Atomic Structure:

s subshells hold 2 electrons (or 1 orbital),
p subshells hold 6 electrons (or 3 orbitals),
d subshells hold 10 electrons (or 5 orbitals),
f subshells hold 14 electrons (or 7 orbitals).

The subshells are arranged in the following order:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, ...

• Atoms are ionised when they lose an electron. The energy required to remove the electron is known as the ionisation energy.
• As each electron is removed from an atom the ionisation energy required increases, so we call the energy required to remove the first electron the first ionisation energy,
• The energy required to remove the second electron the second ionisation energy .

First ionisation energy: the amount of energy needed to remove one mole of electrons from one mole of gaseous M atoms to form one mole of positively charged M+ ions.

E.g: M(g) => M+(g) + e-

Second ionisation energy: the amount of energy needed to remove one mole of electrons from one mole of gaseous ions to form one mole of gaseous M2+ ions.

E.g: M+(g) => M2+(g) +e-

• When electrons in the atom occupy the lowest energy levels, the atomis said to be in its ground state. Most atoms are in their ground state at room tempertature.
• When one or more electrons absorb enough energy to become promoted to a higher energy level, the atom is unstable and is said to be in an excited state.

Factors affecting Ionisation Energies:

• Magnitude of the positive nuclear charge.
• Shielding(Screening) effect of the Inner Electrons.