese. Doyouknow what begins with the same letter, a second letter, written in lower case, is ‘substance it is? You can find the answer in Table 13.1 added. Some common elements and their chemical symbols are given in the following table. NOU CE Name of | Name of j Carer er Creer rt a ish Latin eyaltal oer fev) Cyt Hydrogen H Potassium | Kalium K Helium He Calcium Calx Ca Carbon Iron Ferrum Fe Nitrogen a Copper Cuprum cu Oxygen . ° Zine EE 7 om Sodium | [Na Lead Plumbum Pb Magnesium | Mg Silver Argentum Ag Aluminiam i Al Tin Stannum sn Sulphur s Gold | Aurum Au Chlorine cl Mercury | Hydrargyrum Hg ‘Tho names and chemical symbols of some common eloments chemical symbol (A888 Lato pame iT 22@ TE From Atoms to Materials ® Chemical formulae Hp | araar Us the number of ‘Symbolof hydrogen atoms: hydrogen ina molecule. Fig. 13.5 The chemical formula for hydrogen molecule We have learned that some elements are made up of molecules. A molecule can be represented by a chemical formula. The chemical formula of a molecule indicates the number of atoms it contains. For example, the chemical formula for a hydrogen molecule is Hs. The number ‘2’ is a subscript which indicates that a hydrogen molecule consists of two hydrogen atoms. The following table shows examples of chemical formulae used for the molecules of some elements. ere fone try Nitrogen N: Oxygen oO Fluorine F, Phosphorus Py Sulphur Se Chlorine cl, lodine IL 13.2. Chemical formulae of some molecules Classification of elements There are more than 110 elements. A classification of the elements helps us study them more systematically. Elements can be classified as metals, non-metals and semi-metals based on their physical properties. Metals Do you know what physical properties metals have in common? Let Us look into this on the next page. chemical formula (E485 sem-metal #85 subscpe FR metal 88 on metal 3: Physical property 8TE K} Physical properties of metals = ae a 5 © Physical state: Most metals are solids ©) Appearance: Most metals are shiny 3 at room temperature. Mercury is the in appearance. FS only exception. It is a liquid at room temperature. cord lass e Melting and boiling points: Most metals. °e © Strength: Most metals are strong. metat have high melting points a and high boiling points. Thermal conductivity: Metals are good conductors of © Density: Metals have high densities. Most metals are i denser than water heat. (1 g/cm’), so they will sink in water. @ Electrical rT r Malleability: Metals are malleable. conductivity: haeeeke They can be bent or pressed into Metals are good different shapes. conductors of Ee electricity. 2 a ©) ductility: Metals are ductile, They can be pulled into thin wires ‘metal can be pressed into sheets ‘metal can be pulled into wires shiny 38H thermatconductvey ANKE — ducty HELE maleobilty Re ductile B29 7QTE 13 es @ Non-metals In general, non-metals have the following physical properties. Physical properties of non-metal; @© Physical state: Non-metals are solids or gases at room temperature. Bromine is the only exception. It is a liquid at room temperature. More than half of the non-metals exist as gases at room temperature. ‘oxygen ‘sulphur (oes) (solid) bromine Aiguid) Appearance: Non-metallic solids are usually dull in appearance. Strength: Non-metallic solids are usually brittle. Melting and boiling points: Non-metals usually have low melting points and, boiling points. Density: Non-metals usually have low densities. oo O00 Thermal conductivity: Non-metals are usually poor conductors of heat. Electrical conductivity: Non-metals are usually poor conductors of electricity. A few elements such as boron and silicon are classified as semi-metals. They have properties of both metals and non-metals. boron stlicon Fig. 13.6 Boronand silicon are semi-metals. bromine 5 boron lu 93 sicon 8 brite 8@ TE In general, metals conduct electricity but non-metals do not. Under normal conditions, semi-metals do not conduct electricity. However, when they are mixed with a small amount of an impurity, they conduct electricity quite well. Semi-metals are semiconductors of electricity. For example, silicon is a semi-conductor widely used in the electronics industry to make computer chips. Let us classify some elements into metals and non-metals based on their physical properties in the following experiment. eat CU aaa Experiment video FAs Classifying some elements into metals and non-metals Material and apparatus sandpaper 1 mortar 4 dry cells with battery holder. 