Polyatomic Ions

The -ite and -ate pattern, with one family worked through

Two names differing by one letter, differing by one oxygen. The -ate form has more oxygen than the -ite form: nitrate is NO₃⁻ and nitrite is NO₂⁻, sulfate is SO₄²⁻ and sulfite is SO₃²⁻. Nothing in either name tells you which way round it goes, which is why this is the part of the table people memorise rather than work out.

Chlorine forms all four members, so its oxoanions show the pattern in full:

• ClO⁻ hypochlorite, the active ingredient in bleach
• ClO₂⁻ chlorite
• ClO₃⁻ chlorate
• ClO₄⁻ perchlorate

One oxygen is added at each step, and all four carry a single negative charge. The prefixes extend the pair outward: hypo- is one oxygen fewer than the -ite, and per- is one more than the -ate. Bromine and iodine appear here only as bromate and iodate, since those are the members a school course actually uses.

Charge and formula are separate things

This table holds the formula and the charge in different fields, because they are different pieces of information. Charge comes from the electron count, not from which atoms are present: SO₄²⁻ and ClO₄⁻ both have four oxygens and different charges.

Keeping them apart matters as soon as you build a salt, because the formula of an ionic compound is whatever ratio makes the charges cancel. Ammonium and phosphate give (NH₄)₃PO₄, three singly charged cations against one triply charged anion. The brackets multiply the whole ammonium group rather than just the hydrogen, and dropping them is the most common way this goes wrong. Phosphate’s triple charge is also why it precipitates with most metal cations, and why its three acid dissociation steps, from H₃PO₄ through H₂PO₄⁻ and HPO₄²⁻ to PO₄³⁻, make phosphate buffers work across such a wide range.

Where the molar masses come from

No mass on this page is stored. Each row carries the neutral formula, and the molar mass is computed from it by the same parser the molar mass calculator uses, so every atomic weight has exactly one source and the whole table moves together if that source is revised. A table this size therefore introduces no new numbers that could be individually wrong.

The electron mass is ignored, as it is throughout chemistry. A sulfate ion carries two more electrons than its neutral formula, which adds about 0.0011 g/mol to 96.06 g/mol, about one part in 90,000: below anything but the most careful weighing, and at the level of the last digit shown in the table. You would have to worry about water absorbed from the air long before the electrons mattered.

Both the traditional and the systematic name are given

A student meets one name in a textbook and a different one on the bottle, so where they differ the table lists both. HCO₃⁻ is bicarbonate in conversation and hydrogencarbonate under systematic naming, and it is the buffer that holds blood near pH 7.4. CH₃COO⁻ is acetate or ethanoate, C₂O₄²⁻ is oxalate or ethanedioate, and HSO₄⁻ is hydrogen sulfate or bisulfate. The older names are not wrong and are not going anywhere.