Elements are classified into horizontal periods with their outer most electron in the same outer most shell.
They are also classified into vertical groups with similar chemical and physical properties. However, the
periodic table has its own classification of the elements into s,p,d & f blocks:
Period 3 Trends:
Atomic Radius:
Atomic Radius decreases across period 3. This because the nuclear charge increases across period 3, but
the electrons are added to the same outer most orbital, so the amount of electron shielding remains the
same. This results in a greater electrostatic attraction of the outer most electron to the positive nucleus, so
atomic radius decreases across period 3
Ionisation Energy:
The general trend is the ionisation energy increases across period 3. This is because the nuclear charge
increases, but the atomic radius decreases, whilst the electron shielding stays relatively the same. The
result is a larger electrostatic attraction of the outer most electron to the positive nucleus, giving a larger
ionisation energy.
There are however, 2 exceptions to the rule: Mg -> Al & P -> S both have slight decreases in the ionisaiton
energy.
The outer most electron in Al is added to a new p sub-orbital, which is slightly further away from nucleus &
has slightly more electron shielding, so electrostatic attraction decreases, and so does the ionisation
energy.
In Sulfur, the outer most electron pairs up in the p sub-orbital, giving spin-spin repulsion between
negatively charged electrons. This reduces the electrostatic attraction of the outer most electron to the
positive nucleus, giving a lower ionisation energy.
Melting point:
Melting point really depends on the type of bonding present in to molecule. From Na -> Al the MP
increases, as the ionic radius decreases and more electrons donated to the electron cloud, so the electron
cloud density increases, so the electrostatic attractions between oppositely charged metal ions and
electrons in the metallic bonding is stronger. MP clearly decreases down the groups as the ionic radius
increases but same number of electrons donated to the cloud.
Silicon has the highest melting point as it is a giant covalent structure, meaning all silicon atoms are
covalently bonded together. It requires huge amounts of energy to break all the covalent bonds in the
silicon lattice, so this has the highest MP.
The remaining compounds are all simple molecular. This means that the atoms are covalently bonded
together, but the forces between the molecules are very weak VDWs forces only. Therefore the largest
molecule (S8) has the highest MP, as it has the most electrons so has the most & strongest VDWs. This is
closely followed by P4, then Cl2 and finally Ar has the lowest MP.