Why NH4 is not a molecular ion

Is

Explanation:

Ammonium (NH4+) is a poly-atomic ion. ... NH4+ is not the formula of a neutral molecule, neutral ammonia is NH3. to make the ammonium ion, you add a proton (H+). Thus, Ammonium is NOT a molecular ion. You can create a molecular ion like NH3- by electron bombardment to give it a negative charge.

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the answer is c.the valence of the atoms combine .