We describe elements’ electron arrangements in terms of the number of electrons and their positions in sub-shells and orbitals.
There are four types of electron shell you need to know for AS level chemistry:
s (sharp) -- this has 1 s orbital
p (principle) -- this has 3 p orbitals
d (diffuse) -- this has 5 d orbitals
f (fundamental) -- this has 7 f orbitals
Each orbital can hold a maximum of two electrons.
Each row in the periodic table represents a new major shell, and within the major shells there are sub-shells:
N=1 sub-shells contain only an s orbital; they can hold a total of 2 electrons
N=2 sub-shells contain one s and three p orbitals; they can hold a total of 8 electrons (2+(3Ă—2))
N=3 sub-shells contain one s, three p, and five d orbitals; they can hold a total of 18 electrons (2+(3Ă—2)+(5Ă—2))
N=4 sub-shells contain one s, three p, five d, and seven f orbitals; they can hold a total of 32 electrons (2+(3Ă—2)+(5Ă—2)+(7Ă—2))
We label electrons as follows:
x = orbital name (s, p, d, or f)
y = number of electrons in orbital
1s ↿⇂  2s ↿⇂  2p ↿⇂↿⇂↿⇂  3s ↿⇂  3p↿⇂↿⇂↿
The up and down arrows represent paired and unpaired electrons. Electrons pair up in orbitals. However, if there is more than one orbital available, electrons will go into empty orbitals before they pair up; this is because electrons “prefer” to be alone in orbitals, rather than paired up.
1s ↿⇂ 2s ↿⇂ 2p ↿⇂↿⇂↿⇂ 3s ↿⇂ 3p ↿↿↿
Ionisation energy: the minimum energy required to remove an electron from each atom/ion in one mole of free gaseous atoms/ions
First ionisation energy: the minimum energy required to remove one electron from each atom in one mole of free gaseous atoms, to form one mole of free gaseous ions
Ionisation energy increases if:
there are more protons in the nucleus
there is more nuclear attraction for the outer electrons
the shielding (from other electron shells) remains the same
Ionisation energy decreases if:
there are outer electrons that are further away from the nucleus
there is less nuclear attraction for the outer electrons
there are more shells so there is more shielding
The ionisation energies of the elements across Period 3 show a pattern that links to their electron arrangement
The general trend in first ionisation energy is that it is increasing; however, some of the elements have a lower first ionisation energy than the one before.
The first ionisation energy of aluminium is lower than that of magnesium because aluminium has one electron in a higher outer shell. This electron is further away from the nucleus than the outer electrons of magnesium, so requires less energy to remove because it is not as strongly attracted to the nucleus and there is more shielding between it and the nucleus.
The first ionisation energy of sulphur is lower than the first ionisation of phosphorus because sulphur has one electron that is in a pair in a p orbital. As electrons repel each other, a paired electron requires less energy to remove than an unpaired electron.
The first ionisation decreases down the group because with each extra shell, there is more shielding between the outer electrons and the nucleus and the electrons are further away from the nucleus so less nuclear attraction is acting on them.