Q.(a) What happens when Sodium metal is dropped in Water and Sodium metal is heated in free supply of Air? Write Rxns.

Sodium reacts vigorously with water to give NaOH and H2, and burns in excess air to give sodium peroxide (Na2O2); a diagonal relationship is the resemblance between an element and the element diagonally below-right of it in the next period (e.g. Li resembling Mg), caused by similar ionic size/charge ratios.

1. Reactions of sodium: With water (at room temperature): Sodium is a highly reactive alkali metal (low ionisation enthalpy, low melting point). It reacts vigorously and exothermically with water, floating and often melting into a shiny globule that skitters across the surface, releasing hydrogen gas: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$ The reaction is strongly exothermic — enough heat is released that the evolved hydrogen can catch fire (or even cause a small explosion) if the reaction is fast or the piece of sodium is large. Heated in free (excess) supply of air: When sodium is heated in an excess/free supply of air (oxygen), unlike lithium (which mainly forms the simple oxide Li2O), sodium burns to form the peroxide: $2Na(s) + O_2(g) \xrightarrow{\Delta} Na_2O_2(s) \quad (\text{sodium peroxide})$ (Only a small amount of the simple oxide Na2O forms under limited oxygen; peroxide is the major product on heating in excess air — this size-dependent oxide/peroxide/superoxide preference going down Group 1 is a classic periodic-property trend.)
2. Diagonal relationship: Certain elements of the second period show a marked chemical resemblance to the elements diagonally placed below and to the right of them in the third period, rather than to the other elements of their own group. This is called the diagonal relationship. The classic examples are:

• Lithium (Group 1) resembles Magnesium (Group 2)
• Beryllium (Group 2) resembles Aluminium (Group 13)
• Boron (Group 13) resembles Silicon (Group 14) …