Atomic number and mass number | Chemistry | Fuse School

How do we tell elements apart from each other? Find out in this video from the Properties of Matter chapter of the Virtual School GCSE Chemistry.

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When you look at the periodic table you will
see that each element has its own box, and


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within that box, you will find two numbers.
The atomic number, or proton number, and the


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mass number, but what do these numbers mean?
The atomic number has the symbol 'z',


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this number tells you how many protons are
in one atom of an element. The number is always


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the same for all atoms of a particular element.
Atoms of different elements have different


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atomic numbers, meaning they have different
numbers of protons.


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For example, an atom of Hydrogen has an atomic
number of 1 because it has 1 proton, but an


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atom of Oxygen has and atomic number of 8
because it has 8 protons.


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The next number we look at is the mass number.
The mass number has the symbol A.


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The mass number tells you how many protons
and neutrons are in one atom of an element.


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We need to remember that Protons and Neutrons
each have a relative mass of 1 and that Electrons


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are so small, that their mass does not need
to be considered in the mass number of an atom.


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So if we know the mass number of an element,
and we know the atomic number, we can calculate


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the number of Neutrons in an atom of a particular
element.


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So the Mass Number = The Atomic Number + the
Number of Neutrons. The Atomic Number is just the number of Protons and Atoms, therefore,


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The Mass Number = protons + neutrons.
So if we take oxygen,


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Oxygen has a mass number of 16 and 8 Protons, how many neutrons does it have?


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Remember, Mass Number = protons + neutrons
We can rearrange this to show that


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Neutrons = Mass Number - Atomic Number.
Neutrons = 16 - 8


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= 8
Oxygen therefore has 8 neutrons.


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Let take another example,


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Lithium has a mass number of 7 and an atomic
mass of 3, how many neutrons does it have?


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Mass Number = protons + neutrons
We can rearrange this to show that


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Neutrons = Mass Number - Atomic Number
Neutrons = 7- 3


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= 4
Lithium therefore has 4 neutrons.


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So the atomic number is the number of protons
in an atom and the mass number is the number


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of protons and neutrons in an atom.

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