2 pestle 1 connecting wire 3 lement samples (copper, zinc, tin, aluminium, sulphur) light bulb 1 Procedure 1. Your teacher will provide you with some samples of elements. 2. Use a piece of sandpaper to polish the surface of copper, zinc, tin and aluminium. fe when using sandpaper to polish them 3. Observe whether the appearance of each sample is shiny or dull. Record your observations in the table on the next page. sem:conductor 258© Allanswers 13 eee > 4. Set up a circuit as shown below. Connect copper to the circuit. Does the light bulb light up? Record your observations in the table below. drycelis 5. Repeat Step 4 with other samples. 6. Place copper in a mortar. Use a pestle to hit it a few times. Can you break it into smaller pieces easily? Record your observations in the table below. Weasley [> ceuton goggles. Wash your hands thoroughly, after the experiment. 7. Repeat Step 6 with other samples. Results Zine Tin Aluminium Sulphur NTE at) @ Q Q Q Pree) @ @ Q Q eet EatCeNO) Ceo n ges smaller pieces e: ONO} @ @ @ @ 10@ Allanswers OD TE > Discussion 1. Which element(s) conduct(s) electricity? a 2. Which elements) is/are strong? @ 3. According to your results, classify the elements into metals and non-metals below. @ You have learned the general physical properties of metals and non-metals. However, there are some exceptions to these properties. Let us study the examples below. * Sodium is a metal. However, it is so soft that it can be cut easily with a knife, Besides, it has a low melting point (below 100 °C). © Carbon is a non-metal and can exist in different forms. Diamond, a form of carbon, is the hardest natural substance. I” It is very shiny and has extremely high melting and boiling points. Graphite, another form of carbon, can conduct 13.8. Sodium (a meta) is soft and can be cut easily. electricity. Fig. 13.9 Diamond {a non-metal is the 13.10 Pencil lead is made mainly of graphite. hardest natural substance, ‘The above circuit shows that graphite (a non-metal). ‘can conduct electricity "@ TE © Allanswers BR) om atoms tomaterits Elements can be classified into and eae ( Checkpoint 13.2 The following table shows some information about the physical properties of two elements Xand ¥. eee A Eee Aten ona Gray [eepiicunirs: eee x solid yellow low no y solid silver white high yes Classify X and Y as metal or non-metal. Explain your answers. GSEEEAS SEEEESSSSSSSSEESSUSEEESSEESSSSESSSSSED @® Simple model of an atom . The structure of an atom Scientists once believed that atoms were very tiny ‘solid’ spheres which could never be divided. Later, it was found that atoms are in fact made up of even smaller particles—protons, neutrons and electrons. The centre of an atom is a very tiny and extremely dense region called the nucleus. The nucleus contains tightly packed protons and neutrons. An atom consists mostly of empty space, occupied by electrons, Electrons move very fast around the nucleus. The simple model of an atom is shown in Fig. 13.11 on the next page. proton BF mucous ENE neutron 12, aecron BF@ TE positively charged proton neutron, anatom consists mostly of empty space ‘nucleus containing tightly packed protons ‘and neutrons ‘electron maves very fast ‘round the nucleus Fig. 13.11. The simple model of an atom @ Comparing the masses and charges of protons, neutrons and electrons A proton and a neutron have about the same mass. Protons are positively charged and neutrons have no charge. As a result, the nucleus is positively charged. Electrons are negatively charged and they are much lighter in mass than protons and neutrons. The following table shows a comparison of the proton, neutron and electron ved Parra tere rc Dead Ue mC ae SO of a proton) ona proton) proton P inside the nucleus 1 + neutron a inside the nucleus “1 0 - moves around the negligible electron e \ 1 “1 nucleus Fano ‘Table 13.3 Comparison between the proton, neutron and electron Although an atom has a positively charged nucleus, the atom itself has no overall charge. It is electrically neutral. This is because an atom has an equal number of protons and electrons. The charges of the protons and electrons cancel each other. negligible = H6-Ksh 13© Allanswers Bom xonsto Materals ® Comparing the sizes of a nucleus and an atom The nucleus of an atom is very small even compared with the atom. The radius of an atom is about 20 000 times