History of the Atom
People first began to wonder about what matter was mad eof more than 2,500 years ago. Some though that matter was made of tiny particles that could be cut into half and then in half again many times. Finally, you would have a piece so small that you could not cut it any more. They named the smallest piece the atom, which means "cannot be divided."
Thousands of years ago, people couldn't prove that atoms really existed. How do you describe something that you can't see? Atoms were still a mystery as recently as 500 years ago. Since scientists could not see the atoms, they could not prove that atoms existed.
It took a long time for new ideas about the atom to surface. Finally in the 1700s, scientists started learning more about matter and how it changed when two substances were put together to form something completely new. During this time, scientists realized that some substances could also be broken down. They even discovered that some substances couldn't be broken down into simpler substances, and in this case they called these substances elements. An element is matter made up of only one type of atom, such as oxygen, which is only made up of oxygen atoms.
History of the Atom
The ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today).
John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century.
In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.
In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.
In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.
We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).
Related Links
Understanding how to make Bohr Models
Understanding the Periodic Table and it's families