The electrons in the atoms of two elements X and Y are distributed in three shells having 1 and 7 electrons respectively in their outermost shells. (a) Write the group numbers of these elements in the Modern Periodic Table. (b) Write the molecular formula of the compound formed when X and Y combine with each other. (c) Which of the two is electropositive ?

Element $X$ is Sodium ($Na$), which the NCERT text (Section 3.1.2) identifies as an alkali metal. Alkali metals are highly reactive and electropositive because they easily lose their single valence electron to achieve a stable octet. Element $Y$ is Chlorine ($Cl$), a non-metal. When a metal like $X$ reacts with a non-metal like $Y$, they form an ionic compound. The formula is derived from their valencies: $X$ has a valency of 1 (loses 1 $e^-$) and $Y$ has a valency of 1 (gains 1 $e^-$), resulting in $XY$.