the radius of the nucleus. To get an idea of the comparison between the sizes of a nucleus and an atom, we can use an analogy. Now imagine there was a pea in the centre of a football stadium (Fig. 13.12). If the football stadium was the size of an atom, the nucleus would be about the size of the pea Fig. 13.12 Ifthe football stadium was the size of an atom, the nucleus would be the size of a pea at the centre. Atoms are made up of protons, neutrons and electrons. * The protons and neutrons are packed tightly together in the a (ositvely charged) (no charge) * Inan atom, the number of protons is equal to the number of pea EE 14@ Allanswers OD TE wo @ You have learned the simple model of an atom. Do you know that this model is the result of hard work by many different scientists over the past two centuries? Let us learn more in the following activity. History of the development of the model of an atom Enter the code on our website to watch a video about the main stages in the development of the model of an atom. After watching the video, discuss the questions that follow. John Dalton, Jad. Thomson, Ernest Rutherford, Niels Bohr, ames Chadwick, British scientist British scientist «New Zealand scientist Danishscientist English scientist. (1766-1844) (1956-1940) (1871-1937) (1885-1962) (1891-1974) (® 1. How did Thomson's model differ from Dalton’s model? CD 2. In Rutherford’s gold foil scattering experiment, what evidence did Rutherford find to support his model of the atom? CD 3. who suggested that electrons moved in orbits around the nucleus? (D4. Are the works of these scientists considered to be scientific investigations? Why? gal folscaterng experiment 5B ore 1516 @ TE From Atoms to Materials atomic number FF mass number $6004 @2. Atomic number and mass number Atomic number The atomic number (symbol: Z) of an atom is the number of protons in the atom. Each element has its own unique atomic number. For example, an iron atom has 26 protons. Therefore, the atomic number of iron is 26. A gold atom has 79 protons. Therefore, the atomic number of gold is 79. Fig. 13.13 Theatomic number of ion Fig. 13.14 The atomic number of gold 1826. 1879. @ Mass number The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the atom. For example, an iron atom (with 26 protons and 30 neutrons) has a mass number of 26 + 30 = 56. We can deduce the number of protons, electrons and neutrons in an atom with a given atomic number and mass number. Let us look into the example on the next page. unique 8@ Allanswers OD TE gC Example A particular atom of an element (atomic number (Z) = 13) has a mass number (A) of 27. Give the number of (a) protons (b) electrons and (c) neutrons in the atom. Solution (a) Number of protons in the atom = atomic number of the atom =13 (b)_ Number of electrons in the atom = number of protons in the atom =13 (©) Number of neutrons in the atom = mass number - atomic number = 27-13 =14 Deducing the number of protons, electrons and neutrons in atoms Complete the following table. Na ere’ fe Trey Pvcniite : ee een Hydrogen H 1 7 = = a Helium He > : - 5 . Carbon c 6 a = + Nitrogen N 7 ie is = a. Oxygen oO 3 ie 5 = = Fluorine F 9 i pa = = Sodium Na a a - = a Alu ium AL 13 a7 pa 7 S Potassium K ae. ag = 7 = Calcium Ca 20 7s S = a 718 @ TE From Atoms to Materials © Allanswers * The atomic number (Z) of an atom is equal to the number of SD. in the atom. © The mass number (A) of an atom is equal to the sum of the (and the number of __C___ number of in the atom. e-Checkpoint e Checkpoint 13.3 Floss 1 Write ‘T’ for a true statement and ‘F’ for a false statement. (a) In an atom, the neutrons are moving around the nucleus. (b) An electron is much heavier than a proton. (©) The nucleus of an atom is positively charged. ie) The diagram below shows the simple model of an atom. Label the diagram using these words: proton, electron, neutron, nucleus, empty space. a qg . ® @ o @ @ @o@ oe @ A particular atom of an element has an atomic number of 18 and a mass number of 40. (a) How many protons and neutrons does it have respectively? ‘A. protons: 18; neutrons: 18 B. protons: 18; neutrons: 22 CC. protons: 22; neutrons: 40 D. protons: 40; neutrons: 22 a (b) How many electrons does it have? A. 18 B. 20 Cc. 22 D. 40 3@ TE 13 ®@ Ions El An atom is electrically neutral because it contains an equal number of fle protons and electrons. However, if an atom loses or gains one or more electrons, it