(i) When we move left to right in a period of the periodic table the chemical reactivity of elements first decreases and then increases.

For example- In the 3rd period of elements, sodium is a very reactive element, magnesium is less reactive whereas aluminium is still less reactive. Silicon is the least reactive in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive whereas chlorine is very reactive.

(ii) When we move from left to right in a period the nature of oxides of elements decreases and the acidic nature of oxides increases.

Example- In the 3rd period of the periodic table, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminium and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic whereas chlorine oxides are highly acidic in nature.