Periodicity is the recurring in the and chemical of the elements across each .

The first periodic table in the 1870's was created by

Reading from left to right, the elements are arranged in order of increasing .

The chemistry of each element is determined by its , particularly the outer, electron shell.

Ionisation energy measures how an atom to form ions.

The first ionisation energy is the to remove from of to form one mole of .

The the distance from the nucleus and the , the the nuclear attraction.

The more there are in the nucleus of the atom, the the attraction between the and the electrons.

The second ionisation energy is the energy required to remove 1 electron from 1 mole of to form one mole of .

As you go down a group:
- the atomic radius
- more inner shells so
- decreases
- first ionisation energy

As you go across a period:
- increases
- same shell so shielding
- nuclear attraction
- atomic radius
- first ionisation energy

The fall in ionisation energy from beryllium to boron is due to the start of the of the sub-shell.

The fall in ionisation energy from nitrogen to oxygen is due to the start of in the p orbitals of the sub-shell. The paired electrons each other which makes it to remove an electron.
Nitrogen has stability.

Metallic bonding is the strong attraction between electrons and positively charged

Across period 2 and 3, the melting point between group where there is a sharp between groups 4 and 5. The melting points of group 5 to 8 are all .