becomes charged. A positively or negatively charged ion is formed. When an atom loses one or more electrons, it forms a positively charged ion (Fig.13.15), On the other hand, when an atom m gains one or more electrons, it forms a negatively charged ion 5 (Fig.13.16). a m 2 : & @- loss of anelectron @ S > q atom osttvely charged ion Fig. 13.15 Formation of positively charged ion sain of an electron @ ————> negatively charged ion Fig. 13.16 Formation of negatively chargedion ® in general, atoms of metals tend to lose electrons. For example, in a sodium atom, there are 11 protons and 11 electrons. It is electrically neutral. When it loses one electron, a sodium ion is AP the number of protons and formed. The sodium ion contains 11 protons and 10 electrons, so Electrons ina sodium atom and ® it Has an overall charge of +1. We can represent this process by the sodium on Sodium Sodium following equation: ‘tom fon No.of Bp oon i oa aaa fa > a + e toon sodium atom sodium ion electron lon BF 19QTE 13 ee © Allanswers ®On the other hand, atoms of non-metals tend to gain electrons. A ve numberof protons and electrons ina chine atom and a Chloride ion hiorine Choride ‘atom ion No.of 7 ” protons No.of electrons 7 < NOISN3LX4 20 For example, in a chlorine atom, there are 17 protons and 17 electrons. It is electrically neutral. When it gains one electron, a chloride ion is formed, The chloride ion contains 17 protons and 18 electrons, so it has an overall charge of =1. We can represent this process by the following equation: a + oe > oo chlorine atom electron chloride ion The following table shows some common ions. Element eta) Hydrogen H Ww Oxygen ° oO Fluorine F F Sodium Na Na’ Magnesium Mg. Mg” Aluminium Al ar Sulphur Ss s Chlorine cl a Potassium K K Calcium Ca ca Bromine Br Br 4 Some common ions (pee * When an atom loses one or more electrons, a charged ion is formed. © When an atom gains one or more electrons, a__C).__ charged ion is formed.@ Allanswers OD TE 9 (—_ ecrecipom ) Checkpoint 13.4 J = | 1. Atom X is electrically neutral. It loses two electrons to form an ion, Complete the | following table for atom X and the ion of X. | | Number of protons 12 Qa | Number of electrons oe Qo | m Overall charge Qo Qo | x | om | 2 2. Atom Y is electrically neutral. It gains two electrons to form an ion. Complete the | oO following table for atom Y and the ion of Y. | 6 | 2 | Number of protons Qa | Number of electrons Qa Qa | Overall charge a Qa | ) In daily life, we often use tables to organize information systematically. Take a look at your school timetable, We can clearly tell what lessons will be taught at different times of a day. Scientists have also organized the elements into a table called the ——~eeer | vneeaa [oe Periodic Table. Let us learn more | came | ese | eee | | about the Periodic Table in this as meet ct | ame |e am | aaa | Penome | rack _|. section. neon |e | : Fig. 13.17 Aschool timetable PeodicTable IH 2@ TE © Allanswers From Atoms to Materials @ BH The development of Periodic Table Dating back to the 19th century, 63 elements were discovered, At that time, a Russian chemist, Dmitri Mendeleev (1834-1907), first developed the Periodic Table. He arranged the 63 elements into the Periodic Table. Let us read the story about his discovery. Mendeleev and the Periodic Table Read the story below and discuss the questions that follow. itu Mareoaes Mendeleev was a teacher of Chemistry at @ famous University in Russia, He was writing a textbook for his students. ‘To collect his thoughts for writing the textbook, he wrote the name of each element and each of its properties on aseparate card. © Port otMendeleevs Periodic Table i MoM AP 8 Kou Gaze Ae 188 tne a, One night, he had a dream. The element cards ined up _He also found that if he placed eight elements in a row, in rows. When he awoke, he decided to arrange the _the columns of the table would contain elements with elements in a table according to the mass of atoms, similar properties. He called these columns ‘groups’ D1. In what ways did Mendeleev arrange the elements in the Periodic Table? D2. why did Mendeleev put beryllium (Be), magnesium (Mg) and calcium (Ca) into the same group (Group II) in his Periodic Table? 