How do atoms become ions?

One easy way to do this is to remember the valency of the atom, for most it will be either 1 and 2 (either + or - depending on whether electrons were lost or gained). This concept also related to equivalent factor. Thus Ca2+ will have a valency of +2 (meaning it needs to gain 2 electrons to become stable) and Cl-1 will have a valency of -1 meaning it needs to lose one electron to become stable. Thus 2 Cl atoms will be required to balance the +2 charge on Ca atoms to give CaCl2. Using the equivalent weight concept, the equivalent molecular weight for Ca will be its molecular weight divided by 2, which will react with one equivalent weight of Cl. Since reactions always occur on a molar basis, you need twice the moles of Cl to react with 1 mole of Ca to give CaCl2