Atomic models and structure

Dalton’s atom: a tiny, indivisible particle

An atomic model is a representation that helps us explain the structure of an atom. Models can change when experiments reveal something that the existing model cannot explain.

In 1803, John Dalton proposed that matter was made of tiny atoms. He pictured each atom as a solid, indivisible particle, often represented as a billiard ball. In his theory, atoms of the same element were identical, atoms of different elements were different, and different atoms combined to form new substances.

The important word here is indivisible: Dalton’s model did not contain smaller particles inside an atom. Discovering such particles would therefore require a change to the model.

Discovering electrons changed the model

In 1897, J. J. Thomson discovered the electron using a cathode-ray tube. In the experiment, a beam of electrons was deflected towards a positively charged metal plate and away from a negatively charged plate. Opposite charges attract, so this behaviour showed that electrons were negatively charged.

An electron is a subatomic particle: a particle smaller than an atom that forms part of its structure. Its discovery showed that an atom was not an indivisible solid ball.

Thomson proposed the plum pudding model. In this model, negative electrons were embedded throughout a ball of positively charged material, rather like pieces of fruit within a pudding. Unlike the modern model, it had no small central nucleus: the positive material was spread throughout the atom.

From positive material to a central nucleus

Rutherford’s model, developed in 1909–1911, placed a nucleus at the centre of the atom, with electrons outside it. This was a major change from Thomson’s picture of positive material spread throughout the atom.

The change followed experiments in which positively charged particles were directed at thin gold foil. Most passed through, but some were deflected and a few turned back. These results could not be explained by positive charge spread throughout the atom. Instead, they supported a model in which the atom was mainly empty space, with its positive charge and most of its mass concentrated in a small central nucleus.

In 1913, Bohr developed the nuclear model by proposing that electrons occupied shells at set distances from the nucleus. His calculations agreed with experimental results.

Discovering particles within the nucleus

Further investigations showed that the nucleus itself contained smaller particles. The discovery of the proton, a positively charged particle, explained the nucleus’s positive charge. Positive charge was no longer represented as a continuous material spread through the atom: it belonged to particles within the nucleus.

In 1932, James Chadwick provided evidence for neutrons: particles with no electrical charge. The nuclear model was developed to include both protons and neutrons in the nucleus. Neutrons contribute mass without adding positive or negative charge.

The progression shows how the picture of an atom changed from a single solid particle to a structure containing different subatomic particles.

These drawings are representations, not pictures of atoms taken through a microscope. Their purpose is to show the differences between the models, rather than the true sizes of the particles.

The structure of an atom

In the model used at GCSE, an atom has a central nucleus containing protons and neutrons, surrounded by electrons in shells.

The position of each particle matters: protons and neutrons belong in the nucleus, whereas electrons occupy shells outside it.

A shell represents an energy level occupied by electrons. The circular lines in a school diagram help us show these levels; they are not solid rings of material.

Comparing particle charges and masses

Relative values let us compare the particles without using their extremely small masses in kilograms or charges in coulombs. A proton’s mass is taken as approximately 1, and its charge as +1.

Protons and neutrons have almost the same mass. An electron has only about one eighteen-hundred-and-fortieth of a proton’s mass, so its mass is often described as negligible when considering the mass of an atom.

The charges of a proton and an electron are equal in size but opposite in sign. A neutron has no electrical charge.

Why an atom has no overall charge

An atom is electrically neutral. It therefore contains equal numbers of protons and electrons: each proton’s positive charge is balanced by one electron’s negative charge.

For example, an atom with three protons and three electrons has three positive charges balanced by three negative charges. Its overall charge is zero. The neutrons do not affect this balance because they have no charge.

A tiny nucleus contains most of the mass

The nucleus is very small compared with the overall size of the atom. Most of the space within an atom lies outside the nucleus, in the region occupied by the electrons.

Nevertheless, most of the atom’s mass is concentrated in the nucleus. Protons and neutrons contribute almost all of this mass; electrons contribute very little. This separates two ideas that can otherwise be confused: the nucleus occupies only a tiny part of an atom’s volume, but contains most of its mass.

How the model changed

• Dalton: solid, indivisible atoms.
• Electron discovered: atoms contain smaller particles; Thomson’s plum pudding model placed electrons within positive material.
• Scattering evidence: positive charge and most mass concentrated in a small central nucleus, with electrons outside it.
• Bohr: electrons occupy shells.
• Proton discovered: positive charge belongs to particles in the nucleus.
• Neutron discovered: nucleus also contains uncharged particles with mass.

New experimental evidence can cause a model to be changed or replaced.

Particle properties

Neutral atom: equal numbers of protons and electrons → equal and opposite charges cancel.

Size and mass

The nucleus is very small compared with the atom, but contains most of its mass. Electrons contribute very little mass.