3.1.1 Atomic Structure - Fundamental particles


Students should:
  • be able to describe the properties of protons, neutrons and electrons in terms of relative charge and relative mass
  • know that early models of atomic structure predicted that atoms and ions with noble gas electron arrangements should be stable


Atoms are the fundamental building blocks of matter. However, even atoms are constructed of smaller, sub-atomic particles.

The fundamental sub-atomic particles are protons, neutrons and electrons. The protons and neutrons are held together by strong nuclear binding forces in the nucleus of the atom. The electrons may be considered to be tiny particles that exist in regions of space known as orbitals around the atom.

This model of the atom is precisely that, a model. It is impossible to see atoms and, in order to be able to describe their properties, we use models representing this microscopic world that is invisible to us.

Similarly, the sub-atomic world is a strange place with unusual forces acting over infintesimally small distances. The rules of behaviour that govern the macroscopic world often break down in this strange environment, and it is important to understand that our representations and models are necessarily limited here.


Particle mass

The actual masses of these sub-atomic particles are very small and to the nearest whole number measured relative to the mass of a carbon-12 isotope being equal to 12 units:

Particle location mass/amu
Proton nucleus 1
Neutron nucleus 1
Electron energy shells 0.005

Although these values suggest that the protons and neutrons are identical they do, in fact, have very slightly different masses, which is only of concern to us when considering changes in the structure in nuclear chemistry. Compared to the mass of the protons and neutrons the electrons have negligible mass and can be ignored when carrying out calculations involving mass.

Protons have a mass of 1 atomic mass unit. They are all together in the nucleus, but they cannot repel one another because of the strong nuclear force exerted by the protons and the neutrons. You could consider the neutrons to be the nuclear glue that holds the nucleus together.

This model allows us to use the atomic theory successfully to explain many observations in the microscopic world. Its use in modern particular science has been refined by the introduction of another model, which is rather more difficult to understand, called the quantum model.


Electrical charge

Overall, atoms are neutral, which means that they must have as many positive charges as negative charges.

Protons carry a single positive charge and the electrons carry a single negative charge, so in the neutral atom there are always the same number of protons and electrons.

The electrons are tiny in comparison to the protons and neutrons. The overall charge on an atom is zero, the charges of the electrons cancel out the positive charges of the protons in the nucleus.

Summary of fundamental particle charge and location

Particle location charge
Proton nucleus 1+ (positive)
Neutron nucleus none
Electron energy shells 1- (negative)

Sub-atomic particles


Mass number

The atomic mass number is represented by the symbol (letter) 'A'. This is not to be confused with the relative atomic mass Ar.

The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.

The relative mass is a value that is not necessarily integral that compares a mass to the mass of one atom of a carbon 12 isotope, assigned a value of exactly 12.0000 units.


Atomic number

This is represented by the symbol (letter) 'Z'. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.)

Example: How many protons and electrons does an atom of iron contain?

The atomic number of iron is 26 therefore it contains 26 protons

The number of electrons = number of protons therefore there are 26 electrons


Electronic arrangement (SL)

The electrons are arranged according to energy levels. The lowest energy (most stable) energy level is the one closest to the nucleus. The first energy level can hold up to two electrons. Once it is full, the next energy level may then start to fill up.

When an energy shell is filled up the configuration is particularly stable.

Example: Sodium has an electronic configuration of 2,8,1.

Notice that this is written from the inner shell (2) working towards the outer shell.

See all configurations

Ground state and excited electrons

Electrons can absorb energy and move into higher energy levels. When this happens the atom is said to be 'excited'. An atom in which the electrons are in the lowest possible energy levels is said to be in the 'ground state'.


Electronic arrangement of ions

Ions are formed when atoms either lose or gain electrons to attain a full outer shell. Non-metals gain electrons and turn into negative ions - anions. Metal atoms lose electrons and become positive ions - cations.


quick test