AS-Level Atomic Structure

  • Atoms are the smallest particles of an element.

  • Atoms are made of sub-atomic particles:

    • ​protons (charge +1, relative mass 1)

    • neutrons (no charge, relative mass 1)

    • electrons (charge -1, relative mass 1/1840)

  • The number of protons an atom has is called its ‘atomic number’.

  • The centre of an atom is called a nucleus and contains just protons and neutrons.

  • Electrons are attracted to the positive charge of a nucleus and exist in orbitals.

  • Atoms can lose or gain electrons to become charged ions.


The Structure of an Atom


All substances are made up of atoms – tiny building blocks of our material universe. Atoms are made of three different particles. As these particles are smaller than an atom, we call them sub-atomic particles. They are called protons, neutrons and electrons. Each have different properties and different combinations and amounts of them all can create different atoms.

Protons and neutrons exist in the centre of an atom. The space they occupy is called an atom’s nucleus. Protons have a positive (+1) charge and neutrons have no charge, giving the nucleus an overall positive charge. The amount of positive charge a nucleus has is an atom’s nuclear charge. Of the sub-atomic particles, only protons and neutrons have a mass, this means we can say (virtually!) all the mass of an atom is held very tightly in the nucleus.

Electrons are are negatively charged and are attracted to the positive charge of a nucleus. An attraction between a positive and a negative charge is called electrostatic attraction. The attracted electrons travel around the nucleus of an atom in orbitals. It is the number of electrons around an atom’s nucleus and how those electrons are arranged that determines how an atom is likely to react.


The total space occupied by electrons and the nucleus makes up the overall size of a particular atom. Atomic radius is just a way of describing how big an atom is, by measuring the distance from the nucleus to the electron that is furthest away. As electrons fill orbitals successively (one after the other), the more electrons in an atom, the larger its atomic radius will be.

Orbitals are simply regions of space that electrons are ‘allowed’ to occupy around the nucleus. Orbitals can be different shapes and have different energies.  How they are filled by electrons can help us predict how an atom is likely to react.

Atomic Number

An element is a particular atom that has a specific number of protons. The number of protons an atom has can be used to identify the element and is called the atomic number.  Every atom of the same element has the same number of protons. If you have two atoms each with the same number of protons, they are the same element. If you have two atoms each with a different number of protons, they are different elements.  
















All atoms of an element in the periodic table are neutral. This means they have no overall charge. Remember, protons and electrons have charges (proton = +1, electron = -1). If an atom is neutral, it must have the same number of protons as electrons.


An atom may lose or gain electrons during reactions. An atom is neutral if its number of protons and electrons are the same. Therefore, if the number of electrons in an atom changes the atom becomes charged. A charged atom is called an ion.