As a result of the work done by previous scientists on atomic models (that we discussed in "Models of the Atom"), scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom.
So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by one or more negatively charged electrons. These electrons orbit the nucleus.
Before we look at some useful concepts we first need to understand what electrons, protons and neutrons are.
The Electron
The electron is a very light particle. It has a mass of
The electrons determine the charge on an atom. If the number of electrons is the same as the number of protons then the atom will be neutral. If the number of electrons is greater than the number of protons then the atom will be negatively charged. If the number of electrons is less than the number of protons then the atom will be positively charged. Atoms that are not neutral are called ions. Ions will be covered in more detail in a later chapter. For now all you need to know is that for each electron you remove from an atom you loose
The Nucleus
Unlike the electron, the nucleus can be broken up into smaller building blocks called protons and neutrons. Together, the protons and neutrons are called nucleons.
The Proton
Each proton carries one unit of positive electric charge.
Since we know that atoms are
electrically neutral, i.e. do not carry any extra charge, then the number
of protons in an atom has to be the same as the number of electrons to balance
out the positive and negative charge to zero. The total positive charge of a
nucleus is equal to the number of protons in the nucleus. The proton is much heavier
than the electron (10 000 times heavier!) and has a mass of
The Neutron
The neutron is electrically neutral i.e. it carries no charge at all.
Like the proton, it is much heavier than the electron and its mass is
Note: Interesting Fact :
Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! Also, if protons were the only particles in the nucleus, then a helium nucleus (atomic number 2) would have two protons and therefore only twice the mass of hydrogen. However, it is actually four times heavier than hydrogen. This suggested that there must be something else inside the nucleus as well as the protons. To make sure that the atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass.
| proton | neutron | electron | |
| Mass (kg) | |||
| Units of charge | |||
| Charge (C) |
