ZIMSEC O Level · Combined Science · Form 2, Form 3
Inside the atom
An atom has a tiny central nucleus containing protons (charge +1, mass 1) and neutrons (charge 0, mass 1). Electrons (charge −1, almost no mass) move around the nucleus in shells.
- Atomic number = number of protons (and electrons in a neutral atom).
- Mass number = protons + neutrons.
- Isotopes are atoms of the same element with different numbers of neutrons, e.g. carbon-12 and carbon-14.
Electron arrangement
Shells fill in order: up to 2 electrons in the first shell, then 8, then 8. Sodium (atomic number 11) is 2,8,1.
The periodic table
- Elements are arranged in order of atomic number.
- Groups are vertical columns: elements in a group have the same number of outer electrons and similar properties. Group 1 metals (lithium, sodium, potassium) react strongly with water; Group 7 (the halogens) are reactive non-metals; Group 0 (the noble gases) are unreactive because their outer shells are full.
- Periods are horizontal rows: the period number equals the number of occupied shells.
Bonding
- Ionic bonding: metals lose electrons and non-metals gain them, forming oppositely charged ions (NaCl).
- Covalent bonding: non-metal atoms share pairs of electrons (H₂O, CH₄).
Exam tip: Draw electron arrangements as rings with dots or crosses – and count the electrons twice.
Check yourself
- An atom has 13 protons and 14 neutrons. Give its atomic number and mass number.
- Write the electron arrangement of chlorine (atomic number 17).
- Why are the noble gases unreactive?
Show answers
- Atomic number 13; mass number 27.
- 2,8,7
- Their outer electron shells are full.
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