Using atomic symbols

Mass number and atomic number are two important pieces of information about an atom.

An atom can be represented using the symbol notation:



  • A is the mass number
  • Z is the atomic number
  • X is the symbol of the element

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

Chlorine atom with mass number 35 and atomic number 17.

This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.

Atoms and isotopes

An element's atomic number defines it. An element with 17 protons will always be chlorine.

However an element's mass number can vary, which means that it can have different numbers of neutrons. So chlorine has a mass number of 35, which means it has 18 neutrons, but it can also have a mass number of 37, which means it has 20 neutrons. The different types of chlorine are called isotopes.

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{N} are not isotopes because they are not the same element. They have the same mass number but if the number of protons is different, they are different elements.


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.