ZIMSEC O Level · Chemistry · Form 3
The kinetic particle theory
All matter is made of moving particles. Heating gives them more energy: solids melt, liquids boil. Diffusion and Brownian motion are evidence that particles move.
The atom
- Proton: +1, mass 1, in the nucleus.
- Neutron: 0, mass 1, in the nucleus.
- Electron: −1, mass 1/1840, in shells around the nucleus.
Proton (atomic) number Z = number of protons. Nucleon (mass) number A = protons + neutrons.
Isotopes
Atoms of the same element with the same number of protons but different numbers of neutrons, e.g. ³⁵Cl and ³⁷Cl. They have the same chemical properties.
Relative atomic mass is the average mass of an element's isotopes compared with 1/12 of a carbon-12 atom.
Worked example: Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Find its relative atomic mass.
Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 35.5
Electron configuration
Shells hold 2, 8, 8 electrons (first 20 elements). Potassium (Z = 19) is 2,8,8,1. The outer electrons decide chemical properties.
Check yourself
- How many neutrons are in ²⁷Al (Z = 13)?
- Write the electron configuration of calcium (Z = 20).
- Define isotopes.
Show answers
- 27 − 13 = 14
- 2,8,8,2
- Atoms of the same element with the same number of protons but different numbers of neutrons.
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