C3.1 The periodic table · KS3 Science

Atoms

An atom has a tiny nucleus containing protons (charge +1, relative mass 1) and neutrons (charge 0, relative mass 1). Electrons (charge −1, very small mass) move around the nucleus in shells.
Atoms have no overall charge, because they have the same number of protons and electrons.
The atomic number is the number of protons. The mass number is the number of protons plus neutrons. So neutrons = mass number − atomic number.
Isotopes are atoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14.
Atoms are about 0.1 nm (1 × 10⁻¹⁰ m) across, and the nucleus is about 10 000 times smaller than the atom.

Electrons fill the shells from the inside: up to 2 in the first shell, then 8, then 8. Sodium (11 electrons) is 2, 8, 1; chlorine (17) is 2, 8, 7.
The group number is the number of electrons in the outer shell. The period number is the number of shells.
The model of the atom changed as scientists found new evidence: Dalton (solid spheres), Thomson (discovered the electron: the plum pudding model), Rutherford (the gold foil experiment showed a tiny, positive nucleus), Bohr (electrons in shells) and Chadwick (discovered the neutron).
Mendeleev arranged the elements into a periodic table and left gaps for elements not yet discovered, predicting their properties. The modern table is in order of atomic number.

Group 1, the alkali metals, have 1 outer electron. They are soft, are stored in oil, and react with water to make a metal hydroxide and hydrogen: sodium + water → sodium hydroxide + hydrogen. They get more reactive down the group, because the outer electron is further from the nucleus and lost more easily.
Group 7, the halogens, have 7 outer electrons and exist as pairs of atoms (Cl₂). Fluorine is a pale yellow gas, chlorine a green gas, bromine a red-brown liquid and iodine a grey-black solid. Down the group they get darker, their melting and boiling points rise, and they get less reactive.
A more reactive halogen displaces a less reactive one from its compound: chlorine + potassium bromide → potassium chloride + bromine.
Group 0, the noble gases, have full outer shells, so they are very unreactive. Helium is used in balloons, neon in signs and argon in light bulbs.

The transition metals are in the middle block. They have high melting points and densities, form coloured compounds, and are often used as catalysts (iron, nickel).
Standard form writes very big or small numbers as a number between 1 and 10 multiplied by a power of 10: 3 000 000 = 3 × 10⁶, and 0.0001 = 1 × 10⁻⁴.

Proton: A positive particle in the nucleus.
Neutron: A particle with no charge in the nucleus.
Electron: A negative particle that moves around the nucleus in shells.
Nucleus: The tiny centre of an atom, containing protons and neutrons.
Atomic number: The number of protons in an atom.
Mass number: The number of protons plus neutrons in an atom.
Isotope: An atom of the same element with a different number of neutrons.
Electron shell: An energy level that electrons occupy around the nucleus.
Alkali metal: An element in Group 1 of the periodic table.
Halogen: An element in Group 7 of the periodic table.
Noble gas: An unreactive element in Group 0 of the periodic table.
Transition metal: A metal in the central block of the periodic table.
Displacement reaction: A more reactive element taking the place of a less reactive one in a compound.