Covalent bonding occurs when nonmetal atoms share electrons to form molecules. The dot and cross diagram model represents how electrons are shared in molecules like H2, Cl2, CH4, CO2, and H2O. Covalently bonded substances have properties like low melting and boiling points as the weak intermolecular forces allow the molecules to move apart easily. They also do not conduct electricity as there are no free electrons to move. The periodic table arranges elements in periods and groups based on their atomic structure. The group number represents the number of electrons in the outer shell, which determines an element's chemical properties.

1. 3c Covalent bonding and the Structure of the Periodic Table
Revision: A molecule is ______ or _____________ atoms bonded together
There are two different types of bonding:
1. Ionic bonding (see C3b) metals ______________ electrons to non-metals and
the resulting ions are ___________ to each other forming a bond
2. Covalent bonding - non-metals combine together by ________________ electrons
The “dot and cross” model
For your exam you must be able to:
“Describe the formation of simple molecules containing single and double covalent
bonds by the “dot and cross” model, limited to the molecules
H2 Cl2 CH4 CO2 H2O

2. Revision: By looking at the formula of a molecule, we can tell how many atoms of
each element are in that molecule.
e.g. CO2 1 carbon atom, 2 oxygen atoms
Task: Test yourself on these:
1. Cl2 6.
2. CH4
3. C2H6
4. C6H12 O6 7.
5. C2H5 OH
What properties do covalently bonded substances have?
For the exam you need to be able to list the properties of simple molecules. For a
high grade you must also be able to explain why they have these properties:
Property Explanation of property
Carbon dioxide and water are simple
molecules with __________ ____________
Low melting and boiling points
__________________between molecules*
So they are usually gases or liquids
e.g. CO2 and H2O
If the intermolecular forces are weak then a
low _______________ is needed to break
them so the melting point will be _________.
In order to conduct electricity, electrons
Do not conduct electricity
must be ______ ___ ___________ between
e.g. CO2 and H2O
electrodes. Since simple molecules do not have
free electrons they ____________ conduct
electricity
*You need to understand the difference between the strong covalent bonds holding atoms
together in a molecule and the weak attraction between neighbouring molecules.

3. The Periodic Table
• The rows are called ________________ and there are 7 periods
• The vertical columns are called ________________. There are 8 groups that you
need to be able to number.
• The block between groups 2 and 3 are called the ________________________.
Task: Use a Periodic Table to work out the period and group number of the
following elements:
Element Period number Group number or transition element
Iron, Fe
Sodium, Na
Neon, Ne
Sulphur, S
Nitrogen, N
Gold, Au
There are 2 pieces of information that we can get from the electron arrangement:
1. The group number

4. • This is the same as the number of _______________ in the outer shell
• So, Group 1 have ___ electron in their outer shell
Group 2 have ___ electrons in their outer shell Not this sort of
shell!
Group 7 have ___ electrons in their outer shell
Group 8 (also called Group 0) have a ______ outer shell
e.g. The electron arrangement of calcium is __________. The outer shell (last
number) has ___ electrons so calcium is in Group ______ of the periodic
table.
2. The period number
• By looking at the electronic structure we can tell which period an element is in.
The number of electron shells is the _____________ as the period number.
e.g. Calcium, 2.8.8.2 Calcium has ____ shells so it is in period ____
Task: Work out the group number and period number for the following atoms, then
find them on the Periodic Table and name them:
Atom Electronic Group number Period number Name of element
arrangement
A 1
B 2.4
C 2.8.1
D 2.8.8
E 2.7
F 2.8.3
G 2.8.8.1
Why is the group number important?
• It is electrons that take part in chemical reaction
• But only the electrons in the outer shell are involved

5. • All of the elements in a group behave in the same way because they all have the
same number of electrons in their outer shell – this is the same as the group
number.
e.g. Group 1 are all very reactive metals with water and their electron arrangements
are
Li 2.1
Na 2.8.1
K 2.8.8.1