why is a phosphate group negatively charged?

Question

Referring to the functional group why is it that two oxygens are negatively charged and why does the phosphate have more than 3 bonds when it has a valence of 3?

Answer 1

A few things to remember

1. phosphorus itself has 5 valance electrons, so it can make 3 single bonds and 1 double bond.
2. the oxygen without the double bond can bond with the R- group

In the phosphate functional group, one of the 3 oxygens can then share its electron pair with the R group. This removes a charge from the functional group so it becomes R-PO4 2- as opposed to the phosphate ion which has a charge of 3- . Remember, one of the oxygens has a new bond with the R group, so it's no longer a negative charge.