Combined Science - C1: Atomic Structure.

Structure of the Atom

   All matter is made up of atoms. Each atom has a nucleus (central part) composed of neutrons and protons, collectively called nucleons, which are subatomic particles with neutral and positive charges respectively. Each proton and electron has a relative mass of 1.

Electrons, discovered in 1898 by J.J. Thomson, are subatomic particles with negative charges which move around the atomic nucleus in rings known as shells. Each electron has a negligible relative mass.

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Atomic Number and Mass Number

Atomic Number: All atoms in an element have the same number of protons (and thus electrons). The number of protons in an atom is called the proton number, or the atomic number.

> Mass Number: The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

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Atomic Number and Mass Number

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Isotopes

Whilst two atoms of the same element will always have the same atomic number because they will always have the same number of neutrons, the number of neutrons between two atoms of the same element may vary (and thus so do their mass numbers). Atoms of the same element which have different numbers of neutrons are called isotopes.

Isotopes of the same element have the same chemical properties because they have the same number of electrons.

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Relative Atomic Mass

Relative Atomic Mass: The ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12 

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