22@ Allanswers CD TE ow Actin easing @Mendeleev’s Periodic Table had been much modified over the years (History of te until it became the modern Periodic Table. {development ofthe ( > peters 190 In the modern Periodic Table, there are more than 110 elements. They are arranged in order of increasing atomic number. For example, hydrogen (atomic number = 1) comes first. Helium (atomic number = 2) comes second and so on. THE MODERN PERIODIC TABLE GROUPS le uo vevovw vo ; omen — 2 ae oy sc] Ti | V [Cr [Mn] Fe | Co] Ni [Cu = |) eee |e |e |e oa Py [ze FNb Mo Pte [Ru [Rh | Pa ‘Ag |S ae |e 6 fa] |e [ta |'w [Re [Os |" ir [Pt [Au Hl | zl elelele 7 Mie [se Pe Ps Pe en 13.18 Themodern Periodic Table In the modern Periodic Table, the elements are arranged in order of increasing ‘moder Periodic Table TCM 2324 @ TE From Atoms to Materials @® Groups in the Perio: Table You may notice that there are eight main groups in the modern Periodic Table. Each group has a number (1, I, Ill, LV, V, VI, VIL or 0). Do elements in the same group of the Periodic Table show some common properties? Let us look into this in the following experiment. Experiment 13.2 Do elements in the same group have similar reactions with dilute hydrochloric acid? Material and apparatus test tube 3 wooden splint 3 test tube rack 1 {92s lighter 1 measuring cylinder (10 cm’) 1 spatula 1 stopper 3 testtube holder 1 Procedure 1. Label three test tubes A, B and C. Fill each tube with 5 cm’ of dilute hydrochloric acid. Put the three test tubes in a test tube rack. BD wovrsatereaain SS; cwton + Wear protective gloves. + Handle acid with care. + Wash your hands thoroughly after the experiment, 2. Add two calcium granules to test tube A. Stopper test tube A for one minute. 3. Observe any gas bubbles given out. Also, touch the bottom of the test tube. Is heat released during the reaction? Record your observation in the table on the next page. rou : —— calclum granules magnesium granules carbon powder dilute hydrochloric acid reo SEG hydrochloric acid@ Allanswers oh 4, Remove the stopper from test tube A and immediately place a burning splint over the mouth of the test tube. What happens to the burning splint? Is any gas produced? If yes, what gas is produced? Record it in the table below. 5. Repeat Steps 2 to 4 with two magnesium granules. 6. Repeat Steps 2 to 4 with a spoonful of carbon powder Results eee Tod ir Perret oer Peas Calcium Q Q Magnesium Qa Qa Q Carbon Q | Q Q Discussion 1. Which of the elements react with dilute hydrochloric acid? oe 2. Referring to your answer in question 1, do all the reactions produce the same gas? oe 3. (a) Referring to your answers in questions 1 and 2, are these elements in the same group? @ (b) Referring to the Periodic Table, which group do they belong to? @ ‘Active learning Simulation For example, calcium and magnesium belong to Group Il. They both ( > {interactive Perio Tab! ‘ : react with dilute hydrochloric acid to produce hydrogen. Elements in the same group have ——__C). (similar/different) chemical properties. chemical property (cB ® The elements in the same group have similar chemical properties. 25QTE 13 ee @atthough elements in the same group have similar chemical properties, they differ in reactivities. Let us study this in the following, experiment. Pe TT eed m >< Comparing the action of dilute hydrochloric acid on calcium and mm magnesium Z Material and apparatus a © [eesttuve 2 measuring cylinder (25 cm’) 1-—_ stopwatch 1) Z_ | boitingtuve 2 spatula 1 015g calcium powder mmareerpen water trough 1 05g magnesium powder testtuberack 1 stopperwith # delivery ube 2-—_—dte hydrochloric acid Procedure 7 test tube 1. Make a mark at about 7 cm from the bottom of a test tube as shown on the right. 2. Fill the test tube to the top with tap water. 3. (a) Place the test tube upside-down in a beaker half-filled with water. Put the end of the oo delivery tube under the mouth of the test tube. (b) Fill a boiling tube with 15 cm’ of dilute hydrochloric acid. Using a spatula, add 0.5 g calcium powder to the acid, Immediately fit the stopper with the delivery tube into the boiling tube as shown below. teste GB verter [> Caution Wear protective gloves + Handle acid with care. watertrough «wach your hands thoroughly after the experiment. dilute hydrochloric acid water calcium powder EI reactivity tt 26@ Allanswers OD TE > 4, Using a stopwatch, measure the time taken for the water level to reach the marking. Record your result in the table below. 