Unit 4 chemistry

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Unit 4 - How Atoms are Held Together

This section introduces the idea of covalent bonding, leading to molecules and chemical formulae.

Covalent Bonds and Molecules

Compounds are made up of atoms of more than one element. In Chemistry, compounds can be identified as either covalent compounds or ionic compounds. The next three sections (4, 5 and 6) concentrate almost totally on covalent compounds. Covalent compounds can be distinguished from ionic compounds experimentally (this will be done in later sections (. They can also be distinguished on paper if we know the elements which are chemically joined together to form the compounds.

Ionic Compound: An ionic compound is a metal element chemically joined to a non- metal element.
Example: Sodium Chloride is a compound consisting of a metal (sodium) and a non-metal (chlorine), thus since this is a non-metal chemically joined to a non-metal it is an ionic compound.

Covalent Compound: A covalent compound is two or more non-metals chemically joined to each other.
Example: Hydrogen Chloride is a compound consisting of a non-metal (hydrogen) and another non-metal (chlorine), thus since this is a non-metal chemically joined to a non-metal, it is a covalent compound.

Groups of Atoms

We say atoms are bonded together when they are joined together.
A molecule is a group of atoms joined together by covalent bonds.
A diatomic molecule is a molecule containing two atoms.
Covalent bonding is found in molecules.
The chemical formula gives the number of atoms of each element in a molecule.
Carbon Monoxide is made up of diatomic molecule.
Non-metal atoms bond together to form molecules.


Models are always used initially to determine the full structural formula.
When using the Molymods the spheres are held together by connections. Each connection represents a single covalent bond which is represented as ( - ) two connections holding a pair of shells together represents a double covalent bond which is represented as
( = ).

Covalent Bonding in Elements

Hydrogen is made up of atoms joined by a single covalent bond.
Oxygen is made up of atoms joined by a double covalent bond.
Nitrogen is made up of atoms joined by a triple covalent bond.
Triple bonds (   ) are more difficult to break than double bonds ( = ) : These in turn are more difficult to break than single bonds ( - ).
There are 7 diatomic elements (an element which consists of two atoms):
Iodine, Bromine, Chlorine, Fluorine, Oxygen, Nitrogen, Hydrogen.
I2, Br2, Cl2, F2, O2, N2, H2.
Mnemonic: ( I, Bring, Clay, For, Our, New Home).


Carbon exists in two giant forms which are very different: a) Diamond. b) Graphite.

Structures of Diamond and Graphite: 

 - (Diamond) this consists of strong covalent bonds.
Theses two forms are all called Allotropes (or polymorphs). They are different forms of the same element with different structures and different chemical properties.
Physical Properties: a) Very hard. b) High Melting Point. c) Does not conduct electricity.
Structure: It exists as one


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