The simple nuclear model of the atom

By the end of this lesson

By the end you can describe the atom as a nucleus of protons and neutrons with electrons outside it, and state the charge, mass and size of each part.

The three particles

The simple nuclear model says the atom has a small central nucleus surrounded by electrons.

particle	where it is	charge	mass
proton	in the nucleus	$+e$	about 1 u
neutron	in the nucleus	0 (neutral)	about 1 u
electron	outside the nucleus	$-e$	about $\dfrac{1}{1840}\ \text{u}$

Here $e$ is the elementary charge, the size of one proton's charge: $e = 1.60 \times 10^{-19}\ \text{C}$ . Protons and electrons carry equal and opposite charge, so an atom with equal numbers of them is neutral overall.

Almost all of it is empty

The atom is neutral, but that is a statement about charge, not about how the mass is shared out.

The nucleus is about $10^{-15}\ \text{m}$ across.
The atom as a whole is about $10^{-10}\ \text{m}$ across — roughly 100 000 times larger.

So the nucleus has a diameter about a hundred-thousandth of the atom's, which means the nucleus occupies roughly $(10^{-5})^3 = 10^{-15}$ of the atom's volume. Almost all of the atom's mass sits in that tiny region; almost all of its volume is empty.

The electrons do not sit in fixed orbits like planets on tracks. The word "orbital" means the electrons occupy a region around the nucleus; that is all the AS model needs.

A pea in a cathedral

A well-known comparison: if the nucleus were a pea in the middle of a cathedral, the atom would be the whole building and the electrons would be specks at the walls.

It gives the right feel for the emptiness, and it explains why alpha particles usually pass straight through a foil. It is only a comparison, though: the "walls" are not solid, and the electrons are not stuck in place.

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Recall

Fill in the blanksQuestion 3 · Apply

Answer the 3 questions above to continue.