# 4.1.3 Atomic & Mass Number

### Atomic & Mass Number

#### Atomic Number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)
• Elements in the periodic table are ordered by their atomic number
• Therefore, the number of protons determines which element an atom is
• The atomic number of a particular element is always the same
• For example:
• Hydrogen has an atomic number of 1. It always has just one proton
• Sodium has an atomic number of 11. It has 11 protons
• Uranium has an atomic number of 92. It has 92 protons
• The atomic number is also equal to the number of electrons in an atom
• This is because atoms have the same number of electrons and protons in order to have no overall charge

#### Mass Number

• The total number of particles in the nucleus of an atom is called its mass number
• The mass number is the number of protons and neutrons in the atom
• The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutron = Mass Number – Atomic Number

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

#### Exam Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

### Nuclear Notation

• The mass number and atomic number of an atom are shown by writing them with the atomic symbol
• This is called nuclear notation
• Here are three examples: Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

• The top number is the mass number
• This is equal to the total number of particles (protons and neutrons) in the nucleus
• The lower number is the atomic number
• This is equal to the total number of protons in the nucleus
• The atomic and mass number of each type of atom in the examples above is shown in this table: #### Worked Example

The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?  Step 1: Determine the atomic and mass number

• The gold atom has an atomic number of 79 (lower number) and a mass number of 197 (top number)

Step 2: Determine the number of protons

• The atomic number is equal to the number of protons
• The atom has 79 protons

Step 3: Calculate the number of neutrons

• The mass number is equal to the number of protons and neutrons
• The number of neutrons is equal to the mass number minus the atomic number

197 – 79 = 118

• The atom has 118 neutrons

Step 4: Determine the number of electrons

• An atom has the same number of protons and electrons
• The atom has 79 electrons ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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