Atomic Structure
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Atomic Structure
Atoms have a small, dense nucleus containing protons and neutrons, surrounded by electrons in shells.
The nucleus contains almost all the mass but occupies a tiny fraction of the atom's volume.
Atomic Structure — Key Knowledge
- Proton positive charge, relative mass 1, in nucleus
- Neutron no charge, relative mass 1, in nucleus
- Electron negative charge, negligible mass, orbits in shells
- Nucleus tiny compared to the atom — atom is mostly empty space
Atomic Number and Mass Number
The atomic number is the number of protons; the mass number is the total of protons and neutrons.
Two atoms with the same atomic number are always the same element, regardless of their mass number.
Atomic Number and Mass Number — Key Knowledge
- Atomic number number of protons, defines the element
- Mass number protons + neutrons
- Atoms have no overall charge equal numbers of protons and electrons
Isotopes
Isotopes are atoms of the same element with different numbers of neutrons.
Carbon-12 and carbon-14 are isotopes — both have 6 protons, but carbon-14 has 2 extra neutrons.
Isotopes — Key Knowledge
- Isotopes same number of protons, different number of neutrons
- Different mass numbers but same atomic number
- Same chemical properties because electron arrangement is unchanged
Development of the Atomic Model
The model of the atom changed as new evidence emerged over time.
Each model was revised when experimental evidence could not be explained by the existing theory.
Development of the Atomic Model — Key Knowledge
- Dalton solid sphere model, indivisible atoms
- Thomson plum pudding model — negative electrons embedded in positive sphere
- Rutherford nuclear model — gold foil experiment showed small dense positive nucleus with mostly empty space
- Bohr electrons orbit in fixed shells at specific energies
- Chadwick discovered the neutron in the nucleus
Types of Nuclear Radiation
Unstable nuclei emit radiation to become more stable. There are three main types.
In alpha and beta decay the element changes because the number of protons in the nucleus changes; in gamma emission it does not.
Types of Nuclear Radiation — Key Knowledge
- Alpha α) particle (2 protons + 2 neutrons, equivalent to a helium nucleus
- Beta β) particle (high-speed electron emitted from the nucleus when a neutron turns into a proton
- Gamma γ) ray (electromagnetic radiation with no mass and no charge
Penetrating Power and Ionising Ability
The three types of radiation differ in how far they travel and how strongly they ionise.
There is a trade-off — the more ionising the radiation, the less penetrating it is, because it loses energy faster.
Penetrating Power and Ionising Ability — Key Knowledge
- Alpha stopped by paper or skin, strongly ionising
- Beta stopped by a few mm of aluminium, moderately ionising
- Gamma reduced by thick lead or concrete, weakly ionising
Half-life
Half-life is the time taken for the number of unstable nuclei (or the count rate) to halve.
After three half-lives, the count rate has fallen to one-eighth of the original value.
Half-life — Key Knowledge
- Half-life time for half the unstable nuclei to decay
- Radioactive decay is random cannot predict which nucleus will decay next
- Radioactive decay is spontaneous not affected by external conditions
Contamination vs Irradiation
Contamination means radioactive material has got onto or inside the body; irradiation means being exposed to radiation from an external source.
Contamination is generally a greater long-term hazard because the source remains in contact, continuously irradiating surrounding tissue.
Contamination vs Irradiation — Key Knowledge
- Contamination radioactive atoms on or in the body — source stays with you, ongoing exposure
- Irradiation exposed to radiation from outside — once the source is removed, exposure stops
Nuclear Fission
Fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and neutrons.
Each fission event releases neutrons that can trigger more fissions — this must be controlled to prevent a runaway reaction.
Nuclear Fission — Key Knowledge
- Fission splitting of heavy nuclei such as uranium-235 or plutonium-239
- Chain reaction released neutrons hit other nuclei causing further fission
- Used in nuclear power stations controlled chain reaction to generate electricity
Nuclear Fusion
Fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy.
Fusion releases more energy per kilogram than fission but is not yet viable for power generation on Earth because of the extreme conditions required.
Nuclear Fusion — Key Knowledge
- Fusion joining of light nuclei such as hydrogen
- Requires extremely high temperature and pressure to overcome electrostatic repulsion between positive nuclei
- Occurs naturally in stars the source of a star's energy