Writing Formulae for Ionic Compounds Flashcards

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  • Make sure you understand the information on each card. If something's unclear, click the link to the revision notes at the bottom of the page for more details.

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Note: We may include questions that have multiple correct answers. It's useful to remember specific examples to understand these concepts better.

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In an ionic compound, what must the total charge of all the ions add up to?

The total positive and negative charges must add up to zero.

What charge do Group 1 metal ions have, and how does this help you write ionic formulae?

Group 1 metals form ions with a 1+ charge, which tells you how many negative ions are needed to balance them.

What ion charges do Group 2 metals and Group 7 non-metals usually form?

Group 2 metals form 2+ ions. Group 7 non-metals form 1− (1-) ions.

Describe the steps to write the formula of an ionic compound from the charges on its ions.

Write the symbol and charge of the positive ion, then the negative ion. Work out how many of each are needed so the total charge is zero. Then write the formula with these numbers as small numbers after the symbols, leaving out the charges.

Sodium forms Na+ ions and chlorine forms Cl- ions. What is the formula of sodium chloride, and why?

The formula is NaCl. One Na+ and one Cl- balance to give a total charge of zero, so the ratio is 1:1.

Magnesium forms Mg2+ ions and chlorine forms Cl- ions. What is the formula of magnesium chloride?

MgCl2, because one Mg2+ needs two Cl- ions so the total positive and negative charges are equal.

Aluminium forms Al3+ ions and oxygen forms O2- ions. What is the formula of aluminium oxide?

Al2O3, because two Al3+ ions (total +6) and three O2- ions (total -6) are needed so the charges cancel out.

What is a polyatomic ion, and give one example used in ionic formulae.

A polyatomic ion is a charged group of atoms that stays together in reactions. One example is the sulfate ion, SO4 2-.

When writing formulas, when do you need to use brackets around a polyatomic ion?

Use brackets when you need more than one of that ion, for example in calcium hydroxide, Ca(OH)2, which has two OH- ions.

Sodium forms Na+ ions and the sulfate ion is SO4 2-. What is the formula of sodium sulfate?

Na2SO4, because two Na+ ions (2+) balance one SO4 2- ion (2-).

In the formula MgCl2, what does the small '2' tell you?

It shows there are two chloride ions for every one magnesium ion in the compound.

The ammonium ion is NH4+ and the nitrate ion is NO3-. What is the formula of ammonium nitrate?

NH4NO3, because one NH4+ ion balances one NO3- ion so there is no overall charge.