Difficulty: Introductory | Prerequisites: Basic understanding of atoms, elements, and the periodic table.
This topic sits right at the start of your chemistry journey into compounds. Before you can name compounds, predict reactions, or balance equations, you need to understand how chemical formulas encode what a compound is made of. Ionic compounds, in particular, follow strict charge-balancing rules that govern their formulas. If you have not yet covered the difference between metals, non-metals, cations, and anions, review those first.
Ionic compounds are held together by the attraction between positively charged ions (cations) and negatively charged ions (anions). The formula for an ionic compound must balance to a net charge of zero, which means you need the right ratio of cations to anions. When polyatomic ions like NH₄⁺ or SO₄²⁻ are involved, parentheses and subscripts show how many of each ion are present.
Ionic compound
A compound formed by the electrostatic attraction between oppositely charged ions, typically a metal cation and a non-metal anion. Think of it as: atoms that have gained or lost electrons locking together because opposite charges attract.
Cation
A positively charged ion, formed when an atom loses one or more electrons. In simple terms, metals become cations. Na loses one electron to become Na⁺.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. In simple terms, non-metals become anions. Cl gains one electron to become Cl⁻.
Polyatomic ion
An ion made up of two or more atoms covalently bonded together that collectively carry a charge. Think of it as a "team" of atoms acting as a single charged unit. Examples: NH₄⁺ (ammonium), SO₄²⁻ (sulfate), NO₃⁻ (nitrate).
Charge balancing (electrical neutrality)
The principle that an ionic compound's formula must result in a net charge of zero. You adjust the ratio of cations to anions until the total positive charge equals the total negative charge.
Subscript
The small number written to the lower right of an element symbol or a parenthesised group in a chemical formula, indicating how many atoms or groups of atoms are present. In Ca(NO₃)₂, the subscript 2 means there are two nitrate groups.
Chemical formula
A notation using element symbols and subscripts that shows the types and numbers of atoms in the smallest representative unit of a substance.
Ionic compounds consist of cations (positive ions) and anions (negative ions) held together by electrostatic attraction.
The formula represents the simplest whole-number ratio of ions that produces a net charge of zero.
Unlike molecular compounds, ionic compounds do not exist as discrete molecules. The formula unit is the smallest representative unit.
NaCl: one Na⁺ ion paired with one Cl⁻ ion. Charges: (+1) + (–1) = 0. Balanced.
AlBr₃: one Al³⁺ ion paired with three Br⁻ ions. Charges: (+3) + 3(–1) = 0. The subscript 3 on Br tells you there are three bromide ions per formula unit.
Ca(NO₃)₂: one Ca²⁺ ion paired with two NO₃⁻ (nitrate) ions. Charges: (+2) + 2(–1) = 0. The parentheses around NO₃ with a subscript 2 mean the entire nitrate group is taken twice.
Polyatomic ions are groups of atoms that carry a collective charge. They stay together as a unit in the formula.
When more than one polyatomic ion is needed, place the ion in parentheses and write the subscript outside.
Correct: (NH₄)₂SO₄, meaning two ammonium ions and one sulfate ion.
Incorrect: NH₄SO₄, which implies only one ammonium ion, leaving the charges unbalanced.
Never break up a polyatomic ion with a subscript. The subscript outside the parentheses multiplies the entire group.
Step 1: Identify the ions and their charges. Look these up from the periodic table (for monatomic ions) or from the polyatomic ion table.
Step 2: Find the simplest ratio that makes the total positive charge equal the total negative charge.
Step 3: Write the formula using that ratio, applying subscripts and parentheses as needed.
Example: ammonium sulfate. NH₄⁺ has a +1 charge, SO₄²⁻ has a –2 charge. You need two NH₄⁺ to balance one SO₄²⁻: 2(+1) + (–2) = 0. The correct formula is (NH₄)₂SO₄.
Molecular compound formulas tell you the exact number of each type of atom in one molecule (e.g. H₂O = 2 hydrogens, 1 oxygen).
Ionic compound formulas tell you the ratio of ions, not a specific molecule. There is no single "molecule" of NaCl, just a repeating lattice of Na⁺ and Cl⁻ in a 1:1 ratio.
The compound in question is ammonium sulfate. Here is the step-by-step reasoning.
Identify the ions:
Ammonium: NH₄⁺ (charge = +1)
Sulfate: SO₄²⁻ (charge = –2)
Attempt with NH₄SO₄ (as written, no parentheses, no subscript 2):
This implies one NH₄⁺ and one SO₄²⁻.
Total charge: (+1) + (–2) = –1. This is not zero. The formula is unbalanced.
Correct the formula:
You need two ammonium ions to cancel the –2 from sulfate: 2(+1) + (–2) = 0.
Correct formula: (NH₄)₂SO₄.
The parentheses around NH₄ with the subscript 2 outside mean "take two of the entire ammonium ion."
