The Nuclear Model of the Atom

An atom has a tiny, dense nucleus at its centre. The nucleus is positively charged and holds almost all the mass. Negatively charged electrons move around the nucleus, and most of the atom is empty space. This model explains how atoms behave.

What is inside an atom?

• Proton: charge +1; found in the nucleus.
• Neutron: charge 0; found in the nucleus.
• Electron: charge –1; moves around the nucleus; very light compared with protons and neutrons.

Protons and neutrons make up the nucleus. Electrons occupy the space around it. Atoms can form ions by losing or gaining electrons; the nucleus does not change when ions form.

Proton number and nucleon number

Proton number (atomic number) is the number of protons, written as Z. Nucleon number (mass number) is the total number of protons and neutrons, written as A. We show an atom in nuclide notation as:

$${}^{A}_{Z}\mathrm{X}$$

Number of neutrons is: $$N = A - Z$$

Evidence: alpha particle scattering

In Rutherford’s experiment, a beam of alpha (α) particles (helium nuclei) was aimed at thin gold foil.

• Most α-particles went straight through → the atom is mostly empty space.
• Some were deflected → there is a small, massive centre (the nucleus).
• A very few bounced back → the nucleus is tiny and very dense.
• Deflection away from the centre shows the nucleus is positively charged (like charges repel).

Common misconceptions

• Atoms are not solid balls; they are mostly empty space.
• Electrons are not in the nucleus.
• Changing the number of electrons makes ions; changing protons makes a different element.