Elements like calcium located toard the top of a group have a high attraction for the valance electrons because they have

small atomic radius

Explanation:

Atoms on the top of a group are those with the smallest atomic number of the group they are in. As you go down the group in the periodic table, the atomic radius increases with the presence of additional shells owing to the increase in atomic number (hence electrons, and hence need for more shells to accommodate them). The smaller the atomic radius, the closer the valence electrons are to the nucleus, consequently the force of attraction on the electrons from the positively charged nucleus is stronger. The stronger the pull of the nucleus on the valence electrons, the smaller the radius becomes.