S-Cool Revision Summary

S-Cool Revision Summary

Rutherford alpha particle Scattering experiment

Rutherford's alpha particle scattering experiment changed the way we think of atoms.

Before the experiment the best model of the atom was known as the Thomson or "plum pudding" model. The atom was believed to consist of a positive material "pudding" with negative "plums" distributed throughout.

plum pudding

Particles in the atom

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

Particles in the atom

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.

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

 relative atomic masses

nucleon number: no. of protons + neutrons (Also called the mass number as electrons are relatively light so all the mass is in the nucleus.)

proton number is also called the atomic number as no two elements have the same number of protons.


The number of protons in an atom is crucial. It gives you the charge of the nucleus and therefore it gives you the number of electrons needed for a neutral atom. And the number of electrons governs how an atom behaves and reacts chemically with other atoms. In other words, it gives you its properties. So the number of protons makes the atom belong to a particular element. Change the number of protons and you change the element.