Valency: building compound formulas
Join a metal with a non-metal. See each atom's electron shells, find its valency from its outer electrons, and build the formula: NaCl, MgO, Al₂O₃, CaCl₂ and water.
Core concept
Valency is the combining power of an atom, set by its outer electrons; atoms join so that their valencies balance, which the criss-cross rule turns into a formula
Topic: Atoms and molecules
The electrons of an atom are arranged in shells: 2 in the first, up to 8 in the second, and up to 8 in the third (for atomic numbers up to 20). Atoms are most stable with a full outer shell, so they give away, take or share electrons when they join. Metals with 1, 2 or 3 outer electrons give them away: their valency is 1, 2 or 3. Non-metals with 5, 6 or 7 outer electrons take 3, 2 or 1: that is their valency. In a compound the valencies balance. The criss-cross rule writes the formula quickly: swap the valencies to become the numbers of atoms, and simplify if they can be divided. Aluminium (3) and oxygen: A gas in the air that living things need to breathe and that fire needs to burn. All key words (2) make Al₂O₃.
What you will learn
- Write the electron shells of atoms with atomic numbers up to 20.
- Find the valency of an atom from its outer electrons.
- Use the criss-cross rule to write the formulas of compounds.
- Relate valency to an element's group in the periodic table.
Before you start
- Know that atoms have protons, neutrons and electrons.
Materials
- A periodic table
Equipment
- Bottle caps or beads to model electrons in shells
Procedure
- Join sodium and chlorine and record.
- Join magnesium and oxygen and record.
- Join aluminium and oxygen and record.
- Join calcium and chlorine and record.
- Join hydrogen and oxygen and record.
How this simulation is simplified
- Shells hold 2, then 8, then 8 electrons (enough for atomic numbers up to 20).
- Metals give away their outer electrons; non-metals take enough to fill their outer shell.