electrons and ionisation energy

what are shells made up of?

atomic orbitals - region around the nucleus that holds up 2e- with opposite spins

types of atomic orbitals

s, p, d, f

s-orbital

• electron cloud of a sphere

• can hold up to 2 e-

• greater the shell number, greater radius of s-orbital

p-orbitals

• electron cloud shape of dumb-bell

• one orbital can hold up 2e-

• the subshells (px, py, pz) hold 6e-

• greater shell no. the further the p-orbital is from the nucleus

aufbau principle

electrons fill the lowest energy orbitals first before moving to higher ones

pauli exclusion principle

an orbital can hold a maximum of 2e- and must have opposite spins

hunds rule

electrons fill orbitals of equal energy singly first before pairing up

how many electrons can each orbital hold

s → 2

p → 6

d → 10

f → 14

ionisation energy

how easily an e- loses to form positive ions

first ionisation energy

the energy required to remove one e- from each atom in one mole of a gaseous atom of an element to form one mole of gaseous 1+ ions

units of ionisation energy

kJ mol⁻¹

What are the main factors affecting ionisation energy?

1. Nuclear charge (number of protons)

2. Distance of outer electron from nucleus

3. Electron shielding

4. Electron–electron repulsion (sub-shell effects)

What is meant by successive ionisation energies?

The energies required to remove each electron in turn from the same atom.

Why is the 2nd ionisation energy always higher than the 1st?

Because the electron is removed from a positive ion, so there’s greater attraction between the nucleus and remaining electrons.

What happens to ionisation energy across a period (e.g. Na → Ar)?

increases overall

Why does ionisation energy increase across a period?

• Nuclear charge increases (more protons)

• Same shielding

• Electrons closer to nucleus (atomic radius decreases)
  → Stronger attraction → more energy needed to remove an electron.

What are the exceptions to the trend in ionisation energy across Period 2?

• Boron (B) has lower IE than Be → electron removed from 2p, higher energy sub-shell.

• Oxygen (O) has lower IE than N → due to paired electrons in 2p orbital causing extra repulsion.

What causes a drop in ionisation energy between Be → B and N → O?

• Be → B: change from 2s → 2p, higher energy, easier to remove.

• N → O: electron pairing in p orbital increases repulsion.

What happens to ionisation energy down a group?

decreases

Why does ionisation energy decrease down a group?

• More electron shells → increased shielding

• Outer electrons further from nucleus

• Attraction weaker, despite increased nuclear charge.

Which factor has the greatest influence down a group?

Increased distance and shielding outweigh increased nuclear charge.

What does a large jump in successive ionisation energies show?

A change of shell — the electron is now being removed from a closer, more strongly attracted shell.

How can successive ionisation energies identify an element?

The pattern shows how many electrons are in the outer shell (e.g., a big jump after 3 electrons → element is in Group 3).