Trends in First Ionisation Energy

First ionization energy decreases down a group.

First ionization energy generally increases across a period.

Ionization energy decreases down Group 2 because extra electron shells increase shielding and distance from the nucleus.

The number of protons increases, strengthening the attraction between the nucleus and outer electrons.

No, extra electrons are at similar energy levels, so shielding remains nearly constant.

Small drops occur between groups 2 and 3, and between groups 5 and 6.

The outer electron in group 3 elements is in a 3p orbital, which is higher in energy and farther from the nucleus with additional shielding from 3s electrons.

It provides evidence for the existence of electron sub-shells.

In group 6, electrons start pairing in orbitals, causing electron repulsion that makes it easier to remove one electron.

In sulfur, an electron is removed from a paired orbital, where repulsion makes it easier to remove than in singly-occupied orbitals like phosphorus.

The electron is removed from a singly-occupied orbital.

Increased shielding from inner electrons and greater distance from the nucleus.

The 3p orbital has slightly higher energy and is farther from the nucleus, with additional shielding from 3s electrons.

An increasing nuclear charge raises the ionization energy by strengthening the attraction to outer electrons.