What is the formula of calcium phosphate?
- Calcium is in group 2, so its ion is Ca2+. The phosphate ion is PO43−.
- The charges must cancel. The lowest common multiple of 2 and 3 is 6.
- Three Ca2+ give 6+ and two PO43− give 6−.
Answer: Ca3(PO4)2
The ionic model
Chemistry · Structure 2: Models of bonding and structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Metal atoms lose their outer electrons to form positive ions (cations). Non-metal atoms gain electrons to form negative ions (anions). The ions usually have the electron configuration of a noble gas, so the charge can be found from the group. Transition elements can form more than one stable ion, for example Fe2+ and Fe3+.
An ionic bond is the electrostatic attraction between oppositely charged ions. The ions are packed in a giant lattice, each one attracted to all its neighbours. There are no molecules.
Lattice enthalpy measures the strength of this attraction. The strong attractions explain the high melting points and low volatility. Ionic compounds conduct only when the ions are free to move.
Rules to remember
- Ion charges by group: group 1 is 1+, group 2 is 2+, group 13 is 3+, group 15 is 3−, group 16 is 2−, group 17 is 1−
- Polyatomic ions: NH4+, OH−, NO3−, HCO3−, CO32−, SO42−, PO43−
- In a formula the total positive charge equals the total negative charge; put brackets round a polyatomic ion that is used more than once
- Lattice enthalpy is larger when the ions have higher charges and smaller radii
- Ionic solids do not conduct (ions fixed in the lattice); they conduct when molten or dissolved in water (ions free to move)
- Many ionic compounds dissolve in polar solvents such as water but not in non-polar solvents such as hexane
Common mistake
Saying that molten ionic compounds conduct because of free electrons. The charge is carried by moving ions; there are no delocalized electrons.
Worked example
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