Atoms bond to get a full outer shell. In ionic bonding, a metal transfers electrons to a non-metal, forming oppositely charged ions held by strong electrostatic attraction. Sodium loses one electron to become Na⁺; chlorine gains one to become Cl⁻. Group 1 metals form 1+ ions, Group 2 form 2+, Group 6 form 2− and Group 7 form 1−.
In covalent bonding, non-metal atoms share pairs of electrons, forming molecules such as H₂O, CH₄ and Cl₂, or giant structures such as diamond. In metallic bonding, metal atoms lose their outer electrons to form a lattice of positive ions in a 'sea' of delocalised electrons. The formula of an ionic compound balances the charges: Mg²⁺ and Cl⁻ give MgCl₂.
Worked example
What is the formula of the compound formed between aluminium (Al³⁺) and oxygen (O²⁻)?
- Charges must balance to zero.
- Two Al³⁺ give +6; three O²⁻ give −6.
Answer:The formula is Al₂O₃.
Key words
- ion
- an atom that has gained or lost electrons
- ionic bond
- attraction between oppositely charged ions
- covalent bond
- a shared pair of electrons
- delocalised electrons
- electrons free to move through a metal
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NGSS-referenced: HS-PS1-2