(i) Valence electrons:
On moving from left to right in a period, the number of valence electrons increases from one to eight (except first period where the increase is from one to two only.)
The valency of elements increases from one to four, then decreases to one and becomes zero in the case of inert gases. For example, in compounds Lick, BeCl2, BC13 and CC14, the valency of Li, Be, B and C are 1, 2, 3 and 4 respectively, whereas in compounds.
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NH3, H20, HF the valency of N, O, F are 3, 2 and 1 respectively.
(iii) Zero groups:
At the end of each period is the zero group elements, the inert gas.
(iv) Size of atoms:
Atomic radii of the elements decrease gradually from left to right in a period because the nuclear charge increases but no extra shell is added.
(v) Metallic character:
Metallic character decreases from left to right across a period and non-metallic character increases.
(vi) Ionisation energy:
Ionisation energy increases across a period from left to right. Ionisation energy is the energy required to remove the most loosely bound electron from a neutral gaseous atom.
(vii) Chemical reactivity:
On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases
For example, in third period, Na is very reactive, Mg, A1 are less reactive, Si is least reactive and the reactivity increases from P, S to CI.
The basic character of the oxides of the elements decreases and the acidic nature of oxides of the elements increases.
For example, in the 3rd period starting with sodium oxide, it is highly basic in nature; the next magnesium oxide is less basic then comes aluminium and silicon which give amphoteric oxides.
These are the oxides of phosphorus and sulphur which are acidic. And the last chlorine oxide is highly acidic.
(ix) Electro negativity:
Electro negativity increases on moving across a period from left to right. It is measure of the tendency of the element to attract electrons towards it.