JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Classification of Elements and Periodicity

An element of the second period resembles the element lying diagonally below and to its right in the third period, because the increase in size on moving down a group is largely cancelled by the decrease on moving across a period, leaving the two with similar sizes and similar charge-to-size ratios. Beryllium with aluminium, boron with silicon and lithium with magnesium are pairs of this kind. Lithium and sodium are the first two members of one and the same group, so what they share is the ordinary family likeness within a group and not a diagonal relationship. The set standing apart is $\mathrm{Li}-\mathrm{Na}$.