Atomic structure
The current model of the atom comes from over a century of experiments. You need to know the structure, the history and the meaning of atomic number, mass number, isotopes and relative atomic mass.
The current model
An atom has:
- A tiny nucleus at the centre, containing protons and neutrons.
- Electrons orbiting in shells / energy levels around the nucleus.
The diameter of the atom (~0.1 nm) is about 100,000 times the diameter of the nucleus (~1 fm = 10⁻¹⁵ m). Most of the atom is empty space.
Sub-atomic particles — relative mass and charge
| Particle | Symbol | Relative mass | Relative charge | Location |
|---|---|---|---|---|
| Proton | p | 1 | +1 | Nucleus |
| Neutron | n | 1 | 0 | Nucleus |
| Electron | e | ~1/1836 (≈ 0) | −1 | Shells |
In a neutral atom, the number of protons = the number of electrons (charges cancel).
Atomic number and mass number
- Atomic number (Z) — the number of protons. This defines the element. Carbon always has 6 protons; if it had 7, it would be nitrogen.
- Mass number A — the number of protons + neutrons in the nucleus.
So number of neutrons = A − Z.
For example, ²³₁₁Na has 11 protons, 11 electrons and (23 − 11) = 12 neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (and so different mass numbers).
Common examples:
- Carbon-12 (⁶C with 6 neutrons), carbon-13 (7 neutrons), carbon-14 (8 neutrons).
- Chlorine-35 (¹⁷Cl with 18 neutrons) and chlorine-37 (20 neutrons).
Isotopes have the same chemistry (same electron arrangement) but slightly different physical properties (mass).
Relative atomic mass (A_r)
Because elements often exist as a mixture of isotopes, we use a relative atomic mass — the weighted average of all isotope masses, taking abundance into account:
A_r = (Σ mass × abundance %) ÷ 100
Worked example: chlorine consists of 75 % Cl-35 and 25 % Cl-37. A_r = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
This is why chlorine appears as 35.5 on the periodic table.
History — how the model evolved
- Dalton (early 1800s) — atoms are tiny, indivisible spheres.
- Thomson (1897) — discovered the electron; "plum pudding" model: positive sphere with electrons embedded.
- Rutherford (1911) — gold-foil experiment. Most alpha particles passed through; some were deflected; a few bounced back. This showed the atom was mostly empty space with a small, dense, positively-charged nucleus.
- Bohr (1913) — electrons orbit at fixed energy levels (shells).
- Chadwick (1932) — discovered the neutron in the nucleus.
The current model has electrons in fixed energy levels (shells); inside the nucleus are protons and neutrons.
✦Worked example— Worked example — describe ¹⁹₉F
- 9 protons (atomic number).
- 19 nucleons in total (mass number).
- 19 − 9 = 10 neutrons.
- 9 electrons (neutral atom).
- Electron arrangement: 2,7 (in shells).
⚠Common mistakes
- Mixing up atomic number and mass number. Atomic = protons; mass = protons + neutrons.
- Saying isotopes are different elements. They are the same element with different numbers of neutrons.
- Forgetting electrons have very small mass. Treat their mass as ~0 in mass-number calculations.
- Calculating A_r without using % weighting. Always weight by abundance.
Links
Foundation for C1.3 (history of the periodic table), C1.4 (electronic structure) and all of C2 (bonding).
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