Question

Write the formula of lead phosphate.

Answer 1

Hint: Ionic compound is formed when cation and anion share electrons. A stable inorganic compound is neutrally stable, thus cation and anion combine only when the charges of the cation and anion are balanced. The ionic formula is written in such a fashion that cation is written first followed by the anion. If charges are not balanced then a suitable subscript is added.

Complete answer:
In inorganic chemistry, inorganic compounds are formed when positive and negative ions share their electrons to form an ionic bond. The following are the steps to follow to determine the ionic formula of the compound.
Step 1) Identify the cation or the positive charge species. Positive ion species is the one which has the least electronegativity or most electropositive ion.this cation may be metal .this is generally located on the left-hand side of the periodic table.
Step 2) Identify the anion or the negative charge species .negative ion species is the on which have high electronegativity or least electropositive ion.This anion may be a non-metal such as halogen .this are generally located on the right-hand side of the periodic table.to write a chemical formula anion species is written on the right side of the formula.
Step 3) write a cation first and then write anion on the right side of the cation.
Step4) in the last step adjust the subscripts of the cation and anion such that the charge on the compound is balanced. Write down the formula in the smallest number ratio between the cation and the anion.
We are interested to determine the formula of lead phosphate. Lead has a low electronegativity. It has an atomic symbol as $\text{ Pb }$ .It is an electropositive metal and it has a charge equal to $\text{ +2 }$. Phosphate is a molecule. It has a formula $\text{ PO}_{4}^{3-}\text{ }$ .
According to the step discussed above lead will be written on the left side followed by the phosphate anion. Charges on the ions are not equal thus add subscript as needed to balance the charge.
If charges of the cation and anion are not equal then the total charge on the ion is equal to the product of charge on the ion multiplied by the subscript. Here lead has $\text{ +2 }$ charge and phosphate have $\text{ }-3\text{ }$ charge thus to maintain the neutrality of the compound lead have a subscript as 3 and phosphate as 2.

Thus the formula for lead phosphate is $\text{ P}{{\text{b}}_{\text{3}}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{\text{2}}}\text{ }$ .

Note: Note that the formula can be also determined through the valency of the ions. Lead donates its two electrons and thus have valency equal to 2 and phosphate have valency equal to 3. Thus,
$\begin{matrix}
\text{Ion} & {} & {} & \text{Charge} & {} \\
\text{Pb} & {} & \searrow & \text{2} & {} \\
{} & {} & {} & {} & {} \\
\text{P}{{\text{O}}_{\text{4}}} & {} & \nearrow & \text{3} & {} \\
\end{matrix}\text{ }\Rightarrow \text{ P}{{\text{b}}_{\text{3}}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{\text{2}}}\text{ }$