Taking an example of element of 3rd period discuss the trend of reactivity from left to right.

Explanation

Sodium has one electron in its valence shell, which can be donated most easily. Thus, Na is most reactive. Magnesium has 2 electrons in its valence shell, which it donates easily but comparatively with more difficulty than sodium hence, Mg is less reactive than Na and Al is less reactive than Mg. Silicon has 4 electrons in its valence shell. It is difficult for Si to donate or accept 4 electrons, hence, it is least reactive. Phosphorus, sulphur and chlorine have 5, 6 and 7 valence electrons respectively. They accept 3, 2 and 1 electrons respectively. It is easier to accept 1 electron than 2 or 2 electrons than 3. Hence, phosphorus is least reactive followed by sulphur and chlorine which is most reactive. Argon has 8 electrons in its valence shell hence it is chemically inert.