BGCSE Chemistry

Protons, neutrons and electrons

An atom is the smallest part of an element that can exist — a particle with no overall electric charge, made of a tiny central nucleus containing protons and neutrons, surrounded by electrons arranged in energy levels (shells). Atoms are almost unimaginably small, with a radius of about 0.1 nanometres (1 × 10⁻¹⁰ m), and the nucleus itself is roughly 1/10,000th the size of the whole atom — if the atom were the size of a football pitch, the nucleus would be about the size of a pea. Even so, because protons and neutrons are far heavier than electrons, almost all of an atom's mass sits in that tiny nucleus.

	Proton	Neutron	Electron
Relative mass	1	1	very small
Relative charge	+1	0	−1
Location	nucleus	nucleus	energy levels (shells)

Atoms are electrically neutral because the number of protons always equals the number of electrons, so the positive and negative charges cancel out exactly.

Atomic number, mass number and isotopes

The atomic number (or proton number) is the number of protons in an atom, and it is this number that decides which element an atom is — every atom with 11 protons is sodium, no matter how many neutrons it has. The mass number is the total number of protons plus neutrons. From these two numbers you can work out everything about an atom's structure: the number of protons equals the atomic number, the number of neutrons equals the mass number minus the atomic number, and (for a neutral atom, not an ion) the number of electrons equals the atomic number too.

Atoms of the same element can have different numbers of neutrons — these are called isotopes. Isotopes have the same atomic number but different mass numbers, so they have identical chemical properties (since chemistry depends on electrons) but slightly different masses. Chlorine, for example, exists as ³⁵Cl and ³⁷Cl.

Relative atomic mass

Because most elements are a mixture of isotopes in different proportions, chemists use relative atomic mass (A_r) — the average mass of the atoms of an element, weighted by how much of each isotope is present, on a scale where an atom of ¹²C is defined as exactly 12. This is why chlorine's A_r is 35.5 rather than a whole number.

Electron arrangement and ions

Electrons fill energy levels starting with the one closest to the nucleus: the first shell holds up to 2 electrons, the second and third shells hold up to 8 each. Aluminium (13 electrons) is written 2,8,3. Elements in the same group of the periodic table have the same number of electrons in their outer shell, which is why they react in similar ways.

An ion is a charged particle formed when an atom gains or loses electrons. Losing electrons leaves more protons than electrons, giving a positive ion (a metal atom losing electrons, e.g. Na → Na⁺); gaining electrons gives a negative ion (a non-metal atom gaining electrons, e.g. Cl + e⁻ → Cl⁻). Atoms react by losing, gaining or sharing electrons so as to end up with the same stable electron arrangement as the nearest noble gas.

The periodic table

Elements are arranged in the periodic table in order of increasing atomic number. Vertical columns are called groups and share the same number of outer-shell electrons; horizontal rows are called periods. Metals sit on the left and in the middle (roughly three-quarters of all elements); non-metals sit on the right.

Property	Metals	Non-metals
Conductivity	Conduct heat and electricity	Poor conductors (except graphite)
Appearance	Shiny, malleable	Dull, brittle as solids
Density / melting point	Usually high	Usually low
Ion formed	Positive ions	Negative ions
Oxides	Basic	Acidic

Group 0 (the noble gases — helium, neon, argon) have full outer shells, so they are extremely unreactive and exist as single atoms. Group 1 (the alkali metals — lithium, sodium, potassium) each have one outer electron, which they lose easily to form 1+ ions; reactivity increases going down the group because the outer electron is further from the nucleus and easier to remove. Group 7 (the halogens — fluorine, chlorine, bromine, iodine) each have seven outer electrons and exist as diatomic molecules (Cl₂, Br₂…); they gain one electron to form 1− ions, so reactivity here decreases going down the group, since the incoming electron has further to travel to reach the nucleus. A more reactive halogen can displace a less reactive one from a solution of its salt — chlorine will turn colourless potassium bromide solution orange as it displaces bromine.

The block of elements between Groups 2 and 3 are the transition metals (iron, copper, zinc…). Compared with the alkali metals they are much less reactive, much harder and denser, have much higher melting points, form ions of more than one charge (Fe²⁺ and Fe³⁺), form coloured compounds, and are often useful catalysts.

Mixtures and separation

A mixture contains two or more substances that are not chemically joined, so each substance keeps its own properties and can be separated by physical methods. This is different from a compound, where atoms of different elements are chemically bonded and can only be separated back into elements by a chemical reaction.

Mixture type	Method
Insoluble solid + liquid	Filtration
Dissolved solid (keep solid)	Evaporation or crystallisation
Dissolved solid (keep solvent)	Simple distillation
Miscible liquids (different boiling points)	Fractional distillation
Immiscible liquids	Separating funnel
Dissolved coloured substances	Chromatography