# How do you find the number of ions in a given salt?

Oct 25, 2015

#### Explanation:

There are atoms in ions. Sometimes there is only the 1 atom in an ion. In such an ion the atom would have a positive or negative charge (i.e. a deficiency or excess of electrons): $N {a}^{+}$, $M {g}^{2 +}$, ${X}^{-}$ are examples.

Jan 15, 2016

You have to know the formulas for all the ions.

An ionic compound contains both positive and negative ions.

The only polyatomic ion is ${\text{NH}}_{4}^{+}$, but there are many polyatomic anions.

You must be able to write the formulas corresponding to the names of these ions.

The total charges on the cations must equal the total charges on the anions.

Examples

Strontium iodide

$\text{Sr}$ is in Group 2, ∴ ${\text{Sr}}^{2 +}$; $\text{I}$ is in Group 17; ∴ ${\text{I}}^{-}$.
We need ${\text{2I}}^{-}$ to match the $\text{2+}$ of ${\text{Sr}}^{2 +}$, ∴ ${\text{SrI}}_{2}$.

Sodium carbonate

$\text{Na}$ is in Group 1, ∴ ${\text{Na}}^{+}$; carbonate is ${\text{CO}}_{3}^{2 -}$.
We need ${\text{2Na}}^{+}$ to match the $\text{2-}$ of ${\text{CO}}_{3}^{2 -}$, ∴ ${\text{Na"_2"CO}}_{3}$.

Aluminium sulfate

$\text{Al}$ is in Group 13, ∴ ${\text{Al}}^{3 +}$; sulfate is ${\text{SO}}_{4}^{2 -}$.
We need ${\text{2Al}}^{3 +}$ ($\text{6+}$) to match the $\text{6-}$ on ${\text{3SO}}_{4}^{2 -}$; ∴ "Al"_2("SO"_4)_3.