Science 1.4
Describe aspects of chemistry
Atomic Number & Mass Number
eBook
Science 1.4 Atomic Number & Mass Number
Atomic Number & Mass Number
In this audio we’re going to talk about the atomic number and mass number of atoms.
There are many different kinds of atoms. What makes atoms different is the number of protons they
have in their nucleus. The number of protons an atom has is its atomic number. There are over 100
different atoms with atomic numbers from 1 right up to 100 and over! You might remember that
hydrogen is the atom with the atomic number of 1. Every atom has a different chemical name, and is
represented by a letter symbol. An element is a substance made up purely of just one kind of atom.
Let’s do an example… We’ll take a carbon atom. The carbon atom is represented by the letter ‘C’. The
element carbon is made up of only carbon atoms. Because atoms are so incredibly small a piece of
elemental carbon like a lump of coal is made up of billions and billions of carbon atoms. We said before
that atoms differ by the number of protons they have. Carbon has 6 protons in its nucleus, meaning it
has an atomic number of 6. Easy! Another number you need to know about is called the mass number;
this is also different for every atom. An atoms mass number is equal to the number of protons and
neutrons the atom has. The mass number for carbon is 12, because every carbon atom has 6 protons
and 6 neutrons in the nucleus. Not every atom like carbon happens to have an equal number of
protons and neutrons. For example aluminium has an atomic number of 13, meaning it has 13 protons,
but it has a mass number of 27, meaning it has 14 neutrons (because 27 minus 13 is 14)!
neutron
Mass number
12
Atomic number
6
C
proton
nucleus
electron
Before we finish, let’s do another example. Sodium is an element you’ll end up being really familiar
with. The element sodium is made up entirely of sodium atoms. The letter symbol for sodium is Na.
Weird that it’s not S isn’t it? - Like how ‘C’ is the symbol for carbon, but lots of the symbols for
elements come from the Latin language. But anyway, sodium has an atomic number of 11, which
means what? That’s right - every sodium atom has 11 protons in its nucleus. The mass number of
sodium is 23. You remember that mass number is equal to the number of protons plus the number of
neutrons. Well we already know that sodium has 11 protons, therefore, every sodium atom must have
12 neutrons, because 23-11, is equal to 12. It’s that simple!
© Sound Classroom 2009
2