Which pair of elements is most likely to have similar properties?

Core electrons are also referred to as non-valence electrons. Two different elements have similar chemical properties when they have the same number of valence electrons in there outermost energy level. Elements in the same column of the periodic table have similar chemical properties.

Nonmetals usually react by gaining electrons

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts