Atomic Structure
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Atoms, elements and compounds (1a.1)
An atom is the smallest part of an element that can exist. An element contains only one type of atom. A compound contains atoms of two or more elements chemically bonded together.
Compounds can only be separated by chemical reactions, not physical methods.
Atoms, elements and compounds (1a.1) — Key Knowledge
- Atom smallest part of an element
- Element one type of atom
- Compound two or more elements chemically bonded
Mixtures (1a.1)
A mixture contains two or more substances not chemically bonded. The substances keep their own properties and can be separated by physical methods.
Unlike compounds, no chemical reaction is needed to separate a mixture.
Mixtures (1a.1) — Key Knowledge
- Mixture substances not chemically bonded
- Filtration insoluble solid from liquid
- Distillation liquids with different boiling points
- Chromatography dissolved substances
- Evaporation dissolved solid from solution
Subatomic particles (1a.2)
Atoms contain three subatomic particles: protons, neutrons and electrons.
Protons and neutrons are in the nucleus; electrons orbit in shells around it.
Subatomic particles (1a.2) — Key Knowledge
- Proton +1 charge, relative mass 1
- Neutron 0 charge, relative mass 1
- Electron −1 charge, negligible mass
Structure of an atom (1a.2)
The nucleus is at the centre, containing protons and neutrons. It is tiny compared to the overall atom — about 1/10,000 of the atom's radius — but contains almost all the mass. Most of an atom is empty space.
Atomic radius is about 0.1 nm (1 × 10⁻¹⁰ m).
Structure of an atom (1a.2) — Key Knowledge
- Nucleus centre of atom, protons + neutrons
- Electron shells energy levels around the nucleus
Atomic number and mass number (1a.3)
The atomic number is the number of protons — this defines the element. The mass number is the total of protons + neutrons. In a neutral atom, electrons equal protons.
Change the proton count and you change the element.
Atomic number and mass number (1a.3) — Key Knowledge
- Atomic number number of protons
- Mass number protons + neutrons
Isotopes (1a.3)
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Same atomic number, different mass numbers.
Same chemical properties (same electrons) but different physical properties (different mass). Carbon-12 and carbon-14 are isotopes of carbon.
Isotopes (1a.3) — Key Knowledge
- Isotopes same element, different neutron count
Relative atomic mass (1a.3)
Relative atomic mass (Ar) is the weighted mean mass of an atom of an element, taking into account the abundance of its isotopes, measured relative to carbon-12.
This is why Ar values on the periodic table aren't whole numbers — they reflect the mix of isotopes.
Relative atomic mass (1a.3) — Key Knowledge
- Relative atomic mass, Ar weighted mean of isotope masses relative to ¹²C
Electronic structure (1a.4)
Electrons fill shells from the innermost outward. First shell holds up to 2, second and third hold up to 8 each.
The number of electrons in the outer shell determines chemical properties and group number.
Electronic structure (1a.4) — Key Knowledge
- Electron shells energy levels: 2, 8, 8…
- Electronic structure e.g. sodium = 2, 8, 1
Dalton's atomic model (1a.5)
John Dalton proposed that atoms were solid, indivisible spheres. Each element was made of a different type of atom.
Early 1800s. Replaced when Thomson discovered the electron.
Dalton's atomic model (1a.5) — Key Knowledge
- Dalton's model solid sphere, indivisible
Thomson's atomic model (1a.5)
J.J. Thomson discovered the electron and proposed the plum pudding model: a ball of positive charge with negative electrons embedded in it.
1897. First model to include subatomic particles.
Thomson's atomic model (1a.5) — Key Knowledge
- Plum pudding model positive "dough" with negative electrons scattered through it
Rutherford's atomic model (1a.5)
Alpha particles fired at gold foil. Most passed through, some deflected, a few bounced back. Proved the atom has a small, dense, positive nucleus with mostly empty space around it.
1909–1911. Replaced the plum pudding model.
Rutherford's atomic model (1a.5) — Key Knowledge
- Alpha scattering experiment gold foil experiment
- Nuclear model small dense positive nucleus
Bohr's atomic model (1a.5)
Niels Bohr proposed that electrons orbit the nucleus in fixed shells (energy levels), not randomly.
Explained why atoms emit specific colours of light — electrons moving between fixed energy levels.
Bohr's atomic model (1a.5) — Key Knowledge
- Bohr model electrons in fixed shells/energy levels
Chadwick's discovery (1a.5)
James Chadwick discovered the neutron in 1932 — a particle in the nucleus with no charge and mass 1.
Explained why the nucleus was heavier than the proton count alone.
Chadwick's discovery (1a.5) — Key Knowledge
- Neutron no charge, mass 1, in the nucleus