Bonding - (CCEA)

• There are three types of chemical bonding: ionic, covalent and metallic.
• In ionic bonding, electrons are transferred from a metal atom to a non-metal atom to form oppositely charged ions.
• In covalent bonding, non-metal atoms share electrons in order to achieve a full outer shell.
• In metallic bonding, metal atoms delocalise their outer electrons to form positive ions which are attracted to the delocalised electrons.

Bonding takes place when atoms join together.

There are three types of bonding studied at GCSE.

The type of bonding depends on the type(s) of elements that are being bonded together.

Atoms tend to lose or gain electrons to form a full outer shell.

This is because having a full outer shell of electrons makes an atom stable.

For example, an oxygen atom has an electronic configuration of 2,6.

It can gain two electrons to form a stable full outer shell (the second shell holds a maximum of 8 electrons).

Atoms can also lose electrons to form a full outer shell.

For example, a sodium atom has a electronic configuration of 2,8,1.

It can lose one electron to be left with a full outer shell.

Key fact

Atoms will always choose the ‘easiest’ way to get a full outer shell.

For example, it is easier for a sodium atom to lose one electron to get a full outer shell rather than trying to gain an extra 7 electrons.

Anions and cations

As both the sodium atom and the oxygen atom no longer have the same number of protons and electrons they are no longer neutral and they have an overall charge.

They are now called ‘ions’ rather than ‘atoms.’

An ion is a charged particle.

They are formed when atoms gain or lose electrons to form a full outer shell.

• A negative ion is called an anion. An anion is formed when an atom gains electrons.
• A positive ion is called a cation. A cation is formed when an atom loses electrons.

A molecular ion is a particle made of more than one atom that has an overall positive or negative charge.

For example, the hydroxide ion, OH^-, is a molecular ion.

Key facts

• When a non-metal atom turns into an ion, the element’s name changes to end in ‘-ide.’

For example, a fluorine atom turns into a fluoride ion.

• When writing the charge on the ion, remember to put the number before the positive or negative symbol (2+).

Just write + or – if the charge is 1+ or 1-.

Ionic bonding takes place between a metal and a non-metal.

For example, sodium (metal) and chlorine (non-metal) can form an ionic bond.

To form a stable full outer shell, the sodium atom needs to lose one electron and the chlorine atom needs to gain one electron.

To achieve this, the sodium atom can transfer its outer electron to the chlorine atom.

This forms a sodium ion and a chloride ion that have opposite charges.

The two ions are held together by a strong electrostatic attraction. This is an ionic bond.

When forming an ionic compound:

• electrons are transferred from the metal atom to the non-metal atom.
• this forms oppositely-charged ions.
• the metal ion has a positive charge.
• the non-metal ion has a negative charge.
• an ionic bond is the electrostatic attraction between these oppositely-charged ions.
• ionic bonding is strong and requires substantial amounts of energy to break.

Dot and cross diagrams show how two elements ionically bond.

There are 4 key steps involved in drawing dot and cross diagrams:

1. Draw the electronic configuration of each atom – one element with dots and the other with crosses.
2. Work out how many electrons need to be transferred.
3. Draw the electronic configuration of the ions that are formed.
4. Write the charge of each ion.

Use a dot and cross diagram to show the bonding between magnesium and oxygen.

Covalent bonding occurs between non-metal atoms.

A covalent bond is formed when two atoms share electrons to form a full outer shell.

For example, two chlorine atoms can covalently bond to form a chlorine molecule.

Both atoms have 7 electrons in their outer shells.

Key fact

Only the outer shells of the atoms need to be shown in a dot and cross diagram for covalent bonding.

The outer shells of both chlorine atoms overlap and a pair of electrons is shared.

A covalent bond is a shared pair of electrons.

A covalent bond is strong and takes a large amount of energy to break.

Note the two types of electron pairs in the diagram:

• Bonding pair: a pair of electrons shared between two atoms.
• Lone pair: a pair of unshared electrons in the outer shell of an atom.

The Cl₂ molecule is an example of a diatomic molecule – two atoms covalently bonded.

Here are the dot and cross diagrams for some common molecules.

Key fact

A covalent bond can be represented by a line (–) in a structural formula.

In some cases, more than one pair of electrons can be shared between two atoms.

In an oxygen molecule (O₂), two pairs of electrons are shared between the oxygen atoms.

This is a double covalent bond.

There are three different kinds of covalent bonds:

• a single covalent bond is when two atoms share a single pair of electrons. Represented by a single line (–).
• a double covalent bond is when two atoms share two pairs of electrons. Represented by a double line (＝).
• a triple covalent bond is when two atoms share three pairs of electrons. Represented by a triple line (≡).

The table below shows several molecules with double and triple covalent bonds.

In a metal, atoms are able to form a full outer shell by delocalising their outer electrons.

This means the outer electrons are released from the atom and can move freely.

When lots of metal atoms delocalise their outer electrons, a regular lattice of positive ions is formed, surrounded by delocalised electrons.

Metallic bonding is the attraction between the positive ions in a regular lattice and the delocalised electrons.

Delocalised electrons are free to move throughout the whole structure, they are often described as a delocalised sea of electrons.

When drawing a diagram of a metal’s structure, be sure to draw the ions in regular rows.