The valence electrons of the substance's atoms feel an effective nuclear charge of + 7. If the unknown substance has lower ionization energies than chlorine (Cl), what other characteristics would you expect the substance to exhibit?

1. B ... it has less ioni. energy, less likely to form ion and react, therefore its a halogen under Cl on the periodic table, and has a wider radius due to more shells

2. B ... ionic bond = cation + anion, Mg has charge 2 + and Cl has charge 1 - forming MgCl2

also Mg is a metal and Cl is non metal

but H and Cl ions also bond to form hydrogenchloride. but both H and Cl are non-metals. so B is the best answer.