AQA GCSE Chemistry (8462) · Paper 1
⚛️ Atomic Structure & the Periodic Table
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Atoms, elements & mixtures
Separating mixtures
Inside the atom
Atoms are built from protons, with a charge of +1, neutrons, which are neutral, and electrons, with a charge of −1. Protons and neutrons each have a relative mass of 1 and sit together in the central nucleus, while the much lighter electrons orbit in shells, also called energy levels. Because atoms have equal numbers of protons and electrons, they have no overall charge. An atom is about 1×10⁻¹⁰ m wide, and its nucleus is around 10,000 times smaller than that.
Electron shells fill from the inside outwards, following the pattern 2, 8, 8 for the first 20 elements. Sodium, which has 11 electrons, has the electron arrangement 2,8,1. The number of electrons in the outer shell is central to chemistry, because it decides how an element reacts.
The development of the atomic model
- Dalton described atoms as solid balls that could not be broken down further.
- Thomson discovered the electron and proposed the plum pudding model, with electrons scattered through a ball of positive charge.
- The alpha scattering experiment fired alpha particles at thin gold foil. Most passed straight through, showing that atoms are mostly empty space, but a small number bounced back, showing that an atom's mass and positive charge are concentrated in a tiny nucleus.
- Bohr proposed that electrons orbit the nucleus in fixed shells.
- Chadwick discovered the neutron.
The periodic table
The modern periodic table orders elements by atomic number, which is the same as the number of protons. Elements in the same column, called groups, have the same number of outer electrons, which gives them similar reactions. Elements in the same row, called periods, have the same number of occupied electron shells.
Mendeleev produced a working periodic table in 1869 by ordering the elements by atomic weight. He left gaps for undiscovered elements, and swapped a few pairs around where their properties required it. When later-discovered elements, such as germanium, matched his predictions, his table was accepted as correct. The discovery of isotopes later explained why ordering strictly by atomic weight had caused a few anomalies.
Metals are found on the left and in the centre of the periodic table, and make up most of the elements, while non-metals are found towards the upper right. Metals react by losing outer electrons to form positive ions. Typical metals have high melting points, conduct electricity, are dense, shiny and malleable, while non-metals typically have low melting points, are dull and brittle, and act as insulators.
Groups & trends
Group 1 — the alkali metals
Group 1 contains the alkali metals, such as lithium, sodium and potassium. These are soft metals with a low density, so the first three float on water, and they are stored under oil because they react so readily with air and moisture. Each atom has one outer electron. When added to water, they fizz and move around on the surface, reacting to form a metal hydroxide, which makes an alkaline solution, and hydrogen gas.
Reactivity increases down the group, so potassium reacts violently enough to ignite the hydrogen gas it produces. The reason is that larger atoms hold their outer electron further from the nucleus, so that electron is more easily lost.
Group 7 — the halogens
Group 7 contains the halogens: fluorine, chlorine, bromine and iodine are coloured, diatomic non-metals. For example, chlorine (Cl₂) is a green gas, bromine (Br₂) is a brown liquid, and iodine (I₂) is a grey solid. Each atom has seven outer electrons.
Going down the group, melting and boiling points rise, while reactivity falls. This is because the incoming electron lands further from the nucleus, so it is attracted less strongly. A more reactive halogen will displace a less reactive one from a solution of its salt, producing orange bromine in this example: chlorine + potassium bromide → potassium chloride + bromine.
Group 0 — the noble gases
Group 0 contains the noble gases, such as helium, neon and argon. They already have full outer shells, so they exist as unreactive single atoms. Their boiling points increase down the group.
Transition metals, found in the central block of the periodic table, are studied in Triple Science. Compared with Group 1 metals, they are harder and stronger, have higher melting points, and are far less reactive with water and oxygen. They form coloured compounds, can form ions with different charges, such as Fe²⁺ and Fe³⁺, and make good catalysts.