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Atomic & Mass Number (Edexcel GCSE Physics: Combined Science)
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 a neutral atom has 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 (it is also called the nucleon number)
- The mass number is the total 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 Neutrons = 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
- The mass number of an element can change, which means they are are isotopes
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:
Protons, Neutrons & Electrons 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
Worked example
State the number of protons, neutrons and electrons in Chlorine-35 and Chlorine-36 atoms.
Step 1: Determine the number of protons
- The atomic number is the number of protons
- Both Chlorine-35 and Chlorine-36 have 17 protons
Step 2: Determine the number of neutrons
- The mass number is the number of protons and neutrons
- Chlorine-35 neutrons: 35 - 17 = 18 neutrons
- Chlorine-36 neutrons: 36 - 17 = 19 neutrons
Step 3: Determine the number of electrons
- The number of electrons is equal to the number of protons
- Both Chlorine-35 and Chlorine-36 have 17 electrons
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.
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