Standard Grade Chemistry
Summary Notes
Topic 4 : How Atoms Combine
Learning Outcomes
General







Credit




Atoms can be held together by bonds
Describe the covalent bond in terms of atoms sharing pairs of electrons
A molecule is a group of atoms held together by covalent bonds
The chemical formula gives the number of atoms of each element in a
molecule of a covalent substance
(usually) only atoms of non-metal elements bond to form molecules
a diatomic molecule is made up of two atoms
the elements which exist as diatomic molecules are H2, O2, N2, F2, Cl2, Br2
and I2.
Atoms share electrons to achieve a stable electron arrangement
A covalent bond exists when two positive nuclei are held together by their
attraction for the shared pair of electrons
Draw diagrams to show how the outer electrons form a covalent bond
Draw diagrams to show the shapes of simple two element molecules
Introduction
Most elements are made of atoms which can combine with other atoms to form
molecules.
We say that the bonds form between the atoms. We say that the molecule is held
together by bonds.
Atoms, joined together by bonds, form molecules.
The bonds between atoms are made by electrons.
To understand how this happens we need to know how many electrons are in the outer
shell of an atom.
Why Do Atoms Combine?
The noble gases have two properties in common :
1.
they have the maximum number of electrons in their outer energy level
 helium has 2
 the other have 8
2.
they are all unreactive
Noble gases have a stable electron arrangement. Other atoms which do not have this
stable arrangement often combine with other unstable atoms to achieve this stable
arrangement. It is the combination of unstable atoms which produces a chemical
reaction and results in the formation of compounds.
Atoms can achieve a stable electron arrangements by gaining or losing electrons or by
sharing electrons with other atoms.
Covalent Bonding
This occurs when non-metal atoms join together to form covalent compounds. Atoms in
covalent compounds are held together by covalent bonding. In covalent bonding, the
atoms share a pair of electrons.
The Covalent Bond
A full outer shell of 8 electrons (or two for the first energy level) is stable and atoms
try to end up with a full outer shell if they can.
In molecules formed between atoms of non-metal elements, the bonds are called
covalent bonds. In a covalent bond two atoms share a pair of electrons.
When atoms are held together by sharing electrons we say that a covalent bond has
formed between the two atoms. We usually show a bond in the following way:
F – F
The line stands for the pair of shared electrons.
This is called the structural formula.
Since this molecule is made of two fluorine atoms, we write it this way,
F2
This is the molecular formula for fluorine.
More About Covalent Bonds
We can think of the bond formed between the shared electrons as working in this way :
F
+
positive
fluorine
nucleus
F
e-
e-
negative electronic
charge from the two
shared electrons
+
positive
fluorine
nucleus
The two fluorine nuclei, which are positively charged, are both attracted to the
concentrated negative charge of the shared electrons. This means that they are held
together – they are bonded.
Only non-metal elements form molecules by sharing electrons.
Metals form compounds in a different way. (See Topic 7)
Diatomic Molecules
We have learned that atoms seem to be more stable if they have a full outer shell. For
this reason, many atoms cannot exist on their own. They have to pair up with each other.
Hydrogen
Think about hydrogen. Its atomic number is 1.
It has one proton and one electron in its outer shell. However, the outer shell of
hydrogen needs two electrons to be stable, so 2 hydrogen atoms combine to form a H2
molecule.
Each atom now has two electrons in its outer shell.
We now have a molecule with the structural formula : H – H
and chemical formula : H2
We call a molecule with two atoms in it a diatomic molecule.
H2 is a diatomic molecule – it is made of two atoms only.
Chlorine
Chlorine atoms do the same. They are in Group 7 and have 7 electrons in their outer
shell. To be stable the outer shell needs 8 electrons.
Two chlorine atoms share electrons to form a chlorine molecule which is diatomic.
We now have a molecule with the structural formula
And chemical formula
Cl-Cl
Cl2
All the other elements of Group 7 do exactly the same because they also have 7
electrons in their outer shell.
We get diatomic molecules of :
fluorine
chlorine
bromine
iodine
F2
Cl2
Br2
I2
Oxygen
Oxygen is in Group 6. It has an outer shell of 6 electrons. 6 electrons in an outer shell is
not stable because it is not full. Two oxygen atoms can bond together by sharing two
pairs of electrons.
We get a molecule with the structural formula :
and molecular formula :
O=O
a double bond, formed by sharing
2 pairs of electrons
O2
Nitrogen
Nitrogen is in Group 5. It has an outer shell f 5 electrons. This is not stable because it is
not full.
Two nitrogen atoms can bond together to form a diatomic molecule.
We get a molecule with the structural formula :
and molecular formula :
N = N
a triple bond, formed by sharing 3
pairs of electrons
N2
The following elements exist as diatomic molecules :
hydrogen
fluorine
chlorine
bromine
iodine
oxygen
nitrogen
H2
F2
Cl2
Br2
I2
O2
N2
Most of these are gases, except bromine, a liquid and iodine, a solid.
State Symbols
We can show the state of an element or compound by letters in brackets after the
formula.
(s)
(l)
(g)
(aq)
-
for solid
for liquid
for gas
‘in solution’
We call these letters in brackets state symbols.
Valency
If you want to work out a molecular formula, you do not have to draw electron diagrams.
There are some simple rules to help you do it. Each atom has a certain combining power;
it is called the valency of the atom. The valency is equal to the number of unpaired
electrons in the outer shell.
Group 1
1 unpaired
electron
Group 2
2 unpaired
electrons
Group 3
3 unpaired
electrons
Group 4
4 unpaired
electrons
Group 5
3 unpaired
electrons
Group 6
2 unpaired
electrons
Group 7
1 unpaired
electron
valency = 1
valency = 2
valency = 3
valency = 4
valency = 3
valency = 2
valency = 1
Elements in Group 0 (or Group 8) are unreactive because they have full outer electron
shells. They do not have a valency and do not form bonds.
Rules For Writing Formulae
e.g. sodium chloride
1.
write down the symbols for the elements
Na
Cl
2.
work out the valency of the elements.
write this lightly about the symbols
1
Na
1
Cl
3.
cross over the valency numbers
1
Na
1
Cl
4.
cancel if necessary
Na1
Cl1
5.
Write the formula neatly
NaCl
Shapes Of Molecules
Every molecule has a shape. This is because the atoms which make it up are connected
together in a certain way. For example the hydrogen atoms in a methane molecule are
arranged tetrahedrally around the central carbon. To work out the shape of molecules
you have to imagine each atom in three dimensions.
H
methane
C
H
tetrahedral shape
H
H
water
O
H
H
‘bent’ shape
Word Equation
A word equation can be used to show what is happening in a chemical reaction.
For example :
sodium + oxygen  sodium oxide
Sodium and oxygen are the reactants. Reactants are found on the left of the arrow.
Sodium oxide is the product. Products are found on the right of the arrow.
‘+’

means ‘and’ (or reacts with)
means ‘changes into’
Chemical Equations
In a chemical equation we replace the words with chemical symbols or formulae.
word equation
chemical equation
sodium + oxygen  sodium oxide
Na + O2  Na2O
Chemical equations must then be balanced in order that the same number of each type
of atom appear on each side of the equation.
balanced
chemical
equation
4Na + O2  2Na2O