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In This Chapter You Will Learn :

>> Elements, symbols and their significance >> Atoms and molecules

>> Atomicity and molecular formulae of elements >» Compounds

>> Molecular formulae of compounds

INTRODUCTION
mill ions of subs tanc es in this worl d. Thes e subs tanc es diffe r from each other in
There are
osit ion, properti es and uses beca use they all are made up of different kinds of matter.
their comp
conveniently, they need
They can be pure or impure. To study these substances accurately and
unde r three cate gori es : elem ents , com pou nds and mixt ures on the basis of
to be classified
some similarities and dissimilarities.
tanc es are mad e up of very tiny part icle s call ed mole cule s. Mole cules are formed from
All subs
so small that they can only be
even smaller particles called atoms. Atoms and molecules are
seen through a powerful microscope.
SUBSTANCES
I l
in
Impure ae oaae
Pure substances (Homogeneous)
|

Compounds Mixtures
Elements
r = ee
Noble gases Inorganic Organic Homogeneous Heterogeneous
Metals Non-metals Metalloids
e¢g.: @g. ¢ Sugar, Mixtures Mixtures
CF; e.g. : Bec e.g.: Helium,
Neon Water, Proteins e.g. : Ait, e.g. :
Copper, Hydrogen, Silicon, Sand and Water,
Arsenic Salt Salt, Water,
_ Iron Oxygen Oil and Water, etc.
etc.
Es Carbon
Pure substances ; Pure substances are Elements are made up of same kind of atoms
made up of same kind of atoms or molecules and compounds are made up of the same kind
and have a definite set of properties. They are of molecules.
all homogeneous i.e., their composition iS Impure substances : Impure substances
uniform throughout the bulk. Both elements are made up of two or more pure substances
and compounds are pure substances.
eee al

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mixed together in any proportions. They do At present 118 elements are known, Of
not have any definite set of properties but these, 90 are natural elements (most of them
they retain the properties of constituent are in combined state in the earth’s crust) while
substances. They may be homogeneous or rest 28 have been artificially created. Some
heterogeneous, i.e., their composition is not elements are solids, some are liquids and some
uniform throughout the bulk, e.g. : air, sugar are gases,
solution, sand and stone, etc.
G JA Do You Know ? }
Of the 118 elements, known to us,
| Make a list of five mixtures which you use ‘e radioactive in nature, becaus
in your daily life and give reason why those tions which may be harmful.
|
| substances should be considered as mixtures.
1.2 CLASSIFICATION OF ELEMENTS
1.1 ELEMENTS
Based on their properties, elements are
Elements are the limited number of basic classified into :
substances from which millions of more (ii) non-metals
(i) metals
substances are made.
(iii) metalloids (iv) noble (or inert) gases.
An element is the simplest pure substance.
Each element is made up of only one kind of atom, Metals : Most of the elements known to us
which differs from the atom of any other element. are metals.
Oxygen, hydrogen, sulphur, carbon, iron, 1. They are usually lustrous and hard solids.
gold, silver, efc. are some examples of elements. [Exceptions :- Mercury and gallium are
unit of an element is atom.
The smallest liquids. Sodium and potassium are soft.]
2. Metals are malleable and ductile [Zine is
brittle].
Note : Malleable (can be beaten into thin
‘Robert Boyle was the first scientist to use
sheets).
Ductile (can be drawn into wire without
breaking).
d finition of an element. 3. They have high melting and boiling points.
He defined an element as a basic f
4. They are good conductors of heat and
electricity.
|
5. They are sonorous substances because they
Examples : A piece of aluminium sheet contains only
produce a specific sound when struck.
aluminium atoms and oxygen contains atoms of oxygen
only. Atoms of oxygen and aluminium are different. Examples : Gold, silver, copper, aluminium, zinc, iron,
tin, etc.
Thus, each element has its own unique properties.

_— ain

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* Graphite is another form of carbon which

| ‘is soft, lustrous and a good conductor of |
heat and electricity.
Sittetal calchan. Diamond, due to its brilliant shine is used
as a gem for making jewelleries.
| contains magnesium which Oxygen, hydrogen, chlorine and nitrogen
Ip ‘ to capture the energy from sunlight
are all gases.
shotosynthesis by plants.
Metalloids : These elements show some
properties of metals and some properties of
non-metals. They are hard solids.
Examples : boron, silicon, germanium, arsenic,
Non-metals :
antimony, efc,
1. Non-metals are elements with a dull
surface, i.e., they do not shine. Inert or noble gases : These elements do
2. They are fewer in number as compared not react chemically with other elements or
to metals. They are either soft solids or compounds, so they are known as noble (or
gases, except bromine, which is a liquid. inert) gases. They are found in air, in traces.
There are only six noble gases — helium, neon,
3. Non-metallic solids are brittle in nature.
argon, krypton, xenon and radon.
4. They are all bad conductors of heat and
electricity. [Except carbon in the form of 1.3 SYMBOLS OF ELEMENTS
graphite and gas carbon]. 1. Each element is denoted by a symbol
5. They have low melting and boiling which is usually the first letter of its name in
points [Exception: Carbon in the form of English or Latin [written in capital].
Graphite has a high melting point).
Example : Oxygen is an element. It is denoted by the
6. They are neither malleable nor ductile. symbol ‘O". Similarly, hydrogen is denoted by a s) mbol
7. They are not sonorous, ‘H’. Now-a-days, IUPAC (International Union of Pure
8. They display a variety of colours. and Applied Chemistry) approves names of elements.

