In order to understand many phenomena in the study of chemistry it is important to know how atoms are made up. One of the building blocks of modern chemistry is the current theory of the atom. Over recent centuries chemists have been refining a model of the atom. With some knowledge of these theories it is easier to understand other important aspects of chemistry such as bonding and reactions.
Democritus to John Dalton
In around 400BC the Greek philosopher Democritus proposed the theory that all matter was made up of many small particles. These particles were called “atoms” because they were considered the smallest possible particle which cannot be broken down further. It was not until the early 19th century that this theory was developed by John Dalton (1766-1844). Dalton suggested that elements were made up of identical indivisible atoms and the atoms of different elements were different in mass.
Thompson’s Plum Pudding
Building on these ideas and his recent discovery of electrons, J.J.Thompson proposed a new model of the atom in 1904. He suggested that all atoms are made up of the same basic materials. He suggested that each atom was a positively charged sphere in which the negatively charged electrons were embedded. This is sometimes called the “plum-pudding” model.
Then, a few years later, Ernest Rutherford found some unusual results which pointed him to a more accurate model. He was firing very small, positively charged alpha particles at some very thin gold foil. The majority of these particles passed straight through the gold as if it wasn’t there. A few were deflected very slightly. A very, very few particles were reflected back off the foil. This was astounding. It was like firing a bullet at a piece of tissue paper and having it bounce back.
Nucleus and Shells
Rutherford’s answer to this mystery was a new model of the atom. He suggested that most of the mass of each atom was concentrated in a very small, positively charged nucleus at the centre of the atom. The rest of the atom was actually empty space except for electrons circulating around the nucleus at various levels. The alpha particles that were deflected or reflected were the ones passing near the nucleus.
Protons, Neutrons and Electrons
This is, in fact, the model of the atom we use today. The atoms of each element have a nucleus of a particular weight, based on the number of protons (positively charged particles) and neutrons (uncharged particles) it has. The neutral atoms then have a number of electrons, equal in number to the protons, orbiting the nucleus in a number of different shells. Electrons are many times smaller and lighter than protons and neutrons but have an equal but opposite charge to the protons.
From this theory, chemists have explained the Periodic Table of the Elements and the different types of bonding in molecules among other chemical observations.