Atomic Structure (Protons, Neutrons, Electrons)

Subatomic Particles

Atoms are made up of three main types of subatomic particles:

• Protons: These have a positive charge (+1) and are found in the nucleus at the centre of the atom.
• Neutrons: These have no charge (neutral) and are also located in the nucleus alongside protons.
• Electrons: These have a negative charge (−1) and orbit the nucleus in shells or energy levels.

The charges of these particles are fundamental to how atoms behave and interact with each other.

Atomic Structure Basics

The nucleus is the dense centre of the atom, containing protons and neutrons. It carries almost all the atoms mass but occupies a very small volume.

Electrons move around the nucleus in fixed paths called shells. These shells are arranged at different distances from the nucleus.

Because protons are positively charged and electrons are negatively charged, their charges balance out in a neutral atom. This means the number of protons equals the number of electrons, so the atom has no overall charge.

For example, a carbon atom has 6 protons in its nucleus and 6 electrons orbiting it, making it electrically neutral.

The neutrons in the nucleus add mass but do not affect the charge. They help stabilise the nucleus by reducing repulsion between the positively charged protons.

For more on how electrons are arranged in shells, see the topic Electronic Structure & Electron Configuration.

Atomic Number and Mass Number

Atomic number is the number of protons in an atoms nucleus. It defines which element the atom is.

For example, all atoms with 1 proton are hydrogen atoms; all atoms with 6 protons are carbon atoms.

Mass number is the total number of protons and neutrons in the nucleus.

Since protons and neutrons have approximately the same mass (about 1 atomic mass unit each), the mass number gives the approximate mass of the atom.

The number of neutrons can be found by subtracting the atomic number from the mass number:

Number of neutrons = $\text{Mass number} - \text{Atomic number}$

For instance, if an atom has an atomic number of 11 and a mass number of 23, it has:

• 11 protons
• 23 − 11 = 12 neutrons

The atomic number is unique to each element and determines its chemical properties.

The mass number varies slightly between atoms of the same element due to differing numbers of neutrons (isotopes), but isotopes are covered in a separate topic.

Example: Identifying Protons and Neutrons

An atom has 15 protons and a mass number of 31. How many neutrons does it have?

Number of neutrons = Mass number − Atomic number = 31 − 15 = 16 neutrons.