# How to Determine Group from Successive Ionisation Energy

Let's take a look at the following successive ionisation energies for elements P and Q.

In this video we want to deduce the formula of the compound formed between elements P and Q.

First we need to deduce the group that each element is from based on its successive ionisation energies.

1. Successive Ionisation Energies always increase.

When we supply energy to remove electrons from an atom, the first electron removed will be farthest away from the nucleus.

This is because the attraction between the nucleus and the farthest electron will be the weakest.

Hence least amount of energy is required to overcome this attraction and the first ionisation energy will be the least endothermic.

When more electrons are removed, they are progressively closer to the nucleus hence more energy is required.

This means successive ionisation energies will always increase, or it will always require more energy to remove the next electron.

2. Determine Group of Element P

Let's take a look at the differences in successive ionisation energies for element P.

We are interested in a big jump in IE as it represents a change in a principal quantum shell.

Notice there is a very significant increase or jump between the second and third ionisation energy, as compared to the first and second ionisation energy.

This means that the third electron is significantly harder to remove as compared to the second electron.

Therefore the third electron must have a much stronger attraction to the nucleus and it must be much closer to the nucleus as compared to the second electron.

Hence the third electron must be from an inner principal quantum shell, and the second and earlier electrons are from a valence shell.

Element P has 2 valence electrons and is in Group 2.

3. Determine Group of Element Q

The first big jump in IE is between the fifth and sixth electron.

This means that the sixth electron is from an inner principal quantum shell, experiences a much stronger attraction with the nucleus hence requires much more energy to remove.

The fifth and earlier electrons are from a valence shell, element Q has 5 valence electrons and is in Group 15.

4. Deduce Formula of Compound

P is a Group 2 metal hence will lose 2 electrons to form cation P2+ when forming compounds.

Q is a Group 15 non-metal and will gain 3 electrons to form anion Q3- when forming compounds.

Therefore the formula of compound formed between P and Q will be P3Q2.

Topic: Atomic Structure, Physical Chemistry, A Level Chemistry, Singapore

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