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Atoms consist of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] surrounding a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] that contains protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ].
Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1, while electrons have a relative charge of -1.
The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged in atomic number order.
Electrons are arranged in energy levels or shells, and different energy levels can hold different numbers of electrons. The electronic structure of an atom is a description of how the electrons are arranged, which can be shown in a diagram or by numbers. There is a link between the position of an element in the periodic table and its electronic structure.
The structure of an atom
Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Atoms are made from smaller subatomic particles.
Structure of an atom
At the centre of an atom is a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] containing protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ]. Electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are arranged around the nucleus in energy levels or shells. Make sure you can label a simple diagram of an atom like this one.
Both protons and electrons have an electrical charge. Both have the same size of electrical charge, but the proton is positive and the electron negative. The neutron is neutral.
The electrical charge of particles
| particle | relative charge |
|---|---|
| proton | +1 |
| neutron | 0 |
| electron | -1 |
The total number of electrons in an atom is always the same as the number of protons in the nucleus. This means atoms have no overall electrical charge.
The number of protons in an atom is called its atomic number [atomic number: The number of protons in the nucleus of an atom. Also called the proton number. ] - also called the proton number. Atoms are arranged in the periodic table in order of increasing atomic number. You may need to re-visit the section in AQA GCSE Science on the periodic table to check you recall it.
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