Some elements just seem to want to react. You expose them to the air, sprinkle them with a tiny drop of water, and wham! They’ve reacted. Take sodium, for example. Sodium is a relatively unassuming Group 1 metal with 11 electrons, yet it must be stored in oil to keep it from reacting with any water vapour in the air. As soon as you drop it in a basin of water, it’ll fizz violently, produce a flame, and turn a green universal indicator solution pink. But neon, which contains just one less electron than sodium, will not react with water at all. An element’s reactivity is all to do with its ability to form ions, and for some elements, this is easier than for others.
An ion is an atom that has gained or lost an electron to form a charged particle.
What is ionisation energy?
Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms under standard conditions of 298K and 1 atm.
You should know from Fundamental Particles that ions contain the same number of protons - after all, the number of protons determines what element it is a part of. Ions could contain different numbers of neutrons, as neutrons don’t have much effect on chemical reactivity. But ions always have different numbers of electrons, and they exist because atoms like being in the most energetically stable state. To understand this, let’s have a quick review of electron shells.
Electron shells
In Fundamental Particles, we learnt that electrons orbit the atom’s nucleus in rings called shells, or energy levels. These shells can all hold different numbers of electrons. (For a more detailed look, check out Electron Shells.) Elements are most stable when they have full outer shells of electrons. For some, like sodium, the easiest way to do this is by losing a single electron. It isn’t very hard for sodium to lose an electron, which is why it is so reactive. But for aluminium to have a full outer shell, it must lose three electrons. This is quite a bit harder, which is why aluminium is a lot less reactive than sodium. To understand why, we need to consider ionisation energy.
Types of ionisation energy
Ionisation energy is measured in 
. There are different types of ionisation energy, which we'll explore below.
First ionisation energy
The first ionisation energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms.
An atom’s most loosely held electron is its outer shell electron. After losing an electron, each atom forms a positive ion with a charge of +1.
For example, the first ionisation energy of sodium can be represented by the following equation:
Second ionisation energy
The second ionisation energy is the energy required to remove one mole of the next most loosely held electrons from one mole of gaseous +1 cations.
In other words, it is the energy required to remove the second outermost electron from a gaseous atom.
For example, the second ionisation energy of aluminium can be shown by:
If you continue removing electrons from a species, eventually just a nucleus will remain. This process of repeated ionisation is known as successive ionisation energies.
You should note that the second ionisation energy is not the energy required to remove two electrons from an atom, but rather the energy required to remove the second electron. To find the energy needed to remove the two outermost electrons, you must add up the values of the first and second ionisation energies.
Let’s take our example of aluminium again. We can see that getting from one mole of aluminium atoms to one mole of aluminium ions with a charge of +2 requires 
of energy:
What factors affect ionisation energy?
You'll remember from Electron Configuration that atoms first lose electrons from their outer shells when forming ions. Electrons are all negative, and these negative particles are held in place by electrostatic attraction between themselves and the atom’s positively charged nucleus. Several factors affect the strength of this attraction.
- Nuclear charge.
- The distance of the electron from the nucleus.
- Shielding from inner electrons.
These factors all affect ionisation energy. Let’s explore them.
Nuclear charge
Nuclear charge is a measure of the strength of the positive charge of the nucleus. In other words, it’s a measure of a nucleus's number of protons, as you should know that protons are positive particles with a relative charge of +1. The more protons a nucleus has, the stronger its nuclear charge will be. A stronger nuclear charge increases the attraction between the nucleus and the outermost electron and increases ionisation energy.
Distance from the nucleus
The further the outermost electron is from the nucleus, the weaker the attraction between the nucleus and electron becomes. A weaker attraction decreases the ionisation energy.
Shielding
If we look at sodium again, we know from its electron configuration that it has just one electron in its outer shell. The other ten electrons are all found in inner shells closer to the nucleus.
A sodium atom. Note that it has one electron in its outer shell and ten in inner shells. commons.wikimedia.org
Both elements have their outermost electron in the same sub-shell, and so they are both the same distance from the nucleus. Both have the same number of inner shells - in this case, just the first electron shell. The only other factor remaining is the nuclear charge. As carbon has a nuclear charge of 6 and oxygen has a nuclear charge of 8, oxygen’s outermost electron experiences a stronger attraction to the nucleus than carbon’s, so oxygen has a higher first ionisation energy.
Trends in successive ionisation energies
Ionisation energy increases as you remove more electrons from an atom or ion. For example, the first ionisation energy of sodium is 496 kJ mol-1, but the second is 4563 kJ mol-1. This is because removing the second outermost electron from sodium requires removing a negative electron from a positive ion, where the attraction between the electron and the nucleus will be stronger.
Ionisation Energy - Key takeaways
- Ionisation energy is a measure of the energy required to remove one mole of outer shell electrons from one mole of gaseous atoms.
- Ionisation energy is measured under standard conditions of 1 atm and 298K.
- Factors that influence the attraction between the nucleus and the outermost electron all affect ionisation energy. They include nuclear charge, distance from the nucleus and inner shielding.
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