5. Repeat Steps 1 to 4 with 0.5 g magnesium powder. Record the result in the table below. Results Are CU Rm UR oma ROC Metal Ciara Calcium Magnesium Discussion Which metal, calcium or magnesium, is more reactive? & Elements in the same group differ in @ (Checkpomnt 13.5 f —o Here is part of the modem Periodic Table tou mowovo ove vito H He on ep PPPS Li | Be sic|nloleF|ne fz fafa fis_ fief? fie Na | Mg Al] si} P| s | ci] ar ne oP pa eee pre papa a_i K [cal se} Ti | v [er] Mn| Fe | co} Ni | Cul zn | Ga| Ge | As | Se | Br | Kr Answer the questions below. (a) List three elements that belong to Group 0. ® (b) Which element has an atomic number of 62 @ (©) Which elements are in the same group as sodium? @ — m x si m 2 & So 2 7QTE 13 ees ® Uses of elements Depending on their properties, different elements are used for different purposes. Below are the uses of some common elements. Helium (Group 0) ‘Aluminium (Group i) Filling balloons Making soft drink cans ‘+—— Common elements Lithium (Group 1) Making lithium ion batteries for ‘mobile phones, computers, etc. Magnesium (Group Il) Silicon (Group IV) ‘Added to fireworks to produce Making glass from its compound brillant white sparks ‘ithium ion bartery ERE 28@ TE ‘What properties make these common elements suitable for the uses we see in the photos? oe chrineGroup vi Phosphorus (Group V) Kiling germs in swimming pools and Making matches can electrolytic cell low voltage power supply {as lighter ‘+ Handle acid with care test tube pure water + Wash your hands measuring cylinder (10 cm’) dilute sulphuric acia thoroughly after the experiment stopper 30@ Allanswers > Procedure 1. Label two test tubes A and 8. Fill them with pure water. 2, Set up the apparatus shown on the right. Fill the electrolytic cell with pure water. Add 5 cm’ of dilute sulphuric acid to the pure water. The acid is used to increase the electrical conductivity. Tip An electrolytic cel is used so that electricity can ‘be supplied for the decomposition of water, ‘connected to the power supply 3. Turn on the power supply. 4. After 10 minutes, turn off the power supply. Stopper test tubes A and B quickly. 5. Remove the stopper from test tube A and quickly put a glowing splint into the tube. (a) What happens to the glowing splint? @ (b) What gas is collected in test tube A? 6. cutoecuemiwaay 2 Remove the stopper from test tube B and quickly You may refer to ‘Practise your experimental sls’ at the end ofthis put a burning splint near the mouth of the tube. Pook to revise how to test for gases (a) What happens to the burning splint? SEE eae eee eee (b) What gas is collected in test tube B? e Discus: 1. Can water be broken down into simpler substances by electricity? —__C}___ 2. If water can be broken down into simpler substances, what are they? a q ecrticcel AE 31QTE uk From Atoms to Materials ® In Experiment 13.4, water can be broken down into hydrogen and oxygen by passing electricity through it. Water is made up of two elements—hydrogen and oxygen. We call water a compound. y ets SS water molecules hydrogen molecules ‘oxygen molecules electricity y 13.20 Water is a compound made up of hydrogen and oxygen. (The molecules in the above diagrams are represented by models.) @A compound is a substance made up of two or more elements. It can be broken down into its constituent elements by chemical methods (e.g. decomposition by electricity). However, elements cannot be broken down into anything simpler by chemical methods. Many common substances, such as water, table salt and carbon dioxide, are compounds. The following table lists some compounds and their constituent elements. A We can use chemical formula to representa compound. Look at De SRC ey en «the examples in Table 185 ft water H.0 hydrogen and oxygen table salt Nacl sodium and chlorine carbon dioxide co, carbon and oxygen hy glucose Pee carbon, hydrogen and oxygen quartz SiO, icon and oxygen aoe feo: calcium, carbon and oxygen Table 13.5 Some common compounds and their constituent elements compound (#9 constentlement ARE gWuose 32 guane TE@ TE ©z0 * A compound is a substance made up of two or more elements. * Many common substances, such as water, table salt and carbon dioxide, are compounds. @Does a compound have the same properties as its constituent elements? Let us find out in the following experiment, ( ) Pertti a eed Demonstration Experiment video Investigating the properties of a compound and its constituent elements Material and apparatus boiling tube 2 bar magnet 1 1985 lighter 1 rocksil spatula 1 Bunsen burner 1 testtube rack 1 iron filings stand and clamp 1 heat-proof mat 1 test tube holder sulphur powder Procedure 1. Put two spoonfuls of iron filings into a boiling tube. Add two spoonfuls of sulphur powder in another boiling tube. Observe and record their colours in the table on the next page. 2. Fix the boiling tube containing iron filings with a stand and a clamp. Put a bar magnet near the bottom of the boiling tube and move the bar magnet upwards as shown on the right. Observe if the bar magnet attracts the iron filings and record the results in the same table. 