An ionic bond holds together the atoms of a magnesium chloride atom. Magnesium will give one of its electrons to a one chlorine atom and its other electron to the other chlorine atom. This results in negative Cl- atoms and a Mg^2+ atom. The polar forces then pull these two atoms together to an ionic bond.
Work Step by Step
Another way to look at this is to recognize that magnesium, a metal, bonding to chlorine, a non-metal, is most likely to be an ionic bond due to their type. A metal cation and non-metal anion is the classic ionic molecule.