Compounds, Formulae, and Equations, Formulae and Equations

Full Notes

Ionic Formulae

To write the correct formula of an ionic compound, the total positive and negative charges from each ion must cancel out.

Ionic charge for ions of elements are usually predictable from the periodic table:

• Group 1 metals (e.g. Na, K): form +1 ions
• Group 2 metals (e.g. Ca, Mg): form +2 ions
• Group 6 non-metals (e.g. O, S): form –2 ions
• Group 7 non-metals (e.g. Cl, Br): form –1 ions

Transition metals can have variable charges, often indicated using Roman numerals in their names.

Example Variable charges of transition metals

Iron(II) = Fe²⁺
Iron(III) = Fe³⁺

Common Polyatomic Ions

Polyatomic ions contain more than one atom, and the bonded atoms share an overall charge.

• Nitrate = NO₃⁻
• Carbonate = CO₃²⁻
• Sulfate = SO₄²⁻
• Hydroxide = OH⁻
• Ammonium = NH₄⁺

Writing Ionic Formulae

Balance the charges so the total equals zero:

Example Balancing ionic formulae

Na⁺ + Cl⁻ → NaCl

Ca²⁺ + 2Cl⁻ → CaCl₂

Al³⁺ + 3NO₃⁻ → Al(NO₃)₃

Use brackets when a polyatomic ion appears more than once:
Mg²⁺ + 2OH⁻ → Mg(OH)₂
Ca²⁺ + 2NO₃⁻ → Ca(NO₃)₂

Balanced Chemical Equations

A chemical equation must be balanced so that the number of each type of atom is the same on both sides. Always include correct formulas and state symbols:

Example Balanced equation

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

State Symbols:

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous (dissolved in water)

Ionic Equations

These show only the species involved in the chemical change. Spectator ions (those that appear unchanged on both sides) are not included.

Example:Reaction between NaOH and HCl

Example Precipitation

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Summary

• Ionic compounds must balance total charges to zero.
• Use brackets if polyatomic ions appear more than once.
• Chemical equations must balance and include state symbols.
• Ionic equations include only the reacting species, not spectators.