Ions and Ionic Compounds

The idea

Atoms gain or lose electrons to reach a full outer shell. A charged atom is an ion, and oppositely charged ions stick together.

• Lose electrons → fewer negatives → positive ion (cation). Metals do this.
• Gain electrons → negative ion (anion). Non-metals do this.

$$\mathrm{Na}\to \mathrm{Na}{}^{+}+\mathrm{e}{}^{-}\mathrm{Cl}+\mathrm{e}{}^{-}\to \mathrm{Cl}{}^{-}$$

Put them together and the charges must cancel exactly:

$$\mathrm{Na}{}^{+}+\mathrm{Cl}{}^{-}\to \mathrm{NaCl}$$

Balancing charges

Magnesium forms $\mathrm{Mg}{}^{2+}$; chlorine forms $\mathrm{Cl}{}^{-}$. One magnesium needs two chlorides:

$$\mathrm{MgCl}{}_2$$

Cation	Anion	Formula
$\mathrm{Na}{}^{+}$	$\mathrm{Cl}{}^{-}$	$\mathrm{NaCl}$
$\mathrm{Ca}{}^{2+}$	$\mathrm{Cl}{}^{-}$	$\mathrm{CaCl}{}_2$
$\mathrm{Al}{}^{3+}$	$\mathrm{O}{}^{2-}$	$\mathrm{Al}{}_2\mathrm{O}{}_3$

The crossover trick — and why it works

Swapping the charges to become subscripts works because it guarantees the total positive and total negative charge are equal. Use it, but be able to explain why it works; that is what a test question will ask.

Where this is being used right now

Ions moving is what a battery is. In a lithium-ion cell, $\mathrm{Li}{}^{+}$ ions shuttle from one electrode to the other as it discharges, and back again as it charges — the whole electric vehicle industry rests on that one sentence of chemistry.

That makes battery chemistry one of the fastest-moving fields there is, and it is not only a laboratory field:

Emerging technology	The chemistry	Who works on it
Lithium iron phosphate cells	A cheaper, safer cathode using no cobalt	Chemical engineers; battery plant technicians
Sodium-ion cells	$\mathrm{Na}{}^{+}$ in place of $\mathrm{Li}{}^{+}$ — heavier, but sodium is everywhere	Materials chemists
Battery recycling	Dissolving cells to recover lithium, nickel, and cobalt as salts	Process operators; industrial mechanics
Heat pumps and refrigerants	New working fluids replacing high-warming ones	Refrigeration mechanics and HVAC technicians — a licensed skilled trade

The last row is worth noticing. Ontario is short of the tradespeople who install and service this equipment, the training is an apprenticeship rather than a degree, and the chemistry on that page is the chemistry on this one.

Curriculum connection

C2.7

describe the relationship between the structure of simple compounds and their chemical formulas

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C2.6

investigate and describe physical and chemical properties of elements and compounds, including those that make up common household products

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C1.2

analyse impacts of using emerging chemical technologies in various fields, including in the skilled trades, and assess factors that influence the development of these technologies

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