1.3 Atomic StructureEdexcel IGCSE Chemistry: Revision notes
Section 1
What are atoms and molecules?
An atom is the smallest part of an element that can exist and still show the chemical properties of that element.
A molecule is a group of two or more atoms chemically bonded together — this can be atoms of the same element (e.g. O₂) or different elements (e.g. H₂O).
Section 2
What is the structure of an atom?
An atom has a tiny, dense nucleus at its centre, containing protons and neutrons, surrounded by electrons in shells (energy levels).
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1840 (negligible) | −1 | Shells around nucleus |
In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges balance overall.
Always state relative mass AND relative charge for each particle — mark schemes award marks for each separately.
Section 3
What do atomic number, mass number, isotopes and Ar mean?
- Atomic number: the number of protons in an atom (also equals the number of electrons in a neutral atom); it defines which element an atom is
- Mass number: the total number of protons and neutrons in an atom
- Isotopes: atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass number
- Relative atomic mass (Ar): the average mass of the isotopes of an element, weighted by their relative natural abundance, compared to 1/12th the mass of a carbon-12 atom
Number of neutrons = mass number − atomic number.
Isotopes have the same chemical properties (same number of protons/electrons) but slightly different physical properties (e.g. mass) — students sometimes wrongly say isotopes react differently.
Section 4
How do you calculate relative atomic mass from isotopic abundance?
To calculate Ar from isotopic abundances, use:
Ar = (sum of each isotope's mass number × its % abundance) ÷ 100
For example, chlorine exists as 75% chlorine-35 and 25% chlorine-37:
Ar = [(35 × 75) + (37 × 25)] ÷ 100 = (2625 + 925) ÷ 100 = 35.5
Boron is 20% boron-10 and 80% boron-11. Ar = [(10×20)+(11×80)]÷100 = (200+880)÷100 = 10.8.
Must Know
- Atom = smallest part of an element; molecule = two or more atoms bonded together
- Protons (+1, mass 1) and neutrons (0, mass 1) are in the nucleus; electrons (−1, negligible mass) orbit in shells
- Atomic number = number of protons = number of electrons (neutral atom); mass number = protons + neutrons
- Isotopes = same protons, different neutrons, different mass number, same chemical properties
- Ar = weighted average mass of isotopes relative to carbon-12
- Ar formula: Ar = Σ(isotope mass × % abundance) ÷ 100
That's the notes covered.
Carry on to the next subtopic.