POLYATOMIC IONS: LIST AND EXERCISES - CHEMISTRY - 2025

The polyatomic ions are those containing two or more atoms, so they are also known by the name of molecular ions. In contrast, monatomic ions have only one atom, and are derived from the gain or loss of electrons suffered by the elements of the periodic table.

For example, if we take a look at metals, we will get cations: Na ⁺, Mg ²⁺, Ga ³⁺, Ti ⁴⁺, etc. Meanwhile, non-metallic elements in essence will give us anions: O ^2-, S ^2-, F ^-, N ^3-, etc. In them the ionic charge is fully localized, and to a certain extent the same happens with polyatomic ions; although there are thousands of exceptions.

In the plaster of the ornamentation we find the sulfate ion, which is polyatomic and is also accompanied by calcium and water molecules. Source: Pixnio

In a polyatomic ion, commonly the negative charge rests on the most electronegative atoms, and such a situation would only be possible if there were internal covalent bonds. As there are covalent bonds, we are facing an ionically charged molecule or metal complex. These types of ions are highly prevalent in organic chemistry.

In inorganic chemistry, for example, one of the best known ions is the sulfate anion, SO ₄ ^2-. As can be seen, it has two elements: sulfur and oxygen, which add up to a total of five atoms linked by SO bonds. SO ₄ ^2- is part of gypsum and its mineralogical varieties, widely used since ancient times in construction work.

List of the most common polyatomic ions

Some of the more common polyatomic ions will be mentioned below. Two of them, crucial in the chemistry of solutions, come from the same water.

Hydronium

The hydronium cation, H ₃ O ⁺, is one of the simplest polyatomic cations. The positive charge resides on the central oxygen atom. It is generated when a water molecule gains hydrogen.

Hydroxyl

Also known as hydroxyl, OH ^-, is a polyatomic anion consisting of just two covalently linked atoms, OH. The negative charge is on the oxygen atom, and is generated when a water molecule loses a hydrogen.

Carbonate

The carbonate anion, CO ₃ ^2-, is found in limestone and marble, as well as in chalk on blackboards. Its two negative charges are delocalized by resonance between the three oxygen atoms, carbon being the central atom.

Nitrate

The nitrate anion, NO ₃ ^-, essential for plants, has a structure very similar to that of carbonate. Again, the negative charge is delocalized between the oxygens because they are the most electronegative atoms.

Ammonium

After hydronium, ammonium, NH ₄ ⁺, is the most relevant cation because it is derived from ammonia, an essential gas for countless industrial processes. Nitrogen is the central atom, and despite being the most electronegative, it has a positive charge as a result of losing an electron when forming four NH bonds.

Peroxide

The peroxide anion, O ₂ ^2-, is special because it is diatomic and homonuclear, having an OO bond.

Oxalate

The oxalate anion, C ₂ O ₄ ^2-, is derived from oxalic acid, and is literally a stone in the kidneys.

Phosphate

The phosphate anion, PO ₄ ^3-, has a large magnitude of charge, which is delocalized between its four phosphorous atoms by resonance. It is found in abundant minerals and makes up the crystals of our bones.

Cyanide

The cyanide anion, CN ^-, is also diatomic but heteronuclear. The negative charge resides on the nitrogen atom, and it has a triple bond, C≡N ^-.

Acetate

Acetate, CH ₃ COO ^-, is perhaps the most representative organic polyatomic anion. Note that it has three elements and a more molecular character than the other ions (more covalent bonds). This anion can be obtained from vinegar neutralized with sodium bicarbonate.

Permanganate

So far no polyatomic ion has had a central atom that is not an electronegative non-metallic element. However, in the case of permanganate, the central atom is a transition metal, manganese, MnO ₄ ^-, with the negative charge delocalized between its four atoms.

This anion is easy to recognize because its compounds usually have bright violet crystals, which stain their solutions the same color.

Chromate

Similar to the case of permanganate, chromate, CrO ₄ ^2-, has chromium as its central atom. Unlike MnO ₄ ^-, chromate is divalent, and the color of its solutions is not purple but yellow.

Exercises

Exercise 1

What ions make up the following salt? NH ₄ NaCO ₃

The chemical formula already reveals by itself the presence of the sodium cation, Na ⁺, since it will always be polyatomic and will not form covalent bonds. On the right, the carbonate anion, CO ₃ ^2-, is immediately recognizable; while on the left, the ammonium cation stands out. Therefore, the ions are: NH ₄ ⁺, Na ⁺ and CO ₃ ^2- (sodium and ammonium carbonate).

Exercise 2

What ions make up the following salt and how many of them are there per formula? MgKPO ₄

Again, we look for the monatomic ions first; in this case, potassium, K ⁺, and magnesium, Mg ²⁺. We are left with the phosphate anion, PO ₄ ^3-, visible on the right side of the formula. By formula, we then have one ion of each, whose ratio is 1: 1: 1 (1 Mg ²⁺: 1 K ⁺: 1 PO ₄ ^3-).

Exercise 3

What ions does the following compound have? AlOH ₃. Is there a problem with it?

The formula invites confusion. This could also be written as: AlH ₃ O. Therefore, it would have two cations: Al ³⁺ and H ₃ O ⁺, violating the conservation of ionic neutrality. There must necessarily be negative charges that counteract these four positive charges.

Given this reasoning, the compound AlOH ₃ cannot exist. And what about Al (OH) ₃ ? It still has the trivalent Al ³⁺ cation, but now it does have a well-known anion: the hydroxyl, OH ^-. There must be three OH ^- to neutralize the positive charge of Al ³⁺, and that is why the ratio is 1: 3 (1 Al ³⁺: 3 OH ^-).

Exercise 4

What ions does the following compound have? K ₂ Ti (CN) ₄

From the example of Al (OH) ₃ we know that what is inside the parentheses is a polyatomic anion; in this case, cyanide, CN ^-. Likewise, potassium is a K ⁺ monatomic cation, and having two of it in the formula, they would add two positive charges. We would be missing two other positive charges, which can only come from titanium, Ti ²⁺.

Therefore, K ₂ Ti (CN) ₄ has the following ions: K ⁺, Ti ²⁺ and CN ^-, in a ratio 2: 1: 4 (2 K ⁺: 1 Ti ²⁺: 4 CN ^-).

References

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