In calcium phosphate, $\ce{Ca3(PO4)2}$, since the calcium and phosphate share an ionic bond, and the phosphorus and oxygen share a covalent bond, should the Lewis structure be like the following?

ionic Lewis structure

While I was checking other web pages I noticed there was another Lewis structure for the same molecule:

Lewis structure with covalent bonds between phosphate and calcium

Are they both possible Lewis structures, or is the first structure the only "correct" version of the molecule?

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    $\begingroup$ The first one is definitely more realistic. $\endgroup$ Commented Jan 21, 2016 at 20:33
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    $\begingroup$ Note that calcium phosphate is ionic, not covalent. $\endgroup$ Commented Nov 27, 2016 at 5:21

1 Answer 1


Calcium phosphate crystallises in a number of different modifications of which β-$\ce{Ca3(PO4)2}$ is the stable room temperature and ambient pressure modification. It has a rather complex structure not easily described, consisting of chains and rings with phosphate and calcium ions in specific patterns. It is orthorhombic, but $c \gg b \approx a$.

Any attempt to simplify its structure by drawing it in pseudocovalent depictions like your lower scheme is doomed to fail. Better just use the upper scheme; unless you are expected to know a certain salt’s crystal structure that one is safe.

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    $\begingroup$ Just for reference: M. Yashima et. al, J. Solid State Chem. 2003, 175 (2), 272-277. DOI: 10.1016/S0022-4596(03)00279-2. Unfortunately no ESI, and no deposition in ccdc. $\endgroup$ Commented Dec 12, 2017 at 10:24

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