Ionization energy is controlled by the attraction between the nucleus and the electron removed. Three closely linked factors explain why some electrons are easier to remove than others.

What controls ionization energy?

The size of an ionization energy depends on how strongly a particular electron is attracted to the nucleus. A stronger attraction means more energy is needed to remove the electron. A weaker attraction means less energy is needed.

When explaining the magnitude of an ionization energy, always consider:

• nuclear charge

• electron shielding

• the subshell from which the electron is removed

These factors do not act separately. They combine to determine the overall attraction between the nucleus and the electron.

Nuclear charge

Nuclear charge is one of the main factors affecting ionization energy.

A higher nuclear charge means there are more protons in the nucleus. Since electrons are negatively charged, a more positively charged nucleus attracts them more strongly.

If two electrons are being removed from similar positions in different atoms, and shielding is similar, then:

• higher nuclear charge gives stronger attraction

• stronger attraction gives higher ionization energy

This is why ionization energy often increases when proton number increases, provided the electron removed is in the same type of subshell and experiences similar shielding.

It is important to link the idea clearly: more protons do not directly “cause” higher ionization energy on their own. They increase the electrostatic attraction between the nucleus and the electron being removed.

Electron shielding

However, outer electrons are not attracted only by the nucleus. They are also affected by the presence of other electrons.

Inner electrons lie between the nucleus and the outer electron.

This diagram illustrates the idea of effective nuclear charge: inner-shell electrons partially screen the nucleus so an outer electron experiences a reduced attractive pull. It visually links shielding to a smaller net attraction, helping explain why greater shielding generally lowers ionization energy. Source

They reduce the full effect of the nuclear charge felt by that outer electron.

As shielding increases:

• the attraction between nucleus and outer electron decreases

• less energy is needed to remove the electron

• ionization energy decreases

This means an electron can be easier to remove even if the atom has more protons, because some of the increased nuclear charge is offset by extra shielding.

Inner-shell electrons are especially important in shielding because they are closer to the nucleus and more effectively block the attraction felt by outer electrons. Electrons in the same shell do not shield each other as effectively as electrons in inner shells.

A common exam point is that shielding does not reduce the actual number of protons. The nuclear charge stays the same. Shielding only reduces the effective attraction experienced by the electron being removed.

The subshell from which the electron is removed

Ionization energy is also affected by the subshell containing the electron removed.

Electrons in different subshells do not all experience the same attraction to the nucleus, even when they are in the same main shell. This is because subshells have different energies and different average distances from the nucleus.

For A-Level Chemistry, the key comparison is usually between $s$ and $p$ electrons:

This radial distribution plot for a 2s orbital shows how electron probability varies with distance from the nucleus, including significant density closer in (penetration) and a radial node. The key takeaway is that $s$ orbitals can place electron density nearer the nucleus than higher-$l$ subshells, supporting why $s$ electrons tend to be held more strongly and have higher ionization energies. Source

• an $s$ electron is, on average, closer to the nucleus

• it experiences stronger attraction

• it is harder to remove

• it therefore has a higher ionization energy

By contrast:

• a $p$ electron is, on average, slightly farther from the nucleus

• it is more shielded

• it experiences weaker attraction

• it is easier to remove

So, if the electron removed comes from a $p$ subshell rather than an $s$ subshell, the ionization energy may be lower even when nuclear charge has increased.

This explains why ionization energy does not always follow a perfectly smooth pattern.

This graph plots first ionization energy against atomic number, showing the repeating pattern across periods (peaks at noble gases and minima at alkali metals). The small deviations from a smooth increase within a period are where subshell/repulsion effects matter, motivating explanations that reference shielding and whether the electron removed is from an $s$ or $p$ subshell. Source

A rise in nuclear charge can be outweighed by the fact that the electron removed is now in a higher-energy subshell that is less strongly attracted to the nucleus.

How the factors combine

In real comparisons, all three factors must be considered together.

If nuclear charge increases

Ionization energy tends to increase, because the electron is more strongly attracted.

If shielding increases

Ionization energy tends to decrease, because the outer electron feels less attraction to the nucleus.

If the electron removed comes from a higher-energy subshell

Ionization energy may decrease, because that electron is easier to remove.

The best explanations always connect the factor to the attraction between the nucleus and the electron removed. Phrases such as “stronger attraction”, “more shielding”, and “electron removed from a $p$ subshell rather than an $s$ subshell” are often needed for full credit.

Building a strong exam explanation

When comparing ionization energies, use a clear order:

• identify the electron being removed

• compare nuclear charge

• compare shielding

• compare the subshell

• state whether attraction is stronger or weaker

• link this to whether ionization energy is higher or lower

A high-quality answer is specific. For example, saying “the atom is bigger” is usually too vague on its own. A better explanation links size to shielding, distance, and weaker attraction.

The core idea is simple: ionization energy depends on how strongly the nucleus attracts the electron being removed. Nuclear charge increases that attraction, shielding reduces it, and the subshell determines how strongly the electron is held.