I understand that phosphate is bonded covalently, however, it is characterized as a polyatomic ion. This is making me confused on whether it's polar or non-polar.


There are multiple different things to consider for phosphate $\ce{PO4^3-}$:

  • The bonds themselves. Each of the four $\ce{P-O}$ single bonds is polarised towards the oxygen. Each bond is polar.

  • The molecule as a whole. It is of near-perfect tetrahedral shape thus having the point group $T_\mathrm{d}$. This point group includes the inversion symmetry operation $i$. Therefore, macroscopic properties of a vectorial type (like the overall dipole moment which is a vector more or less pointing from one pole to the other) must, by definition, be $0$. Thus, the entire molecule is not a dipole.

  • The molecule’s total charge. It has an overall charge of $-3$. This charge is located on the surrounding oxygens but the entire thing can be simplified from far away to a ball with the charge in the centre, i.e. the phosphorus atom. We would consider this molecular ion a monopole.

  • $\begingroup$ Located entirely in centre? Really? $\endgroup$ – Mithoron Dec 14 '15 at 19:41
  • $\begingroup$ Symmetry, my dear @Mith; symmetry. $\endgroup$ – Jan Dec 14 '15 at 23:02
  • $\begingroup$ It' symmetrically on oxygens, however not exactly 3 - there's partial charge on P but definitely not such high. $\endgroup$ – Mithoron Dec 14 '15 at 23:06
  • $\begingroup$ @Mith I reworded. Better? $\endgroup$ – Jan Dec 14 '15 at 23:11
  • $\begingroup$ It's not monopole - if sth hasn't dipole moment it still can quadrupole moment $\endgroup$ – Mithoron Dec 14 '15 at 23:29

A poly-atomic ion is a group of atoms that act as a single ion, it has no bearing on the bonding with-in the molecule. The oxygen-phosphorous bonds are covalent but polar, the molecule its-self is charged, but lacks a direction across the molecule of the charge making phosphate non-polar. The molecule its-self is free to bond covalent or ionicly depending on what it reacts with.


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