Formulas and names of ionic compounds
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Question
What is a cation?
Answer
A **positively** charged ion (a metal, or NH_{4}⁺).
Question
What is an anion?
Answer
A **negatively** charged ion (a non-metal, or a polyatomic ion).
Question
What is a polyatomic ion?
Answer
A charged group of **bonded atoms** that acts as a single unit (e.g. SO_{4}²⁻, NO_{3}⁻).
Question
Why is an ionic formula always neutral?
Answer
The ratio of ions is chosen so the **total positive and negative charges cancel** to zero.
Question
Describe the crossover (swap-and-balance) method.
Answer
Write each ion with its charge, **cross over** the charge sizes as subscripts, then **simplify** to the smallest whole-number ratio.
Question
Formula of magnesium chloride?
Answer
**MgCl_{2}** — Mg²⁺ needs two Cl⁻ to balance.
Question
Formula of aluminium oxide?
Answer
**Al_{2}O_{3}** — crossover of Al³⁺ and O²⁻ (6+ balances 6−).
Question
When do you use brackets in a formula?
Answer
When a **polyatomic ion appears two or more times**, e.g. Ca(NO_{3})_{2}, (NH_{4})_{2}SO_{4}.
Question
Charge and formula of the sulfate ion?
Answer
**SO_{4}²⁻** — a 2− polyatomic ion.
Question
Charge and formula of the ammonium ion?
Answer
**NH_{4}⁺** — a 1+ polyatomic cation.
Question
How do you name a simple (binary) ionic compound?
Answer
Cation name first, then the non-metal anion ending in **-ide** (e.g. magnesium ox**ide**).
Question
Formula of calcium nitride?
Answer
**Ca_{3}N_{2}** — Ca²⁺ and N³⁻ crossover (6+ balances 6−).
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Topic 2.1 hub
The ionic model
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