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.

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

The Bohr model describes an atom as having a small, positively charged nucleus at its centre, with electrons arranged in fixed shells around it.

The earlier nuclear model established the central nucleus. Niels Bohr improved the model by suggesting that electrons cannot orbit at just any distance from the nucleus. Instead, they occupy particular orbits, also called shells or energy levels, at fixed distances from it.

In this model, an electron belongs to one of these permitted energy levels rather than being anywhere around the nucleus. This arrangement helped explain experimental observations more successfully than a model that allowed electrons to be at any distance from the nucleus.

Development of the nuclear model

Scientific models of the atom changed as new evidence revealed how its positive charge and mass are arranged. Each model offered an explanation of the evidence available at the time.

From plum pudding to a nuclear model

In the plum pudding model, positive charge was spread throughout the atom, with negatively charged electrons embedded in it. There was no central nucleus.

Rutherford’s nuclear model placed the positive charge and most of the atom’s mass in a tiny central nucleus. This model was supported by experiments in which positively charged alpha particles were directed at thin metal foil. Most passed straight through, but some were deflected and a few bounced back. These results indicated that most of the atom is empty space, while its positive charge and most of its mass are concentrated in a small region.

Protons and neutrons in the nucleus

Later work identified the proton as a positively charged constituent of nuclei. Before the neutron was discovered, nuclei were sometimes modelled as containing protons and electrons.

In 1932, Chadwick discovered the neutron, a particle with no electric charge. Its discovery supported a model in which nuclei contain protons and neutrons. Neutrons add mass to a nucleus without adding positive charge.

This model explains why atoms of the same element can have different masses. They have the same number of protons but can have different numbers of neutrons. Atoms of an element with different numbers of neutrons are called isotopes.