periodicity | Notion

define periodicity
atomic radius across a period
atomic radius down a group
define first ionisation energy
first ionisation energy across a period
first ionisation energy down a group
why does Al have a lower first ionisation energy than Mg
why does S have a lower first ionisation energy than P
melting point from Na to Mg (period 3)
melting point from Mg to Al (period 3)
why does Si have the greatest melting point (in period 3)
Cl2, S8 and P4 lower melting points (period 3)
why does Ar have lowest melting point