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Atoms
This is a diagram of a CD. The diameter of its hole is one-eighth of the diameter of the
CD.
Diagrams of atoms look very similar.
But the nucleus at the centre of an atom is very much smaller.
The diameter of the nucleus is around 100 000 times less than the diameter of the atom.
This is equivalent to a CD with a normal-sized hole and an overall diameter of 1.5 km.
Each atom has a nucleus at its centre, containing sub-atomic particles called protons
and neutrons.
Sub-atomic particles called electrons are arranged around the nucleus.
What are the properties of these sub-atomic particles?
Their masses are incredibly small.
The proton and electron have incredibly small electric charges and the neutron has no
charge.
To make sense of the tiny masses, we use relative masses. The proton’s relative mass
is 1.
The electron is nearly 2000 times lighter than the proton and neutron.
To make sense of the tiny charges, we use relative charges. The proton’s relative
charge is +1.
Protons and electrons have equal but opposite charges.
The nuclear symbol for lithium is 73Li.
The bottom number is the atomic number or proton number, Z.
The top number is the mass number, A.
These numbers mean that this atom has 3 protons, and ...
… the number of protons added to the number of neutrons is 7.
So there are 7 − 3 = 4 neutrons.
3
4
3
There are an equal number of protons and electrons in an atom, so there are 3
electrons.
The three protons are found in the nucleus.
The four neutrons are also found in the nucleus.
The three electrons are arranged around the nucleus.
35
Cl
17
is an isotope of chlorine. It is often called chlorine-35.
Its mass number is 35.
Its atomic number is 17.
So there are 35 − 17 = 18 neutrons.
37
Cl
17
is another isotope of chlorine. It is often called chlorine-37.
Its mass number is 37.
Its atomic number is 17.
So there are 37 − 17 = 20 neutrons.
Both isotopes have 17 protons and 17 electrons but different numbers of neutrons.
Isotopes of an element are chemically identical but have different physical properties.