Here's why:

* Ionic bonds form between a metal and a non-metal. Sodium (Na) is a metal, and citrate (C6H5O7) is a polyatomic ion (a group of non-metal atoms with a charge).

* In sodium citrate, the sodium atom loses an electron to become a positively charged sodium ion (Na+). The citrate ion gains an electron to become negatively charged (C6H5O7-).

* The electrostatic attraction between the positively charged sodium ions and the negatively charged citrate ions is what forms the ionic bond.

Therefore, sodium citrate is held together by ionic bonds.