# Particles in the atom

## Particles in the atom

#### Particles in the atom

Atoms contain 3 types of particles: protons, neutrons and electrons.

It is important to understand that the picture above is a model of the atom. It conveys an impression of what the atom is like, but is not a completely true representation.

As an example of this consider the relative sizes of the nucleus and whole atom. It can be found that a typical nuclear diameter is 1 x 10 -14m while the typical atomic diameter is 1 x 10 -10m. Thus the nucleus is around 10,000 times smaller than the entire atom. You could build a model of an atom by placing a pea on the centre spot of a football stadium (to represent the nucleus) and then placing the electrons somewhere out in the stands. The picture above certainly does not reflect this fact accurately! Molecules are simply combinations of 1 or more atoms so are slightly larger than atoms themselves.

Each of these particles has a mass and a charge.

It is possible to simplfy this information by looking for patterns in the numbers.

Firstly, notice that the electron and proton have equal and opposite charges. A new unit of charge called the elementary charge (e = 1.602 x 10 -19 Coulombs) allows us to assign the values of +1e and -1e, to the charge of these particles.

Secondly, as the mass of the proton, neutron and electron are very small a more convenient unit is the atomic mass unit ( u = 1.660 x 10 -27 kg). Using this new unit we can approximate the masses of the proton, neutron and electron to be 1u, 1u and 0u respectively. The relative atomic masses of the three particles can therefore simply be stated as 1,1,0.

Many different atoms can be built using the 3 particles described above. 91 different atoms occur naturally (the chemical elements) and many more can be found in situations where energy levels are high. It is useful to have a concise way of describing these atoms.

To describe the number of particles in a given atom, we use this notation:

The top number (A) is called the nucleon number (as it is the number of things in the nucleus of the atom) or the mass number (as it is the mass of the atom.)

The bottom number (Z) is called the proton number (as it is the number of protons) or the atomic number (as it is the number that tells you which element the atoms belongs to).

The letters give you a clue as to the name of the element.For example here is an atom of helium:.

Check that you understand by dragging and dropping the correct numbers into this table:

#### How do we know about the protons and neutrons in the nucleus?

It is an established scientific fact that the atom has a small, central nucleus containing protons and neutrons. But how did physicists gather evidence to support this view?

The Rutherford scattering experiment proved that the nucleus was small and positive but it took a different experiment to prove the existence of the protons and neutrons within. Very high-energy electrons have enough energy to actually penetrate into the nucleus itself. In much the same way as the Rutherford experiment showed the substructure of atoms by scattering alpha particles, it is found that high-energy electrons scatter to reveal a substructure for the nucleus. This scattering is due to the protons and neutrons within.