and 
Ionic compounds comprise a large number of ions. Ions are charged particles that can be either positive or negative. Ionic compounds, however, are neutral themselves. What this means is that the ratio between positive and negative ions shall enable their charges to cancel out- to give rise to compounds that are neutral overall.
Finding the chemical formula of ionic compounds therefore requires knowledge about the charge number of each ion, as well as whether each ion is positive or negative.
The IUPAC systematic name of a monatomic ion is expected to indicate whether its positive or not.
Metal ions tend to carry positive charges (cations.) The name of ions they produce are the same as their name as an element, excepting for the oxidation number that are attached to elements with more than one possible charges. Additionally, ions with positive charges are supposed to be placed before those with a negative charge in the systematic name of an ionic compound.
Monoatomic anions with negative charges tend to have names that end in "-ide."
The charge on a monatomic cation of a main group element (which includes all first twenty elements) is usually equal to its group number on the periodic table.
Both magnesium and calcium are on the second row of the periodic table and has group number 2. Their cations shall therefore have a charge of +2.
The charge on a monatomic anion of a main group non-metal element is commonly equal to the group number of this element minus 18, which is expected to be a negative number.
Nitrogen is in group 15; the nitride ion shall therefore have a charge of 15 - 18 = -3. Similarly, chlorine is in group 17 and the chloride ion shall have a charge of 17 - 18 = -1.
Note that there are many other ways to number columns on a periodic table. The previous explanations assume that your periodic table is numbered from the left to the right in order with number 1 through 18.
In order to balance the charge within the ionic compound, the coefficient of the cation shall equal the number of charges on each anion, whereas the coefficient of the anion shall equal the number of charges on the cation.
Each magnesium ion has a charge of 2;Each nitride ion has a charge of 3. (ignoring the sign)
Magnesium nitride should therefore have a formula of
.
Similarly,
Each calcium ion has a charge of 2;Each chloride ion has a charge of -1. (ignoring the sign)
Calcium chloride should therefore have a formula of
.
Note that you should simplify the coefficients such that they are mutually prime. For instance, 
(not correct) should be expressed as 
.
