Five types of atomic models

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Five types of atomic models
An atom consists of a tightly-packed nucleus, with protons and neutrons, with electrons "orbiting" around it. (Jupiterimages/ Images)

Atoms are not easy to study. To understand why, all you have to think about is their size – typically 0.1 nanometres in diameter, or around a ten millionth of a millimetre. Atoms are stable structures built from an entire zoo of smaller particles, balanced by their intricate magnetic make-up. They were once thought to be indivisible, the fundamental building blocks of matter, but as more evidence amassed the basic theory was forced to grow more complicated, accurate and detailed. The journey from the initial inception of the idea to the modern, quantum mechanical understanding shows how science is continuously evolving and developing in order to understand the world we observe around us.

Democritus and Dalton’s billiard ball model

The idea of the atom is thought to come from the Greek philosopher Democritus (460 – 370 BC), who asked a simple question. He wondered what would happen if you continually split something into smaller and smaller pieces. Would you be able to keep splitting forever, or was there a limit? He theorised that there was an indivisible components of matter, for which he used the word “atomos,” which means “not to be cut.” John Dalton (1766-1844) took this idea further, still keeping the idea that they’re indivisible, but also adding that chemical reactions were a result of atomic changes and that all atoms of the same element are equivalent in mass and how they behave. The atoms were believed to be spherical like a billiard ball.

Joseph John Thomson’s plum pudding model

Thomson (1856-1940) discovered the electron, a particle with a negative electromagnetic charge which is smaller than an atom. This contradicted the “indivisible” part of the original atomic theory, with Thomson proposing a model where the small, negatively charged electrons were stuck into some positive matter like chocolate chips on a cookie. It was still thought to be spherical, but Thomson added that the positive and negative charges on an atom cancel each other out.

Ernest Rutherford’s planetary model

Rutherford (1871 – 1937) proposed a much more accurate version of the atom in 1911, based on an experiment he conducted which came to destroy the plum pudding model. He took a sheet of gold foil and fired tiny particles (basically an atom stripped of electrons) at it. Based on the older models, he expected the particles to bounce straight back like a ball thrown at a brick wall. However, the particles mostly passed straight through, which led him to realise that atoms must be largely composed of empty space. He argued that the positive matter from the plum pudding model was actually compressed into a tiny space which contained almost all of the atom’s mass. The light electrons orbited the atom in the same way as the Earth does the Sun (except with electromagnetism as a force, instead of gravity). He also proposed the existence of neutrons, which sit beside the protons (positive particles) in the nucleus (the Sun, in the model) but have no charge.

The Bohr model

Neils Bohr (1885 – 1962) refined the planetary model of the atom, creating the atomic model which is still used when introducing students to the structure of the atom. He proposed that the electrons could only orbit in specifically-defined regions, known as “energy levels.” Electrons closer to the nucleus had a lower energy, and those which were further away had higher energy. He also added that electrons can be “excited” into a higher energy level when struck by a photon of light. This model can be used to describe chemical interactions with impressive accuracy, but isn’t wholly complete.

The quantum model

Louis de Broglie (1892 – 1987), Werner Heisenberg (1901 – 1976) and Erwin Schrödinger (1887 – 1961) made the major recent additions to the atomic model. De Broglie showed that electrons actually possess wave-like qualities – diffracting in the same way light does – which explains why there are specific energy levels for electrons. Heisenberg set out to answer whether or not you could pinpoint an electron, devising his namesake uncertainty principle when he realised that is was impossible to precisely know both the location and the speed of an electron at any one time. Schrödinger completed several calculations which clarified various elements of the theory, which basically leaves the atom as a small, dense nucleus surrounded by a fuzzy cloud of electrons.

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