Examples : Sulphur, phosphorus, carbon, oxygen, 2. However, when the first letter of more
hydrogen, chlorine, nitrogen, efc. than one element is same, the symbol is denoted
by two letters, first letter is written in capital
while the second is written in small letter.
Example : Carbon, cobalt and copper are the elements
whose first letter is ‘C’. Carbon is denoted by the
“Co”
symbol ‘C’. Cobalt is denoted by two letters
Copper is denoted by the symbol *Cu’ [taken from its
Latin name cuprum]. Chromium is denoted by Cr
while chlorine by Cl.

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3. These symbols also represent an atom Chlorine — acl

of that element. — e
Carbon
Example : f Sulphur — S
(i) ‘H’ represents the element hydrogen as well as
one atom of hydrogen.
‘Phosphorus — P

(ii) ‘C’ represents the element carbon as well as one Boron —_ B

atom of carbon.
WSilicon — Si
4. Other symbols have been taken from Bietiur —_ He
the names of elements in Latin, German or
Neon id Ne
Greek.
“Argon —_— Ar
Example : The symbol of iron is Fe from its latin name
Ferrum, sodium is Na from Natrium, potassium is K
from Kalium. Therefore, each element has a name and
Table 1.2 : Names and symbols
a unique chemical symbol. Name in Symbol Name
| in | Symbol
English
Table 1.1 : Names and symbols of some elements = —
derived from latin and greek languages. Chromium Cr Mn
Name in Name in Symbol Cobalt Co Ni
English Latin/Greek Tin Sn Ba
Sodium Natrium Na Tungsten WwW Pt
Potassium Kalium K Radium Ra etl
Magnesium Magnesia Mg
‘Fluorine F » Br
Aluminium Alumen Al
Todine I rs Ge
Calcium Calx Ca
Arsenic As Sb
Tron Ferrum Fe
Krypton Kr Fs Xe
Copper Cuprum Cu
‘Radon Rn |
Zinc Zinke Zn
Compounds : “A compound is a pure
Silver Argentum Ag
substance formed by the chemical combination
Gold Aurum Au of two or more elements in a fixed ratio by
Mercury Hydragyrum Hg mass.”
Lead Plumbum Pb A compound can be broken down into
Hydrogen — H simpler substances. For example : table salt is
a compound which can be broken down into
Nitrogen — N
two elements (more simple substances) sodium
Oxygen — O and chlorine.

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Some common compounds are water, ¢ Compare the texture and colour of the
carbon dioxide, sugar, common salt, chalk, material obtained by the groups.
washing soda, alcohol, efc. * Add carbon disulphide to one part of the
material obtained. Stir well and filter.
The smallest unit of a compound is molecule
¢ Add dilute sulphuric acid or dilute
CHARACTERISTICS OF COMPOUNDS
1.4 hydrochloric acid to the other part of the
1. Compounds are homogeneous. material obtained.
2. The properties of compounds are entirely ® Perform all the above steps with both the
different from those of its constituent elements (iron and sulphur) separately.
elements.
¢ The gas obtained by group | is hydrogen.
3. Compounds can be broken down into (Colourless, odourless, combustable).
their constituent elements only by
chemical means, not by physical means. ¢ The gas obtained by group Il is hydrogen
sulphide. (It is a colourless gas with the smell
4. Compounds have fixed composition of
of rotten eggs).
their own.
5. Energy is either absorbed or liberated You must have observed that the products
during the formation of a compound. obtained by both the groups are different,
though the starting materials were the same.
6. A compound can be represented by a
formula. Group | has carried out the activity involving a
physical change whereas in case of group Il, a
chemical change has taken place.
| F r.
¢ The material obtained by group | is a
What do we get when two or more mixture of the two substances. The
elements are combined ? substances given are the elements — iron
* Divide the class into two groups. Give 5g of and sulphur.
iron fillings and 3g of sulphur powder in a * The properties of the mixture are the same
china dish to both the groups. as that of its constituents.
Group | ¢ The material obtained by group II on heating
* Mix and crush iron fillings and sulphur the two elements strongly we get a
powder. compound, which has totally different
Group Il properties compared to the combining
* Mix and crush iron fillings and sulphur elements.
powder. Heat this mixture strongly till red * The composition of a compound is the same
hot. Remove from flame and let the mixture throughout. We can also observe that the
| cool. texture and the colour of the compound are
Group | and Il the same throughout.
. * Check for magnetism in the material Note : The experiment should be done very
obtained. Bring a magnet near the material carefully under the observation of the concer
ned
. and check if the material is attracted towards teacher.
the magnet.

