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Atomic & Mass Number (AQA GCSE Physics)
Revision Note
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
Examiner 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.
In my experience, students often think that they are learning something new with atomic and mass numbers in GCSE Physics, when in fact these are just the numbers that can be found on the periodic table for every element. You have probably used the periodic table in your Key Stage 3 Chemistry lessons, and if you take GCSE Chemistry or Combined Science. You are already familiar with these numbers, just not necessarily using them in Physics! You will not be given a periodic table in your Physics exam, but don't worry, you will be given the atomic numbers and mass numbers you need to perform any calculations.
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?
ANSWER: D
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
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