# What is the name for "Sn"_3("PO"_4)_2?

Jun 14, 2017

Tin(II) phosphate.

#### Explanation:

The trick here is to recognize the fact that you're dealing with a transition metal, which should let you know that the name of the compound will have to include Roman numerals.

You should also recognize the fact that this compound contains the phosphate polyatomic ion, ${\text{PO}}_{4}^{3 -}$.

So, tin, $\text{Sn}$, is your cation, i.e. the positively charged ion, and the phosphate ion is your anion, i.e. the negatively charged ion.

As you know, ionic compounds are neutral compounds, which implies that the overall positive charge coming from the cations must be balanced by the overall negative charge coming from the anions.

Notice that the chemical formula

"Sn"_ color(blue)(3)("PO"_ 4)_ color(red)(2)

shows that one formula unit of this compound contains

• three cations, color(blue)(3) xx "Sn"^(color(red)(?+)
• two anions, $\textcolor{red}{2} \times {\text{PO}}_{4}^{\textcolor{b l u e}{3 -}}$

Since the phosphate anion carries a $3 -$ charge, you can say that the total negative charge coming from the anions is equal to

$\textcolor{red}{2} \times \left(3 -\right) = \left(6 -\right)$

This implies that the total positive charge of the three tin cations must be equal to $6 +$. Therefore, the charge of a single tin cation is

color(blue)(3) xx (color(red)(?+)) = (6+) implies color(red)(?) = ((6+))/color(blue)(3) = (color(red)(2+))

So, you now know that the cation carries a $2 +$ charge, which tells you that you must use the $\left(\text{II}\right)$ Roman numeral in the name of the compound.

Put all this together to get

$\text{Sn"_ 3("PO"_ 4)_ color(red)(2) implies "tin"(color(red)("II")) color(white)(.)"phosphate}$