The charge of phosphite ( extbf{PO}_3^{3-}) compared to phosphate ( extbf{PO}_4^{3-}) can indeed be confusing at first glance. To understand this, we need to look at the structure and bonding within these ions.
Phosphate has four oxygen atoms and carries a negative charge of 3, which is due to the four oxygen atoms being highly electronegative elements. This results in the phosphate anion ( extbf{PO}_4^{3-}) having a formal charge of -3. Each oxygen typically contributes a -2 charge when bonded, but we also need to account for the phosphorus atom’s contributions.
On the other hand, phosphite consists of only three oxygen atoms and carries a -3 charge as well, denoted as ( extbf{PO}_3^{3-}). The difference in charge states arises from how the electrons are shared and how the overall ion structure is stabilized. In phosphite, the absence of one oxygen atom leads to a different bonding arrangement that results in the entire ion carrying a higher total negative charge.
In conclusion, the difference in the number of oxygen atoms does not directly correlate to the charge because the electronic structure and stabilization effects also play significant roles. Both ions may have differing numbers of oxygens, but their overall charges are determined by the specific arrangement and electronegative character of the atoms involved.