1.1.6

# Ionisation Energy

Test yourself

## Equation for Ionisation Energy

The ionisation energy of an atom can be well defined by its ionisation equation.

### The electrons

• Ionisation is the reaction where one electron is removed from an atom.
• There will be one electron in the product of the equation.
• So we will always have an e term on the right-hand side of the equation.

### The charges

• We must understand the charges on the atom during the reaction.
• During ionisation, we remove one electron from an atom.
• As the negative electron is removed, the atom's charge will increase by 1.
• E.g. the second ionisation of Na converts Na+ → Na2+.

### The state symbols

• Ionisation energies are always defined as being in the gas phase.
• So atomic species of any charge, on either side of the equation, will have the state symbol (g).
• We do not define the state of the electron - it has no state symbol.

### Overall equation

• Putting all these three features together, we gain the ionisation energy equation.
• This is best shown by a few examples.
• The first ionisation energy equation of Na:
• Na(g) → e + Na +(g)
• The second ionisation energy equation of Mg:
• Mg+(g) → e + Mg 2+(g)

## Ionisation Energies

A specific amount of energy is needed to remove an electron from an atom or ion.

### First ionisation energy

• The first ionisation energy is the energy required to remove an electron from every atom in a mole of atomic gas, to produce a mole of unipositive gaseous ions.

### Second ionisation energy

• The second ionisation energy is the energy required to remove an electron from every ion in a mole of unipositive gaseous ions, to produce a mole of dipositive gaseous ions.

### Third ionisation energy

• The third ionisation energy is the energy required to remove an electron from every ion in a mole of dipositive gaseous ions, to produce a mole of tripositive gaseous ions.

### Fourth ionisation energy

• Do you see the pattern?

### Nth ionisation energy

• Overall we can form the equation shown here for the nth ionisation.
• Note the value n, for the nth ionisation, is the value of the charge on the ion in the PRODUCTS.