C3 - Formula of Ionic Compounds
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19 Terms
What is the key rule when writing the formula of an ionic compound?
The charges on the ions must cancel out so the overall charge is zero.
Do ionic compounds ever have an overall charge?
No, ionic compounds always have an overall charge of zero.
What ions make up sodium chloride?
Na⁺ (sodium ion) and Cl⁻ (chloride ion).
What is the formula of sodium chloride and why?
NaCl, because one Na⁺ cancels one Cl⁻.
What would happen if sodium chloride had 2 Na⁺ ions and 1 Cl⁻ ion?
The compound would have an overall +1 charge, which is not allowed.
What ions make up sodium oxide?
Na⁺ and O²⁻.
What is the formula of sodium oxide and why?
Na₂O, because two Na⁺ ions cancel one O²⁻ ion.
What ions make up magnesium iodide?
Mg²⁺ and I⁻.
What is the formula of magnesium iodide and why?
MgI₂, because one Mg²⁺ requires two I⁻ ions to balance the charges.
What ions make up lithium carbonate?
Li⁺ and CO₃²⁻.
What is the formula of lithium carbonate and why?
Li₂CO₃, because two Li⁺ ions cancel one CO₃²⁻ ion.
What ions make up calcium hydroxide?
Ca²⁺ and OH⁻.
What is the formula of calcium hydroxide and why?
Ca(OH)₂, because one Ca²⁺ needs two OH⁻ ions (brackets are needed to show two hydroxide ions, not just two hydrogens).
Why do we use brackets in formulas like Ca(OH)₂?
To show that the subscript applies to the whole ion group, not just the atom next to it.
What ions make up magnesium nitrate?
Mg²⁺ and NO₃⁻.
What is the formula of magnesium nitrate and why?
Mg(NO₃)₂, because one Mg²⁺ needs two NO₃⁻ ions (brackets are needed to multiply the whole nitrate ion by 2).
Why can't we write MgNO₃₂ for magnesium nitrate?
Because that would mean 32 oxygen atoms instead of two nitrate ions, which is wrong.
What type of ions are you always given in an exam question?
You are always given the charges on the ions.
How hard are ionic formula questions in exams?
No harder than the examples: sodium chloride, sodium oxide, magnesium iodide, lithium carbonate, calcium hydroxide, magnesium nitrate.