AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: predict the charge on a simple ion using the…
Full specification wording
Ionic bonding: Students should be able to: predict the charge on a simple ion using the position of the element in the Periodic Table construct formulas for ionic compounds.
The group number predicts the ion.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The group number predicts the ion. Group 1 loses one electron to give 1⁺, Group 2 gives 2⁺, Group 3 gives 3⁺; Group 5 gains three for 3⁻, Group 6 gains two for 2⁻ and Group 7 gains one for 1⁻. In each case the ion reaches the electron configuration of the nearest noble gas. Transition metals vary, so their charge is stated by the Roman numeral in the name, as in iron(III). To build a formula, write the two ions with their charges and add whichever numbers make the total positive charge equal the total negative charge, then reduce to the simplest ratio: magnesium oxide is MgO, not Mg₂O₂, and aluminium oxide is Al₂O₃.
What earns the marks
- The charge on each ion deduced from its group in the Periodic Table.
- The total positive charge balanced against the total negative charge.
- The formula reduced to its simplest whole-number ratio.
- Brackets around a compound ion that appears more than once.
Common mistakes and how to put them right
Giving oxygen as O⁻ instead of O²⁻.
Leaving magnesium oxide as Mg₂O₂ rather than reducing it.
Ignoring the Roman numeral so iron(III) chloride is written as FeCl₂.
Writing a finished formula that still carries an overall charge.
Exam tips
- Write the two ions with charges above them before combining; the cross-over then follows automatically.
- Always reduce the finished subscripts by any common factor.
- For a transition metal, read the Roman numeral in the name as the charge on the ion.
Try the lesson
The start of the Ionic bonding: Students should be able to: predict the charge on a simple ion using the position of the element in the Periodic Table construct formulas for ionic compounds. lesson, made from checked teaching notes for this specification point.
Have a go first
An element in Group 2 of the Periodic Table forms a simple ion. What is the charge on this ion?
Listen
The group number predicts the ion. Group 1 loses one electron to give 1⁺, Group 2 gives 2⁺, Group 3 gives 3⁺; Group 5 gains three for 3⁻, Group 6 gains two for 2⁻ and Group 7 gains one for 1⁻. In each case the ion reaches the electron configuration of the nearest noble gas. Transition metals vary, so their charge is stated by the Roman numeral in the name, as in iron(III). To build a formula, write the two ions with their charges and add whichever numbers make the total positive charge equal the total negative charge, then reduce to the simplest ratio: magnesium oxide is MgO, not Mg₂O₂, and aluminium oxide is Al₂O₃. A strong answer earns marks for these points. the charge on each ion deduced from its group in the Periodic Table. the total positive charge balanced against the total negative charge. the formula reduced to its simplest whole-number ratio. brackets around a compound ion that appears more than once. Exam tip: Write the two ions with charges above them before combining; the cross-over then follows automatically.
Key words
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predicting ionic charge
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noble gas configuration
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Roman numerals
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simplest ratio
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See every definition
- predicting ionic charge
- The group number predicts the ion, as Group 1 loses one electron to give 1⁺ and Group 7 gains one to give 1⁻.
- noble gas configuration
- In each case the ion reaches the electron configuration of the nearest noble gas.
- Roman numerals
- Transition metals vary, so their charge is stated by the Roman numeral in the name, as in iron(III).
- simplest ratio
- The formula is reduced to the simplest ratio, so magnesium oxide is MgO, not Mg₂O₂.
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