The structure of the nucleus

Mass number and atomic number are two important pieces of information about a nucleus.

A nucleus can be represented using the symbol notation:



  • A is the mass number
  • Z is the atomic number
  • X is the chemical symbol of the element
  • A - Z = number of neutrons N

For example, chlorine (Cl) can be shown as:

Chlorine atom with mass number 35 and atomic number 17.

Cl is the chemical symbol for chlorine

A chlorine nucleus will contain:

  • Number of protons Z = 17
  • Number of protons + neutrons A = 35
  • Number of neutrons N = 35 - 17 = 18

The chlorine atom will also have 17 electrons, as an atom is neutral.

Atoms and isotopes

An element's atomic number defines it.

An element with 17 protons will always be chlorine.

However an element's mass numbers can vary, which means that it can have different numbers of neutrons.

So although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons.

The different types of chlorine are called isotopes of chlorine.

Isotopes are forms of an element that have the same number of protons but different numbers of neutrons.

There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium (hydrogen-3):

Table showing the three isoptopes of hydrogen, the first with 0 neutrons, the second with 1 neutron and the third with 2 neutrons.

Carbon has three isotopes: _{6}^{12}\textrm{C}, _{6}^{13}\textrm{C} and _{6}^{14}\textrm{C}. They all contain six protons, but six, seven and eight neutrons respectively.

_{7}^{14}\textrm{N} and _{6}^{14}\textrm{C} are not isotopes.

They have the same mass number, A, but different atomic number, Z, and different chemical symbol - they are not the same element.

If the number of protons changes, then it is a different element.


How many protons does _{6}^{14}\textrm{C} contain?

The atomic number is 6 so _{6}^{14}\textrm{C} contains six protons.


How many neutrons does _{6}^{14}\textrm{C} contain?

Number of neutrons = mass number - atomic number = 14 - 6 = 8 neutrons.


Write down the number of particles in the following nuclei:





  • Number of protons Z = 30
  • Number of protons + neutrons A = 64
  • Number of neutrons N = 64 - 30 = 34
  • Since this is a nucleus there are no electrons

There are 30 protons and 34 neutrons.


  • Number of protons Z = 92
  • Number of protons + neutrons A = 238
  • Number of neutrons N = 238 - 92 = 146
  • Since this is a nucleus there are no electrons

There are 92 protons and 146 neutrons.


  • Number of protons Z = 1
  • Number of protons + neutrons A = 3
  • Number of neutrons N = 3 - 1 = 2
  • Since this is a nucleus there are no electrons

There is 1 proton and 2 neutrons.