Revision notes · Atomic structure and the periodic table
A simple model of the atom, symbols, relative atomic mass,
Atoms, elements and compounds4.1.1.1
Definition: An atom is the smallest part of an element that can exist; an element is a substance made of only one type of atom; a compound is formed when two or more elements are chemically combined.
All substances are made of atoms, and how those atoms combine determines what a substance is.
- •There are about 100 different elements, each represented by a one- or two-letter chemical symbol (e.g. O, Na, Cl).
- •Elements are arranged in the periodic table.
- •In a compound, atoms of different elements are chemically bonded together in fixed proportions.
- •Compounds can only be separated back into elements by a chemical reaction, never by physical means.
⚠️ Common mistake: Thinking a mixture and a compound are the same thing — a compound's atoms are chemically bonded in fixed ratios; a mixture's substances are not chemically combined and can vary in proportion.
🧠 Remember: Compound = chemically joined, fixed ratio. Mixture = just mixed, no fixed ratio.
Mixtures4.1.1.2
Definition: A mixture consists of two or more elements or compounds that are not chemically combined together.
The chemical properties of each substance in a mixture are unchanged, so mixtures can be separated using physical processes.
| Method | Used to separate |
|---|---|
| Filtration | An insoluble solid from a liquid |
| Crystallisation | A soluble solid (solute) from a solution, by evaporating the solvent |
| Simple distillation | A solvent from a solution (collects the solvent) |
| Fractional distillation | A mixture of miscible liquids with different boiling points |
| Chromatography | Substances with different solubilities, based on how far they travel |
⚠️ Common mistake: Forgetting that separation techniques for mixtures are all physical processes — no new substances are made, and the separation can be reversed.
🧠 Remember: Mixture in, physical method, same substances out.
The development of the model of the atom (common content with physics)4.1.1.3
The model of the atom has changed over time as new experimental evidence was discovered.
- •Before the discovery of the electron, atoms were thought to be tiny solid spheres (Dalton's model).
- •J.J. Thomson's discovery of the electron led to the 'plum pudding' model — a ball of positive charge with negative electrons embedded in it.
- •Rutherford's alpha particle scattering experiment showed that most of an atom's mass and its positive charge is concentrated in a tiny central nucleus, leading to the nuclear model.
- •Niels Bohr adapted the nuclear model by proposing that electrons orbit the nucleus at specific distances, in fixed shells/energy levels.
- •Later experiments showed the nucleus could be further divided into protons and neutrons.
⚠️ Common mistake: Thinking the plum pudding model included a nucleus — it didn't; the nucleus was only proposed after the alpha scattering experiment overturned that model.
🧠 Remember: Plum pudding → Nuclear (Rutherford) → Shells (Bohr) — each model was replaced by new evidence.
Relative electrical charges of subatomic particles4.1.1.4
Definition: An atom has a small central nucleus, made of protons and neutrons, surrounded by electrons arranged in shells.
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | very small (1/1835) | Shells (orbiting nucleus) |
- •Atoms have no overall electrical charge, because the number of protons equals the number of electrons.
- •The atomic number is the number of protons in an atom; the mass number is the total number of protons and neutrons.
⚠️ Common mistake: Forgetting that the number of protons must equal the number of electrons in a neutral atom — this is what makes atoms overall uncharged.
🧠 Remember: Atomic number = protons. Mass number = protons + neutrons.
Size and mass of atoms4.1.1.5
Atoms are extremely small, and almost all of their mass is concentrated in the nucleus.
- •Atoms have a radius of about 0.1 nanometres (1 × 10⁻¹⁰ m).
- •The radius of the nucleus is around 1/10,000 of the radius of the whole atom.
- •Almost all of the mass of an atom is concentrated in the nucleus, since electrons have negligible mass.
⚠️ Common mistake: Confusing the size ratio of the nucleus to the atom with the mass ratio — the nucleus is tiny in size, but holds almost all the mass.
🧠 Remember: Nucleus: tiny in size, (almost) all the mass.
Relative atomic mass4.1.1.6
Definition: Relative atomic mass (Ar) is the average mass of the atoms of an element, taking into account the relative abundance of its different isotopes, compared to 1/12th the mass of a carbon-12 atom.
Most elements exist as a mixture of isotopes, so relative atomic mass is a weighted average, not a whole number.
- •Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
- •Relative atomic mass = (sum of isotope mass number × % abundance) ÷ 100.
⚠️ Common mistake: Assuming relative atomic mass should always be a whole number — because it is a weighted average across isotopes, it is often a decimal (e.g. chlorine, Ar = 35.5).
🧠 Remember: Ar is a weighted average, not just the mass number of one atom.
Electronic structure4.1.1.7
Definition: Electronic structure describes how electrons are arranged in shells around the nucleus of an atom.
- •Electrons occupy shells, starting with the lowest energy shell available (closest to the nucleus) first.
- •The first shell holds up to 2 electrons; the second and third shells hold up to 8 electrons each.
- •The arrangement of electrons in an atom can be written as numbers separated by commas, e.g. sodium (11 electrons) is 2,8,1.
- •The number of electrons in the outermost shell determines an element's chemical properties, and its group number in the periodic table.
⚠️ Common mistake: Filling shells out of order — always fill the innermost (lowest energy) shell completely before starting the next one.
🧠 Remember: Fill shells innermost first: 2, then 8, then 8.
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