• To understand a simple nuclear model of the atom; consisting of protons, neutrons and electrons
  • To have an appreciation of the relative sizes of the atom and nucleus
  • To identify the terms proton number, nucleon number and isotopes
  • To understand and be able to extract information from nuclear notation; ^{A}_{Z}X for the representation of nuclei

Currently the most widely accepted model of the atom is the Bohr (Rutherford-Bohr) Model. This consists of neutron and protons in the nucleus with electrons orbiting in discrete energy levels around the nucleus. Although neutrons were not mentioned on the previous page, they had been sought after since the time of Rutherford; it had been known that the atomic mass number A of a nuclei is a bit more than twice the atomic number Z for most atoms and that the majority of all the mass of the atom is concentrated in the tiny nucleus.

Bohr atomic model of a nitrogen atom.
Encyclopedia Britannica, Inc

Atomic Notation

Physicists and Chemists needed a general approach to displaying the contents of an element, since atoms are neutrally charged but contain both positively and negatively charged particles, the number of electrons must equal the number of protons. Displaying a charge of zero is not helpful, as all elements would show the same. So the approach used was to display what was in the nucleus, this way the charge was accounted for and so was the majority of the mass.

Atomic notation is displayed in the following way;


where X is the element symbol
A is the Atomic mass, frequently reffered to as the nucleon number.
Z is the Atomic number, frequently reffered to as the proton number.

The nucleon number combines the total mass of all the protons and neutrons in the nucleus of an element, as such include the atomic number within it. The difference between the atomic mass and the atomic number is the number of nuetrons present in an element. The following equation can then be written for the number of neutrons in an element;

A = Z + N

where N is the number of neutrons.

Any particular element is defined by the atomic number, this means that all atoms with a particular atomic number will be reffered to as a type of that particular element. For example, any element with an atomic number of 2 will be classed as the element Helium, or at least a type of Helium. This is where the term isotope is used.


Isotopes are variants of a certain type of element, in which the atomic number remains the same but the number of neutrons differs (and therefore the atomic mass differs).

The image below shows an example of the possible isotopes for Hydrogen, ^{1}_{1}H , Hydrogen itself has just a proton in its nucleus. If a neutron were to fuse together with a neutron the atomic number would not change but the atomic mass would, the result would therefore be ^{2}_{1} H , this is a variant of the element Hydrogen. Some Isotopes have their own name, in the case of ^{2}_{1}H , it has been name deuterium. Similarly, if an additional neutron fuses with deuterium the result would be ^{3}_{1} H  which is also known as tritium.

Further reading:

  • The discovery of the neutron; link
  • More on Isotopes, Britannica – link