Ionisation Energy is the amount of energy needed to remove an electron from an atom. When an atom gains or loses an electron, it becomes a charged particle called an ion. This process is known as ionisation. The energy required to cause this change is called Ionisation Energy. It is an important concept in chemistry and is used to explain many physical and chemical properties of elements. Understanding Ionisation Energy can help us understand how elements interact with each other and with the environment.

What is ionisation energy?

Ionisation energy is a crucial concept in chemistry, and it refers to the energy required to remove a specific number of electrons from an atom. Specifically, it's the energy needed to remove one mole of electrons from one mole of gaseous atoms under standard conditions. You may recall from your study of fundamental particles that ions consist of the same number of protons, which determine the element it belongs to. However, ions can have varying numbers of neutrons without affecting their chemical reactivity. The key difference between ions is their number of electrons, which determine the atom's stability. To better understand this, let's quickly review 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 refers to the minimum amount of energy required to remove one mole of an atom's outermost or most loosely held electron from one mole of gaseous atoms. This process results in the formation of a positively charged ion with a charge of +1.

To give you an idea, let's take the example of sodium. The first ionisation energy of sodium can be represented by the following equation:

Na(g) → Na^+(g) + e^-

This equation shows that when one mole of sodium atoms in the gaseous state loses one electron, it forms one mole of positively charged sodium ions and one mole of free electrons.

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 simpler terms, it refers to the energy required to remove the second outermost electron from a gaseous atom that has already lost one electron.

For instance, the second ionisation energy of aluminium can be represented by the following equation:

Al^+(g) → Al^2+(g) + e^-

As you continue removing electrons from a species, eventually, only the nucleus remains, and this process is known as successive ionisation energies.

It's important to note that the second ionisation energy does not refer to the energy required to remove two electrons from an atom, but rather the energy needed to remove the second electron. To calculate the energy required to remove the two outermost electrons, you need to add up the values of the first and second ionisation energies.

Let's use the example of aluminium again. To go from one mole of aluminium atoms to one mole of aluminium ions with a charge of +2, it would require the sum of the first and second ionisation energies. 

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.

‍

When comparing the first ionisation energies of carbon and oxygen, we can observe that both elements have their outermost electron in the same sub-shell and the same number of inner shells. However, oxygen has a higher first ionisation energy than carbon due to its higher nuclear charge of 8, which results in a stronger attraction between the nucleus and the outermost electron.

Furthermore, successive ionisation energies increase as more electrons are removed from an atom or ion. For example, the first ionisation energy of sodium is 496 kJ mol-1, while the second ionisation energy is significantly higher at 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.

In conclusion, ionisation energy is a measure of the energy required to remove one mole of outer shell electrons from one mole of gaseous atoms, and it is influenced by factors such as nuclear charge, distance from the nucleus, and inner shielding.

Ionisation Energy

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.

What is the first ionisation energy of an element?

The first ionisation energy is the energy required to remove one mole of the outermost electrons from one mole of gaseous atoms under standard conditions.

Why is the second ionisation energy greater than the first?

The second ionisation energy is greater than the first because an electron is removed from a positive ion, which requires more energy.

What is the second ionisation energy?

The second ionisation energy is the energy required to remove one mole of the next outermost electrons from one mole of gaseous cations that have a charge of +1.

‍