Atomic structure and isotopesEdexcel International A Level Chemistry: Revision notes

Every atom has a tiny, dense nucleus containing protons and neutrons (together called nucleons). Electrons move around the nucleus in shells (energy levels). Almost all of the atom's mass is in the nucleus, but almost all of its volume is the space occupied by the electrons.

A neutral atom has equal numbers of protons and electrons, so the positive and negative charges cancel.

Masses and charges of subatomic particles are compared with a proton:

• Proton: relative mass 1, relative charge +1
• Neutron: relative mass 1, relative charge 0
• Electron: relative mass 1/1836 (about 0.0005), relative charge −1

Because an electron's mass is so small, the mass of an atom is effectively the mass of its protons and neutrons.

The atomic number (proton number), Z, is the number of protons in the nucleus. It defines the element: every atom of carbon has 6 protons.

The mass number, A, is the total number of protons and neutrons in the nucleus.

number of neutrons = A − Z

The symbol is written with the mass number as a superscript and the atomic number as a subscript, for example ²³₁₁Na: 11 protons, 12 neutrons and, in the neutral atom, 11 electrons.

In an ion the number of electrons differs from the number of protons. A positive ion (cation) has lost electrons; a negative ion (anion) has gained them. The number of protons and neutrons does not change.

Worked example: ³²S²⁻ (Z = 16)

• protons = 16
• neutrons = 32 − 16 = 16
• electrons = 16 + 2 = 18 (it has gained two electrons)

Worked example: ⁵⁶Fe³⁺ (Z = 26)

• protons = 26, neutrons = 56 − 26 = 30
• electrons = 26 − 3 = 23 (it has lost three electrons)

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

For example, chlorine has ³⁵Cl (18 neutrons) and ³⁷Cl (20 neutrons); both have 17 protons and 17 electrons.

Each isotope is named by its mass number: carbon-12, carbon-13 and carbon-14.

Chemical properties depend on the electronic configuration, particularly the outer electrons. Isotopes of an element have the same number of electrons, so they have the same chemical properties and undergo the same reactions.

Physical properties that depend on mass (density, rate of diffusion, boiling temperature) differ slightly between isotopes, because the extra neutrons make the atoms heavier. For example, heavy water, ²H₂O, is denser than ordinary water.