The Atom: Early Theories

The modern concept of the atom can be traced back to Democritus in 465 BC.  Unlike most Greeks of his time, he believed that everything was made up of small, indivisible shapes that he called atomos.

For the next 2 thousand years, most thinkers, philosophers and alchemists followed the basic idea set down by Aristotle.  He argued that everything was composed of some combination of the 4 elements- fire, earth, air and water.  This belief slowly began to break down during the Renaissance. This was when belief and debate began to be replaced with experimentation and observation.

Antoine Lavoisier (1743-1794) began to pull the ideas of the previous 200 years when he clearly stated the Law of Conservation of Matter.  From his experiments, he found that even if the appearance of a substance may change, the total mass of the materials before a change will always equal the total mass after the change. Thanks to Einstein, we now realize that we can make nuclear changes that will convert matter into energy, but Lavoisier's concept is still valid for chemical changes.
Law of Conservation of Matter- matter can neither be created nor destroyed by ordinary chemical means.

In the early nineteenth century, Dalton further defined matter by clearly stating that all matter is composed of atoms. He said an atom is the smallest unit of an element that can exist alone or in combination with other atoms of the same or different elements.

1. All matter is made up of very small particles called atoms.
2. Atoms of the same element are all chemically alike: atoms of different elements are chemically different.
3. The atoms of different elements have different average masses.
4. Atoms are not subdivided in chemical reactions, they unite in simple ratios to form compounds.

Over the next century, many others made contributions to our concept of the atom. Many different types of atoms (elements) were identified and investigated.  By the end of the nineteenth century, about 60 elements had been discovered and described.  Dmitri Mendeleev was the first to publicly attempt to organize the elements and look for patterns.  He organized the known elements by atomic weight and their known properties.  He was even able to predict the properties and atomic weights of elements that had not been discovered yet, but there were problems with his table.  There were several elements that seemed to be in the wrong place.  One of the most glaring was iodine.  Iodine clearly had properties similar to bromine, but it fell in the same row with oxygen.

Henry Gwyn Jeffreys Mosley (1887-1915):  Mosley began to work on the inconsistencies found in Mendeleev's table at Oxford. He arranged the elements in order of their atomic numbers horizontally and formed columns of elements with similar properties. Tragically for the development of science, Moseley was killed in action during WW1 at Gallipoli in 1915. 

The modern periodic law states that if the elements are arranged by increasing atomic number, their properties will reoccur at regular intervals.