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Atomic structure and bonding

Chemistry · WAEC and JAMB · SS2 and SS3

The foundation of the whole subject. Almost every later topic — periodicity, bonding, electrolysis — is an application of where the electrons are.

What you need to know

  • Atomic number = number of protons = number of electrons in a neutral atom.
  • Mass number = protons + neutrons. Neutrons = mass number - atomic number.
  • Isotopes have the same atomic number but different mass numbers, so the same chemistry but different mass.
  • Electron configuration fills 2, 8, 8, 18 for the first shells; the outermost shell decides chemical behaviour.
  • Ionic bonding: a metal transfers electrons to a non-metal, forming oppositely charged ions held by electrostatic attraction.
  • Covalent bonding: two non-metals share pairs of electrons.
  • Metallic bonding: a lattice of positive ions in a sea of delocalised electrons, which explains conductivity and malleability.
  • Dative or co-ordinate bonding: both shared electrons come from the same atom, as in NH4+.

Key terms

Relative atomic mass
The weighted average mass of the isotopes of an element compared with one-twelfth of a carbon-12 atom.
Electronegativity
The tendency of an atom to attract a shared pair of electrons towards itself.
Allotropy
The existence of an element in more than one form in the same physical state, such as diamond and graphite.

Worked example

Chlorine has two isotopes, Cl-35 (75%) and Cl-37 (25%). Calculate its relative atomic mass.

  1. Multiply each mass by its abundance as a fraction.
  2. (35 x 0.75) + (37 x 0.25)
  3. = 26.25 + 9.25

The mistake to avoid

Ionic compounds conduct electricity when molten or in solution, NOT when solid. In the solid the ions are fixed in the lattice and cannot move.

In the exam

Draw electron diagrams with the shells as circles and the shared pairs clearly between the nuclei. Marks are given for the correct number of electrons in the outer shell.