Atomic Model

The atomic model, which has changed over time, is the model used to describe the structure and composition of the atom. The atom as a component of the universe has been under extensive study to understand how atoms make up the universe.

The concept of the atom

The concept of the atom comes from a Greek philosopher named Democritus. He stated that all matter is made of indivisible particles called atoms surrounded by empty space. There were also some other theories until our modern idea of the atom was formulated in the 19th and 20th centuries.

The composition of the atom

In the classical model, the atom is composed of smaller particles with an electrical charge known as electrons and protons. The atom also features a third, neutral kind of particle known as neutrons. Atomic models seek to understand how these particles make up the atom. The classical atom composition is as follows:

Modern models of the atom see the positive charge as concentrated in a small space at the centre, i.e., in the atom’s nucleus. Here, protons and neutrons are held together thanks to the strong nuclear force, which prevents the protons from repelling each other.

What are the five models of the atom?

There are five principal models of the atom that have been proposed over time, each being related to the understanding of the atom at the time. The models are: Dalton’s atomic model, Thomson’s atomic model, Rutherford’s atomic model, Bohr’s atomic model, and the quantum atomic model.

Dalton’s atomic model

John Dalton was an English scientist who proposed the first modern atomic model. He proposed that all matter is made of atoms, which are indivisible. Here are some of the properties Dalton associated with the atom:

• All atoms of the same element have the same mass.
• Atoms cannot split into smaller particles.
• When any chemical reaction occurs, atoms rearrange.
• Molecules are composed of several kinds of atoms of each different element, and chemical compounds have different ratios of elements.

Figure 1. Dalton’s atomic model proposed that atoms were indivisible and different for each element. Source: Manuel R. Camacho, StudySmarter.

Thomson’s atomic model

With the discovery of electrons by British scientist J. J. Thomson, it became clear that the atom consisted of even smaller particles that were responsible for moving electrical charge.

Scientists during Thomson’s time thought that atoms were essentially neutral. Thomson proposed that atoms had small negative particles floating above a fluid of positive charge. This model is also known as the plum pudding model.

Figure 2. Thomson’s atomic model proposed a soup charged positively with the electrons floating on top. Source: Manuel R. Camacho, StudySmarter.

Rutherford’s atomic model

A New Zealand scientist named Ernest Rutherford designed some experiments together with German scientist Hans Geiger. The experiments, carried out by a student named Ernest Marsden, fired particles against a thin foil made of gold.

If the atom was a solid blob made of positive charge with some electrons on top, as Thomson’s atomic model proposed, most of the fired particles would not reach the other side of the foil. However, the experiment proved that Thomson was wrong. The atom was almost empty inside, as not many particles fired against the foil impacted the nuclei of the atoms.

Rutherford proposed that the atom contains a nucleus, with all the positive charges concentrated in the centre. In the model, the electrons were orbiting around the centre.

Figure 3. Rutherford’s atomic model proposed that electrons move around the nucleus in orbits. Source: Manuel R. Camacho, StudySmarter.

Bohr’s atomic model

Rutherford’s model did not gain full acceptance. Knowing that moving charges release energy as electromagnetic radiation, electrons should lose their kinetic energy. After losing their kinetic energy, electrons should then fall into the nucleus attracted by the electrostatic force. Inconsistencies in Rutherford’s atomic model led a Danish scientist named Niels Bohr to propose a new one.

Bohr’s atomic model was similar to Rutherford’s. The difference between the two concerns the question of how electrons move. According to Bohr, electrons can only travel in certain orbits, depending on their energy level, and they can move up and down the orbits releasing or absorbing energy. The rules proposed by Bohr are as follows:

• Electrons can occupy certain orbits, depending on their energy level.
• Each orbit has a certain energy level.
• When jumping between orbits, energy must be absorbed or released by the electrons.
• The energy emitted as a form of radiation can be calculated by the difference in energy levels between the orbits. This energy is said to be quantised.

Figure 4. Bohr’s atomic model proposed that the electrons move around the atom in orbits and also jump to different orbits, depending on their energy level. The energy of each level has a fixed value, and electrons jump up and down, absorbing or releasing radiation. Source: Manuel R. Camacho, StudySmarter.

Bohr’s model could explain a hydrogen atom whose electron is unique in not interacting with other electrons orbiting the atom. However, it failed to explain more complex elements or effects.

The quantum atomic model

The quantum atomic model is the most detailed model so far of how the atom is composed and how it works. It was developed with contributions by Erwin Schrödinger, Werner Karl Heisenberg, and Louis de Broglie. The model is an extension of Bohr’s model by adding the concept of wave-particle duality, and it is able to explain more complex atoms than hydrogen.

The quantum model proposes that matter can behave as waves and that electrons move around the atom in orbitals. The orbital is a region in which there is a higher probability of an electron moving. In this model, electrons cannot be located precisely, and the orbitals are defined as clouds of probability.

Figure 5. An atom showing four orbitals, i.e., clouds where electrons could be present. Source: Manuel R. Camacho, StudySmarter.