The Greek word atomos means “indivisible”.
John Dalton (England), early 1800s
Many changes have been made to Dalton’s model of the atom but his five key observations proved vital for later developments.
1. Elements consist of tiny particles called atoms.
2. Atoms of the same element are identical in size, mass and other properties.
3. Atoms cannot be split, created or destroyed.
4. The atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. During chemical reactions atoms are combined, separated or rearranged.
*Dalton did ground-breaking work on colour blindness. As a tribute it’s sometimes referred to as Daltonism.
Sir Humprhy Davy and Michael Faraday (England), Baron Jöns Jacob Berzelius (Sweden), George Johnstone Stoney (Ireland), early 1800s.
At about the same time but independently of one another in Britain and Sweden it is found chemical compounds change when they come into contact with an electrical current. Material is therefore electrical by nature. Stoney speculates electricity consists of different parts or particles and calls them electrons.
*In 1815 Davy developed the Davy-lamp which made it possible for mineworkers to work in places where there are flammable gasses.
*Faraday had very little formal education but he still enjoys recognition as one of the leading scientific experimentalists.
*In 1835 Berzelius (then 56) married 24-year-old Elisabeth Poppius.
*Craters on Mars and the moon were named after Stoney, who also did a lot of work in the field of astronomy.
Sir Joseph John Thomson (England), late 1800s
Stoney’s theory is confirmed. Because material is neutral, positive particles that must neutralise the electrons are searched for. These particles, the same size as electrons but 1 836 times heavier, are called protons. Thomson proposes an atom as a whole is positive, with electrons randomly arranged in it (like raisins in a muffin). This explains the neutral nature of the atom.
*Thomson won the Nobel Prize for Physics in 1906.
*He was an exceptionally gifted teacher – seven of his research assistants won the Nobel Prize in Physics. His son, George, also won the prize in 1937.
Sir Ernest Rutherford (England), 1911
Thanks to the discovery of radioactivity and radio-active elements by Pierre and Marie Curie in France, Rutherford can prove Thomson’s theory wrong. By bombarding alpha particles with a positive charge on a thin sheet of gold leaf he discovers the gold leaf does not let all the particles through. A handful of particles are deflected at unusually large angles. This leads to Rutherford’s theory of the atom: the positive nucleus is small compared to the surrounding area in which the electrons freely move.
*Rutherford is known as the originator nuclear physics .
*In 1908 he was awarded the Nobel Prize for Chemistry.
*Element 104 on the periodic table, Rutherfodium, was named after him.
Niels Henrik David Bohr (Denmark), 1913
Rutherford was unable to explain why the electrons were not attracted to the positive nucleus but Bohr discovers the electrons move in fixed paths (or energy levels) around it. Electrons further away from the nucleus have higher energy levels.
*In 1922 Bohr wins the Nobel Prize for Physics. His son Aage follows in his footsteps: the prize is awarded to him also in 1975.
*Bohr was one of the group of scientists who worked on the so-called Manhattan Project which resulted in the first atom bomb being made.
Today: quantum mechanical model
The most accurate model of the atom that currently exists is based on the work of various scientists such as Einstein, Planck, Schrödinger, Heisenberg and Chadwick. According to this model electrons have similar properties to waves and particles.
But because a wave is “spread out” (not in a fixed place at a fixed time) it’s impossible to determine an electron’s speed and precise position. Today we start from the supposition that electrons move around the nucleus in paths (orbitals) or even as an electron cloud. We also now know there are neutral parts in the nucleus – neutrons.
Sources (text and pictures): en.wikipedia.org; Platinum Series: Physical Science Grade 10, Maskew Miller Longman; Oxford Successful Physical Science Grade 10, Broster, Horn & James.
DRUM in the classroom, January 2012
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