Question

Which is the Lewis structure for H3PO4?
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A central upper P is single bonded left, right, above, and below to upper Os. The O above the P is single bonded to upper H on the left and the right, and has two electron dots above it. The O below the P is single bonded to an H below, and has pairs of electron dots to the left and right.
A central upper P is double bonded to an O above, and single-bonded to an upper O single-bonded to an upper H to the left and the right. The O above the P has three pairs of electron dots, to the left, above, and to the right; the O's to the right and left have pairs of dots above and below.

A central upper P is bonded to an upper H above, an upper O below, and upper O's bonded to upper H's to the left and the right. The O below the P has three pairs of electron dots, to the left, right, and below; the other two O's have pairs of dots above and below. A central upper P is double bonded to an O above, and single-bonded to an upper O single-bonded to an upper H to the left and the right. The O above the P has three pairs of electron dots, to the left, above, and to the right; the O's to the right and left have pairs of dots above and below.

Answer 1

The correct Lewis structure is the one showing P double-bonded to one O and single-bonded to three O–H groups (i.e. P=O plus three P–O–H bonds).

Reason: H3PO4 has three acidic hydrogens on three hydroxyl oxygens, and phosphorus uses an expanded octet (P has 5 valence electrons and makes four P–O bonds, one of them a double bond to satisfy octets/ minimize formal charges).