3. Repeat Step 2 with sulphur powder instead of iron filings. 4, Pour the iron filings into the boiling tube containing sulphur powder. Tap the boiling tube to mix them thoroughly. Record the colour of the mixture in the same table. sronmicg #5 3334 @ TE © Allanswers From Atoms to Materials 2 5. Repeat Step 2 with the mixture of iron filings and sulphur powder. 6. Place a piece of rocksil in the mouth of the boiling tube containing the mixture, Heat the mixture strongly with a non-luminous flame as shown. Turn off the Bunsen burner immediately when the mixture glows or smoke comes out of the boiling tube. BB wenesserersen SS can + Perform this step in the fume cupboard, + During heating, do not point the mouth of the boiling tube to anyone, A compound called iron(!!) sulphide has been formed from iron and sulphur in the boiling tube. 7. Put the boiling tube in the test tube rack and allow the iron(|t) sulphide to cool. Observe and record the colour of iron(ll) sulphide in the table below. 8. Repeat Step 2 with ironill) sulphide instead of iron filings. Results EUs Corrs iron sulphur powder mixture of iron filings and sulphur powder iron(|!) sulphide 01818 0 010/10 0 Discuss Do iron, sulphur and ironill) sulphide have the same properties? Using the experimental results, explain your answer. @ rot spice Bec)@ Allanswers OD TE F Wwe windscusstne experimental @ In Experiment 13.5, we could see that the colour of iron) sulphide tesuls about the mature of ron fiingsandsubhurpowderin. iS different from the colours of iron and sulphur. Also, iron is eee attracted by a magnet but iron(II) sulphide is not. The properties of a compound usually differ from those of its constituent elements, We can also notice that in the reaction between iron (element) and sulphur (element), a new substance—iron( ll) sulphide (compound)— is formed. A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. Learn more saps) Not all compounds are made up of molecules In Fig. 13.20 on p.32, you can see that water existsas__-—* ~The properties of a compound are usually different from the mecha over eos) properties of its constituent elements. eno ea * A chemical change is a change in which one or more new sulphide are made up of ions but not molecules. substances are formed. You will learn more about this if you take Chemistry in senior forms. ° Checkpoint 13.6 1. Which of the following is a group of compounds? A. copper, water, sodium chloride B. water, sodium chloride, carbon dioxide CC. oxygen, copper, water D. water, carbon dioxide, oxygen 3 2. Which of the following statements concerning compounds is INCORRECT? A. A compound is made up of at least two elements. B.A compound can be broken down into its constituent elements by chemical methods. C. When an element is broken down chemically, compounds are formed. D. The properties of a compound are usually different from its constituent elements. 3 9 chemical change (cA838(E 35NOISN3LX4 36 @ TE From Atoms to Materials: Writing chemical equations One or more substances are formed during a chemical reaction. For example, when carbon burns in oxygen (or air), carbon dioxide is formed In the reaction between carbon and oxygen, one carbon atom reacts with one oxygen molecule to form one carbon dioxide molecule. Fig. 13.21. Carbon busin ‘oxygen to form carbon dioxide. =» carbon 1 oxygen 1 carbon dioxide stom molecule molecule Fig. 13.22 cartoon illustrating the formation of carbon dioxide from carbon and oxygen @We can use a word equation or a chemical equation to represent this reaction. A word equation for this reaction is shown below. carbon + oxygen ——+ carbon dioxide The word equation tells us which substances are involved in a chemical reaction. For a better understanding of a chemical change, we use a chemical equation. The chemical equation for the reaction between carbon and oxygen can be written as: C(s) + Ox(g) ——+ COAg) Teer eStore Write the reactants on the left- Write the product on. hand side. If there are two or the right-hand side. ‘mote reactants, put a plus (+) ‘sign between them. You may notice that the numbers of carbon atom and oxygen atoms are the same on both sides of the equation. We say that this chemical equation is balanced. chemical equation (LARTER balanced@ TE @ Let us study the four steps in writing a chemical equation. 