# 3.1 Structure

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## Structure of the atom

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 $9,11×{10}^{-31}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ . Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it can't be broken down into anything smaller. The electron also carries one unit of negative electric charge which is the same as $1,6×{10}^{-19}\phantom{\rule{2pt}{0ex}}\mathrm{C}$ (Coulombs).

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 $-1$ of charge and for each electron that you add to an atom you gain $+1$ of charge. For example, the charge on an atom of sodium after removing one electron is $-1$ .

## 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 balanceout 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 heavierthan the electron (10 000 times heavier!) and has a mass of $1,6726×{10}^{-27}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ . When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons.

## 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 $1,6749×{10}^{-27}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ (slightly heavier than the proton).

## Interesting fact

Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this becauseif 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 havetwo 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.

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Properties of longitudinal waves