Atomic Structure, Isotopes and the Periodic Table
Atomic structure, isotopes and the Periodic Table
The GCE O-Level exam tests subatomic particles, atomic and mass numbers, electron arrangement, isotopes, relative atomic mass and periodic trends. You must link position in the Periodic Table to electronic structure.
Core rules
- An atom is neutral: protons equal electrons. The proton number, Z, identifies the element. The mass number, A, is protons plus neutrons, so neutrons = A - Z.
- Electrons occupy shells in the order 2, 8, 8 for the first twenty elements. Ions form by losing or gaining electrons; the nucleus does not change.
- Isotopes are atoms of the same element with the same proton number but different neutron numbers. They have similar chemical properties because chemical behaviour depends mainly on outer electrons.
- Relative atomic mass is the weighted average of isotope masses compared with carbon-12. Use percentage abundances.
- In the Periodic Table, group number equals outer-shell electrons and period number equals number of occupied shells. Group I reactivity increases down the group; Group VII reactivity decreases down the group.
Worked example
Chlorine consists of about 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
Reasoning: Ar = (75 x 35 + 25 x 37)/100 = (2625 + 925)/100 = 3550/100 = 35.5. The answer lies between 35 and 37 and closer to 35 because chlorine-35 is more abundant. Relative atomic mass is a weighted average, not the mass number of the most common isotope.
Traps that cost marks
Mass number is not relative atomic mass. Isotopes do not have identical physical properties such as density or rate of diffusion. When writing ion electron configurations, adjust electrons but keep the proton number. Do not confuse group number with period number. Transition metals often show variable oxidation states and act as catalysts.
Key points
- Z = protons; A = protons + neutrons.
- Ions differ from atoms only in electron number.
- Isotopes have the same Z but different A.
- Relative atomic mass is a weighted isotope average.
- Group number gives outer electrons; period gives shells.