C4: Atoms

Combining

CHEMISTRY
SECONDARY 3 ZEST
 Atoms Combining:
4.1 Compounds, Mixtures & Chemical Changes
• All substances can be classified into one of these
three types: elements, compounds and mixtures
Element
• A substance made of atoms that all contain
the same number of protons (one type of atom)
and cannot be split into anything simpler
• There is a limited number of elements and all
elements are found on the Periodic Table
• i.e. hydrogen, carbon, nitrogen
 Atoms Combining:
4.1 Compounds, Mixtures & Chemical Changes
Compound

• A pure substance made up of two or more

elements chemically combined

• There is an unlimited number of compounds

• Compounds cannot be separated into their elements by

physical means

• i.e. copper (II) sulphate (CuSO4), calcium carbonate

(CaCO3), carbon dioxide (CO2)
 Atoms Combining:
4.1 Compounds, Mixtures & Chemical Changes
Mixtures
• A combination of two or more
substances (elements and/or
compounds) that are not chemically
combined
• Mixtures can be separated by physical
methods such as filtration or
evaporation
• i.e. sand and water, oil and water, Particle diagram showing elements, compounds and mixtures
sulphur powder and iron filings
 Atoms Combining:
4.1 Compounds, Mixtures & Chemical Changes
The Signs of a Chemical Change
• In chemical reactions, new chemical products are formed
that have very different properties to the reactants
• Most chemical reactions are impossible to reverse
• Energy changes also accompany chemical changes and
energy can be given out (exothermic) or taken in
(endothermic)
• The majority of chemical reactions are exothermic with
only a small number being endothermic
 Atoms Combining:
4.1 Compounds, Mixtures & Chemical Changes
Physical Change
• Physical changes (such
as melting or evaporating) do
not produce any new chemical
substances
• These changes are often easy to
reverse and mixtures produced
are usually relatively easy to
separate
Exercise 4.1: Textbook page 43
Answers to Exercise 4.1: Textbook page 43
 Atoms Combining:
4.2 Why do Atoms form Bonds?
• The Periodic Table can be used to make several
predictions about whether atoms will form bonds and
what type of bonds they might form with each other
• On the far right-hand side of the Periodic Table is the
group of elements called the noble gases
• Atoms of these elements (i.e. helium, krypton, neon)
have full outer electron shells
• These atoms are stable and very rarely form bonds with
other atoms
 Atoms Combining:
4.2 Why do Atoms form Bonds?
The Formation of Ions
• An ion is an electrically charged atom
or group of atoms formed by
the loss or gain of electrons
• This loss or gain of electrons takes
place to gain a full outer shell of
electrons
• The electronic structure of an ion will
be the same as that of a noble gas –
such as helium, neon and argon Formation of positively charged sodium ion
 Atoms Combining:
4.2 Why do Atoms form Bonds?

Formation of negatively
charged chloride ion
 Atoms Combining:
4.2 Why do Atoms form Bonds?
• Metals: all metals lose electrons to other
atoms to become positively charged ions

• Non-metals: all non-metals gain electrons

from other atoms to become negatively
charged ions

• The positive and negative charges are held

together by the strong electrostatic forces
of attraction between opposite charges

• This is what holds ionic compounds together

Electrostatic forces between the positive Na ion and negative Cl ion

• Question 1: What are ions and how are they formed? Give 2 examples of ions.
• Question 2: What is the relation between group number in PT and electrons of atoms?
• Question 3: Why do elements of Group I & II tend to lose their electrons?
• Question 4: Why do elements in Group VI & VII tend to gain electrons from other elements?
• Question 5: When writing equations for the ions Na+ and Cl-, what is the difference?
• Question 6: How many electrons Mg will lose and O will gain and why?
• Question 7: Other than Na & Mg, name 2 more elements which will lose electrons and other than Cl
and O, name 2 more elements which will gain electrons.
Exercise 4.2: Textbook page 45
Answers to Exercise 4.2: Textbook page 45
Additional Exercises: Question 1

Answer: B
Additional Exercises: Question 2

Answer: B
Additional Exercises: Question 3

Answer: D
Additional Exercises: Question 4
Topical Test, Term 2 21/22

• Date & Day: 21st October 2021, Thursday (Week 3)

• Covers: Chapter 4 (subtopics will be announced later)

• MYE: 22nd November 2021 – 3rd December 2021

 Example:

Atoms Combining: Sodium Chloride,

NaCl

4.3 The Ionic Bond

The Formation of Ionic Bonds, Groups I to VII
• Sodium is a group 1 metal, so it will lose one outer electron to
another atom to gain a full outer shell of electrons

• A positive sodium ion with the charge +1 is formed

• Chlorine is a group 7 non-metal, so it will need to gain an electron to

have a full outer shell of electrons

• One electron will be transferred from the outer shell of the sodium
atom to the outer shell of the chlorine atom

• A chlorine atom will gain an electron to form a negatively charged

chloride ion with a charge of -1
 Atoms Combining:
4.3 The Ionic Bond
• Magnesium is a group 2 metal, so it will lose two outer
Example: Magnesium Oxide, MgO
electrons to another atom to have a full outer shell of
electrons

• A positive ion with the charge +2 is formed

• Oxygen is a group 6 non-metal, so it will need to gain

two electrons to have a full outer shell of electrons

• Two electrons will be transferred from the outer shell of

the magnesium atom to the outer shell of the oxygen
atom
Diagram showing the dot-and-cross
diagram of magnesium oxide • Oxygen atom will gain two electrons to form a negative
ion with charge -2
 Atoms Combining:
4.3 The Ionic Bond
The Lattice Structure of Ionic Compounds
• Lattice structure refers to the arrangement of the atoms of a
substance in 3D space

• In lattice structures, the atoms are arranged in

an ordered and repeating fashion

• The lattices formed by ionic compounds consist of

a regular arrangement of alternating positive and negative ions

The lattice structure of NaCl