Atomic Structure
Section: Nuclear Physics | Syllabus: Cambridge IGCSE Physics (0625)
The Nuclear Model of the Atom
An atom consists of a positively charged nucleus, with negatively charged electrons in orbit around it.
- Nucleus: a very small, dense, positively charged region at the centre of the atom
- Electrons: negatively charged particles that orbit the nucleus
- The nucleus occupies only a tiny fraction of the atom's total volume, but contains almost all of its mass
A positively charged nucleus with negatively charged electrons in orbit around it
Evidence for the Nuclear Model: Alpha Particle Scattering
Extended Only
The scattering of alpha (α) particles by a thin sheet of metal provides evidence that supports the nuclear model of the atom.
The Experiment
- A narrow beam of alpha particles - which are positively charged and relatively heavy - was directed at a thin sheet of gold foil
- A circular fluorescent detector screen surrounding the foil recorded the angle at which each alpha particle was scattered
Alpha particle scattering by a thin gold foil
Evidence from Alpha Scattering
| Observation | Conclusion |
|---|---|
| Most alpha particles passed straight through the foil undeflected | (a) the atom is mostly empty space, with a very small nucleus |
| Very few alpha particles (about 1 in 8000) bounced back or were deflected through large angles | (b) these particles hit something very small, dense and massive - the nucleus contains most of the mass of the atom |
| Some alpha particles were deflected through small angles | (c) they passed close to something positively charged and were repelled - the nucleus is positively charged |
Composition of the Nucleus
The nucleus is composed of protons and neutrons; these particles are collectively called nucleons.
| Particle | Location | Relative Charge | Relative Mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Orbiting the nucleus | −1 | Negligible |
Proton Number and Nucleon Number
Every element is uniquely defined by the number of protons in its nucleus.
- Proton number (atomic number), Z: the number of protons in the nucleus of an atom
- Nucleon number (mass number), A: the total number of protons and neutrons in the nucleus
- Number of neutrons in a nucleus = A − Z
Nuclide Notation
Atoms are represented using nuclide notation, AZX, where X is the chemical symbol of the element, A is the nucleon number (written above) and Z is the proton number (written below).
The nuclide notation of an element, showing the nucleon number (A), proton number (Z), and chemical symbol (X)
- Carbon, 126C: proton number = 6, nucleon number = 12, so number of neutrons = 12 − 6 = 6, and (in a neutral atom) number of electrons = 6
- Uranium, 23892U: proton number = 92, nucleon number = 238, so number of neutrons = 238 − 92 = 146, and (in a neutral atom) number of electrons = 92
Carbon-12 nuclide notation showing 6 protons and 6 neutrons in the nucleus, and 6 electrons orbiting the nucleus
Relationship to Nuclear Charge and Mass
Extended Only
- The relative charge on a nucleus = +Z - equal to the proton number, since each proton has charge +1 and each neutron has charge 0
- The relative mass of a nucleus ≈ A - equal to the nucleon number, since protons and neutrons each have relative mass 1
Isotopes
An element may have more than one isotope. Isotopes are atoms of the same element with the same proton number but different nucleon numbers - the same number of protons, but different numbers of neutrons.
- Same number of protons (same Z) - so isotopes of the same element
- Different number of neutrons (different A)
- Same chemical properties, since these depend on the number of electrons
- Different physical properties, e.g. different masses, and some isotopes may be radioactive while others are stable
Carbon-12, carbon-13 and carbon-14 are isotopes of carbon - all have 6 protons, but different numbers of neutrons
- 126C: 6 protons, 6 neutrons - stable, the most common isotope of carbon
- 136C: 6 protons, 7 neutrons - stable, but rare
- 146C: 6 protons, 8 neutrons - radioactive, used in carbon dating
Ions
Atoms can form ions by gaining or losing electrons.
| Ion Type | Formation | Charge |
|---|---|---|
| Positive ion | Atom loses electrons | Positive (+) |
| Negative ion | Atom gains electrons | Negative (−) |
Worked Example: Sodium Ion Formation
Museli is asked to explain how a sodium ion, Na⁺, forms from a neutral sodium atom.
- Neutral sodium atom: 11 protons, 11 electrons (overall charge = 0)
- The atom loses 1 electron: Na → Na⁺ + e⁻
- Sodium ion (Na⁺): 11 protons, 10 electrons (overall charge = +1)
- The nucleus never changes when an ion forms - only electrons are lost or gained, so the proton number stays the same
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