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Inside the Nucleus

The nucleus is so dense that almost all of an atom's mass (over 99.9%) is concentrated in a tiny central region with a radius approximately 10,000 times smaller than the radius of the atom itself.
The nucleus is built from two types of subatomic particles, collectively called nucleons.
These nucleons are positively charged protons and neutral neutrons, both of which have a relative mass of 1 unit (u).
Electrons orbit outside the nucleus and have a negligible mass (roughly 1/2000 the mass of a nucleon), so they do not significantly contribute to the atom's total mass.
The number of protons is the defining feature of an element; if the proton count changes, the atom becomes an entirely different element.

Atomic and Mass Numbers

Scientists use specific numbers to describe exactly what is inside a nucleus.

The Atomic (Proton) Number ( $Z$ ) represents the exact number of protons in the nucleus and determines the element's position on the Periodic Table.
The Mass (Nucleon) Number ( $A$ ) is the total combined number of protons and neutrons in the nucleus.

Feature	Carbon-12 ( $^{12}_{6}\text{C}$ )	Carbon-14 ( $^{14}_{6}\text{C}$ )
Atomic Number ( $Z$ )	6	6
Mass Number ( $A$ )	12	14
Proton Count	6	6
Neutron Count ( $A-Z$ )	6	8

Students often describe isotopes simply as 'atoms with a different mass number', but fail to explicitly state they are atoms of the SAME element.

For a 2-mark definition of isotopes, examiners expect the 'Gold Standard' answer: you must state they have the 'same number of protons' AND a 'different number of neutrons'.

On exam papers, the mass (nucleon) number is always the larger of the two numbers next to the chemical symbol, even if the printing layout looks slightly different to standard notation.

Edexcel exams use 'mass number' interchangeably with 'nucleon number', and 'atomic number' interchangeably with 'proton number', so be prepared to recognise both pairs of terms.

A collective term for any subatomic particle found inside the nucleus, meaning either a proton or a neutron.

A subatomic particle found in the nucleus of an atom, possessing a positive charge and a relative mass of 1.

A subatomic particle found in the nucleus of an atom, possessing no charge (neutral) and a relative mass of 1.

A subatomic particle with a negative charge and negligible mass, which orbits the nucleus of an atom in shells.

The number of protons in the nucleus of an atom, which determines the element's identity.

The total number of protons and neutrons in the nucleus of an atom.

Atoms of the same element that have the same number of protons but different numbers of neutrons.

An atom that has gained or lost electrons, resulting in an overall electrical charge, without changing its nuclear structure.

The process by which an unstable nucleus releases energy and/or particles to become more stable.

A system of writing the chemical symbol with the mass number as a superscript and the atomic number as a subscript, both to the left of the symbol.

The mass of a subatomic particle compared to the mass of a nucleon (defined as 1 unit).

Characteristics of a substance that relate to its chemical reactions, determined by electron configuration.

Characteristics such as mass, density, and boiling point that do not involve changing the substance's chemical identity.