Nuclear model from alpha scattering

In the alpha particle scattering experiment, a beam of positively charged alpha particles was directed at a thin metal foil. Scientists observed how the particles travelled after encountering the foil.

Most alpha particles passed straight through the foil without being deflected. This showed that most of each atom is empty space: the particles could pass through without encountering anything that significantly changed their path.

A small number of alpha particles were deflected through large angles, and a very few bounced back. Since positive charges repel each other, these particles must have come close to a concentrated positive charge. The fact that so few were strongly deflected showed that this region is very small. The large changes in direction also showed that most of the atom’s mass is concentrated there.

The nuclear model

The results led to the nuclear model of the atom. In this model, a tiny, dense nucleus at the centre contains most of the atom’s mass and all of its positive charge. Electrons are outside the nucleus, and most of the atom is empty space.

This differed from the plum pudding model, which had positive charge spread throughout the atom, with electrons embedded in it. That spread-out positive charge could not explain why some alpha particles were deflected through such large angles or bounced back.

Plum pudding model

The plum pudding model pictured an atom as a sphere of positive charge with negatively charged electrons embedded throughout it. The positive charge was spread across the atom, rather than concentrated in one place.

Before electrons were discovered, atoms were thought to be tiny solid spheres that could not be divided. The discovery of electrons showed that atoms contain smaller parts, so the solid-sphere idea had to change. The plum pudding model accounted for these smaller, negatively charged electrons by placing them within a sphere of positive charge.

Bohr model

Bohr proposed that electrons occupy specific orbits, or shells, around the nucleus rather than orbiting at any random distance from it. Each permitted shell is at a fixed distance from the nucleus.

His model kept the earlier nuclear model’s idea of a small central nucleus, but gave a more organised picture of where electrons could be found around it. Bohr’s calculations agreed with experimental observations, so his model was accepted as a better description than the nuclear model alone.

Discovery of protons and neutrons

Later experimental evidence showed that the positively charged nucleus is not a single, indivisible object: it contains smaller particles.

The positive charge of a nucleus could be divided into a whole number of equal positive charges. These charges are associated with particles called protons. Every proton carries the same amount of positive charge, so a nucleus with more protons has a greater positive charge.

About 20 years after the nucleus became an accepted scientific idea, James Chadwick’s experimental work provided evidence for another particle in the nucleus: the neutron. Neutrons have no electric charge, so they do not contribute to the nucleus’s positive charge. This evidence refined the nuclear model: the nucleus contains both positively charged protons and uncharged neutrons.