Common Polyatomic Ions

Polyatomic ions are charged species (molecules) composed of two or more atoms bound together. Common polyatomic ions possess distinct structures and net charges. They can be either negatively charged (anions), or positively charged (cations). These ions can be formed from combinations of nonmetals, metalloids, and metals. For example, the sulfate ion (SO₄²⁻) consists of one sulfur atom bonded to four oxygen atoms and carries a charge of 2-.

How to balance polyatomic ions:

In the context of balancing equations, these ions can be considered a single atom. This means that in the molecule H₂SO₄, you would consider (SO₄)²⁻ as one atom of (SO₄)²⁻. If you’re unsure whether something is a polyatomic ion, you can often look to the products for a matching ‘atom’—in this case, an ‘atom’ of (SO₄)²⁻.

In the unbalanced equation:

Na + H₂SO₄ → _Na₂SO₄ + _H₂

2Na +1 H₂SO₄ → 1Na₂SO₄ + 2H₂ OR 2Na + H₂SO₄ → Na₂SO₄ + 2H₂

There is an ‘atom’ of (SO₄)²⁻ on the reactant and product side. Therefore, you would balance Na, H, and (SO₄)²⁻.

Note: For this simulation, you are not required to balance charges. When balancing ionic equations, the total charge needs to be balanced on both sides of the reaction. This is done by balancing the electrons transferred during the reactions. For example, when balancing the electrons for (SO₄)²⁻ , Na atoms become Na+ ions and, therefore, lose electrons, while the H+ ions from H₂SO₄ accept the electrons to form H2. Na + Hٰٰٰ⁺ + (SO₄)²⁻→ Na⁺ + (SO₄)²⁻ + H2