Introduction to atoms
What is an atom?
An atom is the smallest form of a chemical particle that retains the properties of the particle.
The word 'atom' comes from the Greek word 'atomos', meaning 'unable to be cut'. The original meaning of atom was the smallest, indivisible form of a chemical particle. Now that we know how to divide atoms into sub-atomic particles, the definition of an atom includes the concept that the particle must retain its chemical properties.
History of atomic theory
Atomism is the philosophy of the existence of atoms. The first recorded theories come from India. They were put forward by a handful of ancient religious philosophers, many of whom pondered on the nature of substances of the material world as part of their spiritual quest. In the sixth century BC, a Hindu sage named Kanad hypothesised that all matter was composed of nine elements: earth; water; light; wind; ether; time; space; mind and soul. Around the same time, another philosopher named Pakuda Katyayana questioned the 'root of all things' and suggested that everything could be classified into seven categories: earth; water; fire; air; pleasure; pain and soul.
At this stage, Indian atomists developed ideas about how atoms could combine, react and move, and that they existed in groups. They also suggested that they could obtain energy by splitting these particles.
A century later, Greek philosophers Democritus and Leucippus postulated that all atoms were of the same material but that they came in various shapes and sizes, which contributed to their various properties.
In the eleventh century, al-Ghazali, an Islamic philosopher from the Asharite school of philosophy, wove the Indian and Greek theories together and suggested that there may be particles smaller than atoms and that atoms were the only eternal, material things in existence.
In the 1800s, chemist John Dalton considered previous theories. He proposed that atoms of a particular element were the same size and weight and unlike any other type of atom. He also believed that different atoms could combine in ratio to make different chemical compounds. We generally regard Dalton as the founder of modern atomic theory from which other theories have since developed. See image 1.
Structure of an atom
An atom contains three sub-atomic particles. At the centre is a nucleus, which contains protons, positively charged particles, and neutrons, which are particles with no charge. Surrounding the nucleus are moving electrons, which are negatively charged particles.
An exception to this structure is the simplest atom, hydrogen, which comprises a nucleus of one proton and no neutrons, surrounded by one electron.
- Proton: a positively charged sub-atomic particle
- Neutron: a sub-atomic particle with no charge
- Electron: a negatively charged sub-atomic particle
Protons and neutrons have about the same mass. Although smaller and of very little mass, electrons occupy the bulk of the space with their movement around the nucleus. See image 2 and image 3.
The simple atomic model
There are a number of atomic models corresponding with the various theories regarding the way atoms operate. Earlier models depicted the protons as a cloud at the centre of the atom, ringed by electrons, or like a star system with planet-like electrons orbiting the nucleus 'sun'. However, these models proved incorrect as the field of quantum mechanics developed.
One simple, widely accepted representation of an atom is the electron shell model. Shells containing the requisite number of electrons surround the nucleus with increasing diameters. Imagine that a golf ball is the nucleus containing protons and neutrons, then place the golf ball inside a tennis ball, then the tennis ball inside a cricket ball and so on. The 'shells' are the balls that surround the original golf ball. The electrons move around on these shells, attracted to the positive charge of the protons in the nucleus. See image 4.
At this introductory stage, atoms are represented in a two-dimensional manner, as shown above with the hydrogen and helium examples in 'Structure of an atom', though atoms are actually three-dimensional, as in the electron shell model.
The standard two-dimensional model of an atom is one where dots, usually of differing colours or shades, represent the protons and neutrons in the nucleus and electrons sit on defined rings around the nucleus. This model suffices for this introduction to atoms. Increasingly inclusive models will be developed later. See Animation 1.
