Question

What is the formula of a compound that contains na+ and po43- ions? question 8 options: na3po4 napo4 na2po3 na3po3 na3p?

Answer 1

Since sodium (Na) is in Group I of the periodic table, it has an ionic charge of +1.
The phosphate ion (PO4) 3- has an ionic charge of -3 (as stated). 
To balance the charges completely and form a stable molecule, three Na+ ions will combine with the PO4 3- ion. 
Na3PO4

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Answer 2

Answer: $Na_3PO_4$.

Explanation:

For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.  

Here sodium is having an oxidation state of +1 called as $Na^+$ cation and phosphate $PO_4^{3-}$ is a polyatomic anion with oxidation state of -3. Thus they combine and their oxidation states are exchanged and written in simplest whole number ratios to give neutral $Na_3PO_4$.

The cations and anions being oppositely charged attract each other through strong coloumbic forces and form an ionic bond.