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1. Write the symbols of helium, silver, krypton, 6. Name the main metal present in t
antimony, barium. following:
2. Write the names of the following elements Na, (a) Haemoglobin (b) Chalk
C, Kr, U, Ra, Fe, Co.
(c) Chlorophyll (d) Chocolate wrappers
3. Define :
(b) Compounds (ec) Hammer (f) Door-knob
(a) Elements
(g) Electric wires
4. What do you understand by :
(a) metalloids (b) noble gases Which clement exists as a soft solid as well as a
hard solid ? Give one example for each type c
5. List four differences between metals and non-
solid
in which it exists.
metals.
—

You have studied in the beginning of

the chapter that all substances are made up
of very minute particles called atoms and
molecules. Will you not like to know about
them in detail ?

1.5 AN ATOM
“An atom is the smallest particle of an
element that exhibits all the properties of that
element. It may or may not exist independently rs ' ep so small that it ¥
ou
but can take part in every chemical reaction. ons of them, just to covera
Example : Take a small piece of zinc and grind it
into smaller pieces. All these pieces show properties
1.6 A MOLECULE
of zinc. On grinding them further they break up into “A molecule is the smallest particle ofa
very fine particles which still show the properties of pure substance (element or compound) which
zinc. But, there comes a stage when the particles has independent existence. It exhibits all the
cannot be further subdivided into particles exhibiting properties of that pure substance” or a molec
properties of zinc. These indivisible particles are the is a group of two or more atomne See
atoms of zinc. chemically bonded together by at fore
“In other words atoms are the smallest possible Molecules are of two types:
units of an element”.
1. Molecules of an element.
Atoms of same element are all identical. 2. Molecules of Ome i i

They differ from the atoms of other elements. Mk

a
1.6.1 Molecules of an element.
That is why different elements differ in mr
ae

their properties. Te 0 mon est |

sen ce one
combine to form a molecule of ann.

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TN
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Triatomic molecules contains three atoms.

atoms of certain elements, like oxygen,
nitrogen, chlorine, efc., cannot exist Examples : Ozone O,.
independently. So they join to form molecules Polyatomic molecules contain more
that have independent existence. To form than three atoms.
molecules, atoms always join in whole numbers. Examples : Phosphorus (P,), sulphur (Sg), etc.
Examples :
1.6.3 Molecular formula of an element
1. Two atoms of hydrogen join to form one
molecule of the element hydrogen. The molecular formula of an element is
the symbolic representation of its molecule. It
® -@o-— ee indicates the number of atoms present in tt.
| ATOM OF HYDRO. | ATOM OF HYDRO- OF
| MOLECULE
GEN GEN HYDROGEN Example : A molecule of chlorine is represented by
join
‘Cl,’ which indicates that two atoms of chlorine
2. Eight atoms of SEPARATE
shows that
ATOMS OF to form one molecule of chlorine. It also
sulphur join to * SULPHUR
the atomicity of chlorine is 2.
form a molecule
of sulphur. Table 1.5 ; Names, symbols and state of the
A SULPHUR MOLECULE
molecules of common elements
1.6.2 Atomicity
Name of | Symbol of | Number of atoms | State
The number of atoms of an element that molecules | in one molecule
of that element
join together to form a molecule 2 Gas
of that Hydrogen H,
element is known as the atomicity Gas
Nitrogen N, 2
molecule. Depending upon the atomicity, the Oxygen O, 2 Gas
molecules of elements can be divided into : Fluorine F, 2 Gas
Gas
(a) monoatomic molecules Chlorine Cl, 2
Br, 2 Liquid
(b) diatomic molecules Bromine
l, 2 Solid
Iodine
(c) triatomic molecules O, 3 Gas
Ozone
(d) polyatomic molecules. Phosphorus P, 4 Solid

Monoatomic molecules : They contain Sulphur Sp 8 Solid

only one atom. Atoms of metals and metalloids

do not combine with their own types of atoms. From the above it is clear that :
ogen
So, their atoms are regarded as their molecules (i) ‘H’ represents one atom of hydr
of
too. Similarly, atoms of inert gases exist freely and ‘H,’ represents one molecule
under all conditions. All these elements are said hydrogen.
hydrogen
to have monoatomic molecules. (ii) ‘2H’ represents two atoms of
s of
Examples : Na, Zn, Mg, etc., noble gases - He, Ne, and 2H, represents two molecule
Ar, Xe, etc. hydrogen.
left hand
Diatomic molecules of an element If a numeral is written on the
number of
contains two atoms of the same type. side of a symbol it represents the
Examples : H,, 0, N3, Cl, ele atoms or molecules.