1 Write the word equation for the reaction. 2 Write the chemical formulae of the reactants and products. Balance the equation so that the number of atoms (of each element) on the left-hand side is equal to the number of atoms (of each element) on the right-hand side. Write the state symbols ('s' for solid; formula to give the chemical equation. S Example Write a chemical equation for the reaction below. " for liquid; “g’ for gas) after each chemical mn Pad 3S nm 2 2 o 2 37 Hydrogen and oxygen react to form water. Solution Use the four steps given above to write the chemical equation, Write the word equation for the reaction, hydrogen + oxygen ——» water Write the chemical formulae of the reactants and products. H, +O, ——+ H,0 There are two oxygen atoms on the left-hand side but only one oxygen atom on the righthand side. So, put ‘2’ in front of H, +O) ——» 2H,0 (still unbalanced) There are two hydrogen atoms on the leftthand side but four hydrogen atoms on the right-hand side. So, put ‘2’ in front of H,on the left-hand side. The equation is now balanced. 2H, + O; ——* 2H,0 (balanced chemical equation) (Note that ‘1’ is understood and should be left out. Therefore, Os is written instead of 10..) Write the state symbols. 2H(g) + Og) ——® 2H,0(€)@ TE © Allanswers 13 ees ‘Active leaning simustion s balanced when there is an equal number e Samia of atoms of each ___C__ on both sides of the equ: Writing a chemical equation Use the four steps listed on the previous page to write a chemical equation for the reaction below. NOISN3LX4 A piece of sodium burns in a gas jar of chlorine and sodium chloride (white powder) is formed. Write the word equation: Q Write the chemical formulae of the reactants and products: Q Balance the chemical equation: Write the state symbols: @ 0 9 i Checkpoint 13.7 = Write the chemical equations for each of the following word equations. (a) magnesium + oxygen ——> magnesium oxide (Hint: The chemical formula of magnesium oxide is MgO.) @ (b) nitrogen + hydrogen ——> ammonia (Hi oe : The chemical formula of ammonia is NH;.) 9 3813.28. Amixture of iron (element) and sulphur (element) rminture 1 physial change HABE @ TE eB Mixtures 1. What are mixtures? A mixture consists of two or more substances that are mixed together without the formation of a new substance. The substances that make up a mixture can be elements or compounds. Air and bronze are examples of mixtures. carbon dioxide molecule oxygen molecule nitrogen molecule Fig. 13.23. Airis a mixture of elements (eg. oxygen and nitrogen) and compounds (9. carbon dioxide and water) tinatom ‘copper atom Fig. 13.24 This sculpture is made of bronze. Bronze is a mixture of two elements: copper and tin @In Experiment 13.5, we mix iron filings and sulphur powder together. A yellowish grey mixture is obtained. Mixing iron filings and sulphur powder is a physical change. A change in which no new substances are farmed is called physical change. It involves changes in shape, volume, appearance or states of matter (solid, liquid and gas). bronze Bi 39@ TE From Atoms to Materials ri © Amixture consists of two or more substances mixed together without the formation of a new substance. A physical change is a change in which no new substances are formed. Properties of a mixture Do the constituent substances in the mixture still retain their original properties? Let us recall the experimental results of Experiment 13.5. When we put a magnet near the mixture of iron and sulphur, only iron is attracted by the magnet but not sulphur. The iron and sulphur retain their original properties in the mixture. Fig, 13.26 Ironis attracted by the magnet but sulphur is not, A vounave eames evaporation Mixtures can be separated into their constituent substances fration and dstilation in Unit 2 ; : eee by physical methods (e.g. evaporation, filtration, distillation, magnetic attraction). For example, we can separate the iron filings from the sulphur powder by putting a bar magnet near the mixture. consent ubstance S540 40,