🎓 Prepared by students from Boğaziçi University

What Are Chemical Formulas?

A chemical formula is a symbolic representation of a compound showing the types of atoms and their proportions. It tells you which elements are in a substance and how many atoms of each are present.

Short answer

A chemical formula uses element symbols and subscripts to show the composition of a compound — for example, H₂O means two hydrogen atoms and one oxygen atom bonded together.

Reading a Chemical Formula

1
Element symbol (e.g., H)
Identifies the type of atom
↓
2
Subscript number (e.g., 2)
Shows how many atoms of that element
↓
3
Parentheses with subscript (e.g., Ca(OH)₂)
Groups polyatomic ions; subscript multiplies atoms inside
↓
4
Full formula interpretation
H₂O₂ = 2 hydrogen + 2 oxygen atoms

Step-by-step worked examples

Interpret the formula NaCl.

Na = sodium atom
Cl = chlorine atom
No subscript = one atom of each
NaCl = 1 sodium + 1 chlorine atom.

How many atoms of each element in Ca(OH)₂?

Ca = 1 calcium atom
(OH)₂ = subscript 2 multiplies atoms inside
O: 1 × 2 = 2 oxygen atoms
H: 1 × 2 = 2 hydrogen atoms
Total: 1 Ca + 2 O + 2 H.

Write a formula for a compound with 2 nitrogen and 5 oxygen atoms.

Nitrogen symbol: N
Oxygen symbol: O
2 nitrogen: N₂
5 oxygen: O₅
Formula: N₂O₅

Flashcards

Quick quiz

Q1.In the formula CO₂, how many oxygen atoms?

Correct answer: B. The subscript 2 after O means 2 oxygen atoms.

Q2.What does subscript 1 mean in a chemical formula?

Correct answer: B. A subscript 1 is implied and rarely written — if no subscript appears, there is one atom.

Q3.How many atoms are in Mg(NO₃)₂?

Correct answer: D. Mg = 1; (NO₃) with subscript 2 = 2×N + 2×(3 O) = 2 N + 6 O. Total: 1+2+6=9... wait: 1 Mg + 2 N + 6 O = 9 atoms.

Q4.Write the formula for 3 carbon and 8 hydrogen atoms.

Correct answer: B. 3 carbon = C₃; 8 hydrogen = H₈ → C₃H₈ (propane).

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Common mistakes

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No subscript means zero atoms. — Correct: No subscript means one atom (often not written).

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H₂O has three atoms total. — Correct: H₂O has three atoms: two hydrogen + one oxygen.

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Parentheses in a formula are just for grouping, subscript after doesn't multiply. — Correct: A subscript after parentheses multiplies everything inside — Ca(OH)₂ = 1 Ca + 2 O + 2 H.

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Element symbols are arbitrary. — Correct: Each element has a specific symbol (H = hydrogen, O = oxygen, Na = sodium), international standard.

FAQ

How do I read a chemical formula?

Identify each element symbol and its subscript number. No subscript = 1 atom. Subscripts after () multiply the atoms inside.

What is the difference between H and H₂?

H is an atom; H₂ is a molecule of two hydrogen atoms bonded together.

Can subscripts be other numbers besides 1 and 2?

Yes, formulas can have any subscript — Al₂(SO₄)₃ has subscripts 2 and 3.

Why is it important to write formulas correctly?

An incorrect formula means a different compound with different properties.