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1.7 MOLECULES OF COMPOUNDS A molecule of a compound can be broken

When atoms of two or more elements into its constituent elements.
join together a molecule of a compound is Examples :
formed.
1. Mercuric oxide is a solid compound. When it
Examples : is heated, it decomposes to give mercury and
1. Two atoms of hydrogen and one atom of oxygen oxygen, which are elements.
combine to form a molecule of water.

©-0-
oe & eo
= 6 -@ ——-~ e@t_——_ #

A MOLECULE AN ATOM OF AN ATOM OF

OF MERCURIC MERCURY OXYGEN
1 ATOM | ATOM | ATOM | MOLECULE OF OXIDE
WATER

One atom of carbon combines with two atoms 2. When electric current is passed through
th

of oxygen to form a carbon dioxide molecule. acidulated water it ionises to give hydrogen
and oxygen gases in the ratio of 2 : 1 [by
© -@ -eo-—-v90060 volume].
| ATOM 1 ATOM | ATOM 1 MOLECULE OF From whatever source water is taken and
CARBON DIOXIDE
ionised, the ratio of gases hydrogen and oxygen”
is found to be the same.

d
{1 MOLECULE
=-0-0-0
{TWO ATOMS OF HYDROGENAND
in all respects but differ in tel prop i ie OF WATER] | ATOM
OF OXYGEN]

from the atoms of which they are made,— Note : Molecules of different compounds show
different properties. E.g. :- Water molecules and sugar
Accordingly, a water molecule is a liquid,
molecules are different.
but hydrogen and oxygen atoms are gaseous.
Similarly, carbon dioxide is a gaseous Of the 118 elements known to mankind,
compound but carbon is a solid element. till now, millions of compounds are formed.
eg —_ i

r Pure substances are broadly classified into elements and compounds.

r Elements are made up of same kind of atoms.
rr Compounds are made up of different kinds of atoms.
@ Atoms are the smallest unit ofbee!
r Molecules ace the sneies ae co
‘pone s :

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—
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1. Define : Why is the molecular formula of sodium is Na,

(a) Atom (b) Molecule while that of oxygen is O,?
(c) Atomicity What does each of the following represent.
2. Give an example for each of the following and (a) 30 (b) H,
also give their formulae :
(c) 5Cl (d) 2Cl,
(a) A diatomic molecule of an element
(e) H,O () H,0;
(b) A polyatomic molecule of an element
(c) A diatomic molecule of a gaseous
Which of the following substances fall in the
compound.
category of a “pure substance” ?
Ice, milk, iron, hydrochloric acid, calcium oxide,
(d) A triatomic molecule of a liquid compound
mercury, brick, wood, air.
(e) Adiatomic molecule of a solid compound.

IVENS |
TYPE QUESTIO
| OBJECT
1. Fill in the blanks : Z. Give one words for the following :
(a) Molecules consisting of two atoms of an
(@) — sseesceeeseeeess...., are Malleable and ductile.
element.
(b) A non-metal which is a good conductor (b) The smallest unit of an element.
of heat and electricity is .............0... (c) The elements which do not react
chemically.
i tel Wexihisaiveessént BENGE ais vaicaites bx9* are pure
(d) The elements which resembles both
substances. metals and non-metals in their properties.
(d) A water molecule contains ............060 3. Write true or false for the following statements:
TIAL actsadecasnressiwes elements. (a) Molecule is the smallest particle
(e) 2H represents ..............+..... of hydrogen. of matter

BED arapacceiaysvevivis on heating gives two (b) Atoms of same element are all
alike
SOMONE ss seivssecviaiaes BMD ccendeuicoasas es
(c) 20 represents two molecules of
(g) The formation of a compound involves a oxygen
ANSE IEDs caisaveeresssuves
(d) Both chalk and bones contain
(h) The smallest unit of a compound is calcium
Oe See e eee eee eee eee
(e) Compounds are made up of
same kind of atoms.
(i) The atomicity of helium is ..................

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3, A water molecule contains hydrogen and oxygen

MULTIPLE CHOICE QUESTIONS
in the ratio (by volume)
1. The smallest particle of a compound showing 2a
(a) Lez (bo)
all its properties is
(a) an atom (b) an element (Chime (d) 1:1
(c) amolecule (d) none of these
4. A liquid non-metal is
2. A molecule of ozone has atomicity (b) Germanium
(a) Mercury
(a) one (b) two
(d) four (c) Neon (d) Bromine
(c) three
a
er =. ee
=e eee

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