The neutron number refers to the total number of neutrons in the nucleus. If the number of neutrons is larger than the number of protons, the atom has a neutron excess.
Reading the neutron number
To identify the neutron number, you need to read the element’s nuclide notation. The neutron number is located in the lower right corner of the nuclide notation, as in this example of Iron 56:
\[^{56}_{26}Fe_{30}\]
However, the neutron number is not always specified and thus may need to be calculated.
Figure 1. Tritium, which is an isotope of hydrogen. Its nuclide notation indicates that there are three particles in the nucleus: one proton and two neutrons. Source: Manuel R. Camacho, StudySmarter.
Calculating the neutron number
To calculate the neutron number, use the nuclide notation as shown below:
\[\text{Neutron number} = (\text{Mass number - Atomic number})\]
In the case of Iron 56, this looks as follows
\(\text{Neutron number} = (56 - 26) = 30\)
Neutron excess
The nuclide notation defines the number of particles in the nucleus in the upper-left corner, while the number of protons is given in the lower-left corner, as in this example of Iron 56:
\[^{56}_{26}Fe_{30}\]
Iron has 26 protons, as indicated by the atomic number in the lower-left corner, and a total of 56 particles in the nucleus, as shown in the upper-left corner. You might expect the number of protons and other particles in the nucleus to be equal, but that is not the case. Multiplying 26 by two would give you 52, whereas the atomic number 56 indicates that there are four additional particles in the atom’s nucleus. There is, therefore, a neutron excess of four.
Neutrons and isotopes
An atom with either a lack or an excess of neutrons is called an isotope. Some isotopes break up after some time, which produces radiation in the form of particles such as photons, beta particles and others. There can be many isotopes for a single atomic element.
Figure 2. Hydrogen isotopes with different numbers of neutrons in the nucleus. From left to right: no neutron, one neutron, and two neutrons. Source Manuel R. Camacho, StudySmarter.
What is a neutron?
A neutron is a particle located in the centre of an atom alongside protons. Both neutrons and protons are also known as nucleons. Neutrons have a neutral charge and are one of two types of particles that contribute a greater amount of mass to an atom. The characteristics of neutrons are shown below:
| Particle | Mass in kilograms | Electrical charge in coulombs | Location |
| neutron | \(1.67 \cdot 10 ^{-27}\) | 0 | Nucleus |
Neutrons and the atom’s mass
Neutrons are one of two types of particles that make up most of an atom’s mass. The more neutrons an atom has, the heavier it is. Neutrons have a mass that is 1836.15 times larger than that of an electron and are just a bit heavier than protons.
Neutrons and the atom’s charge
Neutrons have a neutral electrical charge. Because of this, when an atom has a neutron excess, it does not change its electrical nature.
Neutrons and the atom’s relative charge
When thinking about atomic charges, it helps to work with the concept of relative charge. The relative electrical charge of a neutron is 0. A neutron’s relative charge is indicated as below:
\[0^n\]
Neutron Numbers - Key takeaways
- The neutron number indicates how many neutrons there are in total in an element's atom.
- The difference between the neutron number and the atomic number indicates the neutron excess.
- Neutrons are one of two particles that add a significant amount of mass to an atom.
- Neutrons are almost 1836.15 times heavier than electrons.
- Neutrons and protons have similar masses.
- Neutrons have a neutral charge and only add mass to an atom.
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