Atomic Structure


Sub-atomic Particles

In the centre of every atom is a nucleus.

Inside the nucleus, there are protons  and neutrons.

Orbiting the nucleus are electrons.

Protons are POSITIVE (relative charge = +1)

Neutrons have NO CHARGE (relative charge = 0)

Electrons are NEGATIVE (relative charge = -1)

Every atom contains equal numbers of protons and electrons, and their charges cancel each other out, so that atoms have no overall charge.

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Atomic Number & Periodic Table

All atoms in the same element have the same number of protons (and therefore electrons).

The number of protons in an element is called the atomic number for that element.

In the periodic table, the elements are arranged in order of atomic number:

Periodic Table (

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Mass Number

An element's mass number is the number of protons plus the number of neutrons.

If we have the mass number and the atomic number of an element, we can find the number of protons, neutrons and electrons that element has:

Argon has an atomic number of 18 and a mass number of 40.

The atomic number is 18 so it has 18 protons.

Atoms have the same number of electrons as protons so it also has 18 electrons.

The mass number (protons + neutrons) is 40. We know there are 18 protons, so the number of neutrons is 40 - 18, which is 22. There are 22 neutrons.

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