Question

Why is the formula for calcium nitride Ca3N2?

Answer 1

As main groups elements calcium and nitrogen it is possible to derive their oxidations in compounds directly from their group numbers on the periodic table in the new IUPAC system. Start by looking up the position of the two elements on a modern IUPAC periodic table:

• Calcium $_{20}\text{Ca}$ is found in group $2$ of the periodic table; it is located to the left end of the periodic table and tend to behave as a metal. As a main group metal the oxidation state $_{20}\text{Ca}$ is expected to demonstrate in compounds equals to its group number, $2$.
• Nitrogen is found in group 15 of the periodic table. It is located close to the right end of the periodic table and tend to behave as a nonmetal; the oxidation state it tend to demonstrate, especially in ionic compounds, equal to $\text{its group number} - 18= 15 - 18 = -3$

The systematic name "calcium nitride" contains two words, indicating that the substance in question exists as a neutral compound rather than a charged ion. Atoms in this compound shall be arranged in a way that allow their charges to balance each other, such that the overall compound is electrostatically-neutral. Assuming that calcium and nitrogen atoms in this compound are present at a $n(\text{Ca}) : n(\text{N}) = 1 : x$ ratio. For the charge to cancel out

$(+2) \times 1 + (-3) \cdot x = 0 \\ x = 2/3$

Thus the ratio $n(\text{Ca}) : n(\text{N}) = 1 : 2/3$

It is necessary to simplify this expression such that

• All numbers in this ratio are integers
• Numbers in the ratio are coprime, for example, use $2 : 3$ instead of $4 : 6 = 2 \times 2 : 2 \times 3$

$n(\text{Ca}) : n(\text{N}) = 1 : 2/3 = 3 \times 1 : 2/3 \times 3 = 3:2$

Write the ratio of the atoms in the compound as subscripts of the respective element. Hence the formula $\text{Ca}_3 \text{N}_2$.

Note that the group number rule is more than a rule of thumb; however, it can take understandings of periodic trends in electron configurations of the elements along with the octet rule to see why this rule makes sense.