What causes ionisation energy to vary across the periodic table?

Ionisation energy varies across the periodic table due to atomic size, nuclear charge, and electron shielding.

Ionisation energy is the energy required to remove an electron from an atom or ion. It is an important concept in chemistry, as it helps us understand the reactivity and stability of elements. The variation in ionisation energy across the periodic table is influenced by three main factors: atomic size, nuclear charge, and electron shielding.

Firstly, atomic size plays a significant role. As you move across a period from left to right, the atomic size decreases. This is because the number of protons in the nucleus increases, pulling the electrons closer to the nucleus and reducing the size of the atom. As a result, the electrons are more strongly attracted to the nucleus, making them harder to remove and thus increasing the ionisation energy.

Secondly, the nuclear charge also affects the ionisation energy. The nuclear charge is the total charge of all the protons in the nucleus. As you move across a period, the nuclear charge increases due to the addition of more protons. This increased positive charge attracts the electrons more strongly, again making them harder to remove and increasing the ionisation energy.

Lastly, electron shielding or screening effect also influences the ionisation energy. Electron shielding refers to the ability of inner shell electrons to shield outer shell electrons from the full positive charge of the nucleus. As you move down a group, the number of electron shells increases, leading to more shielding. This means the outermost electrons are less attracted to the nucleus, making them easier to remove and thus decreasing the ionisation energy.

In summary, the variation in ionisation energy across the periodic table is a result of the interplay between atomic size, nuclear charge, and electron shielding. Understanding these factors can help you predict the reactivity and stability of different elements, which is crucial in many areas of chemistry.