Compound | Ions | Charges | Balanced? |
|---|---|---|---|
NaCl | Na⁺, Cl⁻ | +1, –1 | Yes |
AlBr₃ | Al³⁺, 3 Br⁻ | +3, 3(–1) | Yes |
Ca(NO₃)₂ | Ca²⁺, 2 NO₃⁻ | +2, 2(–1) | Yes |
NH₄SO₄ | NH₄⁺, SO₄²⁻ | +1, –2 | No (–1) |
(NH₄)₂SO₄ | 2 NH₄⁺, SO₄²⁻ | 2(+1), –2 | Yes |
Students often think that NH₄SO₄ and (NH₄)₂SO₄ are the same thing. They are not. The parentheses and subscript 2 double the entire ammonium ion, changing the formula's meaning and charge balance entirely.
Students sometimes believe the subscript inside a polyatomic ion (like the 4 in NH₄) can be changed to balance charges. It cannot. That 4 is part of the ion's identity. You balance by changing how many of the whole ion you use, not by altering the ion itself.
Some students assume that if individual elements are present in the right types, the formula must be correct regardless of subscripts. Subscripts determine ratio and charge balance; getting them wrong means you have written a different (and possibly non-existent) compound.
Confusing molecular formulas with ionic formulas is common. Students may try to read NaCl as "one molecule of sodium chloride." Ionic compounds form lattices, not discrete molecules. The formula gives a ratio, not a molecule count.
⚠️ Charge balancing is tested constantly. Expect questions that give you a cation and anion and ask you to write the correct formula. If you cannot balance charges, you will lose marks on nearly every ionic compound question.
⚠️ Parentheses around polyatomic ions are a classic exam trap. If a question shows Ca(NO₃)₂ and asks how many oxygen atoms are present, the answer is 6 (2 nitrate groups, each with 3 oxygens). Missing the parentheses means you will count wrong.
⚠️ The CER (Claim, Evidence, Reasoning) framework may appear on exams. You will be asked to state a claim, support it with evidence (what is observably wrong), and connect it with reasoning (the scientific principle that explains why). Practise writing all three steps clearly.
⚠️ Know your common polyatomic ions and their charges cold. Ammonium (NH₄⁺), sulfate (SO₄²⁻), nitrate (NO₃⁻), hydroxide (OH⁻), and carbonate (CO₃²⁻) come up repeatedly.
True or false: The formula NaCl represents one molecule of sodium chloride. (False. Ionic compounds form lattices, not molecules. NaCl is a formula unit showing a 1:1 ratio.)
Fill in the blank: To balance the charges in ammonium sulfate, you need ____ ammonium ions for every one sulfate ion. (Two.)
True or false: The subscript 4 in NH₄⁺ can be changed to balance the overall compound's charge. (False. The 4 is part of the polyatomic ion's identity.)
Fill in the blank: In Ca(NO₃)₂, there are ____ total oxygen atoms per formula unit. (Six.)
True or false: AlBr₃ is electrically neutral because Al³⁺ contributes +3 and three Br⁻ ions contribute 3(–1) = –3. (True.)
Q: Given the ions K⁺ and SO₄²⁻, write the correct formula for potassium sulfate.
A: K₂SO₄. Two potassium ions (+1 each) balance one sulfate ion (–2): 2(+1) + (–2) = 0.
Q: How many total atoms are in one formula unit of (NH₄)₂SO₄?
A: 15 atoms. Each NH₄ group has 1 nitrogen + 4 hydrogens = 5 atoms, and there are two of those groups (10 atoms). The sulfate has 1 sulfur + 4 oxygens = 5 atoms. Total: 10 + 5 = 15.
Q: A student writes the formula for calcium chloride as CaCl. Using the CER framework, explain why this is incorrect.
A: Claim: CaCl is incorrect. Evidence: Calcium forms Ca²⁺ and chlorine forms Cl⁻. One Cl⁻ only provides –1 charge, leaving a net charge of +1. Reasoning: Ionic compounds must be electrically neutral. You need two Cl⁻ ions to balance Ca²⁺, giving the correct formula CaCl₂.
Q: What is the difference between a subscript inside a polyatomic ion and a subscript outside parentheses?
A: A subscript inside a polyatomic ion (like the 4 in NH₄) tells you how many of that atom are part of the ion itself. A subscript outside parentheses (like the 2 in (NH₄)₂) tells you how many of the entire polyatomic ion are present in the formula unit.
Q: Which of the following formulas is correctly balanced: MgOH₂, Mg(OH)₂, or Mg₂OH?
A: Mg(OH)₂ is correct. Mg forms Mg²⁺ and hydroxide is OH⁻. You need two hydroxide ions to balance: (+2) + 2(–1) = 0. The parentheses indicate two complete OH groups.
This material connects directly to naming ionic compounds (nomenclature), which is typically the next step after writing correct formulas. If you can balance charges, naming becomes straightforward.
Charge balancing also lays the groundwork for balancing chemical equations later in the course. The same principle of conservation (charges must balance, atoms must balance) runs through both topics.
Understanding polyatomic ions here will pay off when you reach acid-base chemistry. Many common acids (sulfuric acid, nitric acid) are built from the same polyatomic ions you are learning now.
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