Formulas and names
A formula says which atoms and how many, and a subscript applies only to the symbol in front of it: H₂SO₄ has two hydrogens, one sulfur and four oxygens. Brackets multiply everything inside: Ca(OH)₂ is one calcium, two oxygens and two hydrogens. To name an ionic compound you put the anion first and then the cation, with the valence in brackets if it has more than one: FeCl₂ is iron(II) chloride and FeCl₃ iron(III) chloride. Binary covalent compounds use prefixes: CO is carbon monoxide and CO₂ dioxide. The trick that solves almost everything is to cross the charges: Al³⁺ with O²⁻ gives Al₂O₃.
Where this sits
- Comes after
- Bonds
- Opens up
- Reactions and balancing
Exercises from this rung
Generated by the app, from easiest to hardest
How many atoms are there in total in H₂SO₄?
- Seven
- Three
- Eight
- Four
Why Two hydrogens, one sulfur and four oxygens: seven. A subscript applies only to the symbol in front of it, and where there is no number there is a silent one, which is the sulfur's case. The usual slips are reading the 2 as if it covered everything, or forgetting the sulfur because it carries no number.
easierHow many oxygens are there in Al₂(SO₄)₃?
- Twelve
- Nine
- Four
- Seven
Why The subscript outside the brackets multiplies everything inside: three SO₄ groups are three sulfurs and twelve oxygens. In total, aluminium sulfate has two aluminiums, three sulfurs and twelve oxygens. Brackets are the part of nomenclature that trips up most people, always for the same reason: applying the number only to the symbol next to it.
in betweenWhat is the formula of the sulfate ion?
- SO₄²⁻
- S²⁻
- SO₃²⁻
- SO₄⁻
Why Sulfate is SO₄²⁻ and sulfite, with one oxygen fewer, is SO₃²⁻: the -ate ending carries more oxygens than -ite. That is the pair to fix, because it repeats across the whole of nomenclature: nitrate NO₃⁻ and nitrite NO₂⁻, chlorate ClO₃⁻ and chlorite ClO₂⁻. Plain S²⁻ is sulfide, with no oxygen at all.
harder