Question

What is the formula for aluminum nitrite?

Answer 1

Aluminum nitrite is an ionic compound having a molar mass/molecular weight of around 164.998 $g/mol$ . This compound is generally formed through the combination of an aluminum cation and a nitrite anion.

Complete answer:
As the name suggests, the given compound aluminium nitrite comprises a metal and a group of nonmetals. We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. aluminum nitrite. Aluminum nitrite possesses a polyatomic ion (i.e. group of nonmetals (nitrite in this case) after the metal (Aluminum) so we'll use a table of names for the common polyatomic ions, along with the Periodic Table. Rules to write formula of a ternary ionic compound are listed below:
1. Using the periodic table, write down the symbol for the metal along with its charge.
In the present case, the symbol of metal Aluminum is $Al$ having a charge +3.
2. Using the Common ion table, identify the symbol as well as charge of the polyatomic ion.
In the present case, polyatomic ion nitrite is written as $N{O_2}$ having a charge -1.
3. Check if the charges are balanced
In the present case it becomes $A{l^{3 + }}NO_2^{1 - }$ . The charges are not balanced as summation of charges is non-zero i.e. $( + 3) + ( - 1) = + 2 \ne 0$
4. If unbalanced charges are there, add subscripts following the criss-cross method such that net charge for the compound is zero.
In the present case, we will add subscripts to both metal and non-metal such that the net charge becomes zero. So it becomes ${(A{l^{3 + }})_1}{(NO_2^{1 - })_3}$ and now the net charge is balanced i.e. $(1 \times ( + 3)) + (3 \times ( - 1)) = 0$
Hence, the chemical formula for Aluminum nitrite is $Al{(N{O_2})_3}$.

Note:
While writing the chemical formula of an ionic compound, never write the subscript '1'. And it should be noted that we can also have two polyatomic ions like $N{H_4}N{O_3}$ in the same compound. In this case, we have to find and write both of the names as identified from the Common Ion Table.