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An ionic compound is built from Al3+ cations and O2- anions. Which ratio of ions leaves the compound neutral?
AThree cations to two anions
BOne cation to one anion
CTwo cations to one anion
DTwo cations to three anions
Answer & Solution
Correct answer: D. Two cations to three anions
1. The total positive charge has to cancel the total negative charge exactly.
2. Two Al3+ ions carry 2 times +3, which is +6 in total.
3. Three O2- ions carry 3 times -2, which is -6 in total.
4. Since +6 and -6 add to zero, two cations to three anions is the neutral ratio.
5. A one to one ratio would leave +3 and -2, a net of +1, so it cannot be right.
_Source: OpenStax Chemistry (CC BY 4.0), Chs 7 and 17, section 7.1 Ionic Bonding_
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