The Energy of an Electron

The ionisation energy of an atom is the energy needed to remove an electron, so it is a measure of how strongly that electron is held in its orbital.The energy of an electron depends on

• the orbital (the energy of p orbitals is greater than the energy of s orbitals)
• the nuclear charge
• the shielding or repulsion of inner shell electrons.

Hydrogen and helium both have electrons in the 1s orbital. In helium each electron shields the other from the nuclear charge. This increases the energy of the electrons. However, helium has two protons. The effect of the increased nuclear charge is greater than the effect of shielding.

H    1s1  1st I.E.  1310 kJmol-1

He   1s2  1st I.E.  2370 kJmol-1

With helium and lithium, the second quantum shell is at a higher energy than the first. The increased distance from the nucleus and the increased shielding is more significant than the increased nuclear charge.

He    1s2       1st I.E.  2370 kJmol-1

Li    1s2 2s1   1st I.E.    519 kJmol-1