Matter is composed of very small particles which may be atoms, molecules or ions.
The History of the Atom
The Greek philosophers first proposed that there were indivisible particles which they called ‘atoms’ which means ‘indivisible’ in Greek. This is where we get the word atom.
In 1808, an English chemist called John Dalton was able to provide indirect evidence for the existence of atoms.
Dalton’s Atomic Theory:
- All matter is made up of very small particles called atoms.
- All atoms are indivisible. They cannot be broken down into simpler particles.
- Atoms cannot be created or destroyed. (Law of conservation of mass).
Discovery of the Electron
William Crooks showed that cathode rays travels in straight lines.
The negative metal inside the tube is called the cathode. Because the rays come from the cathode, they are called cathode rays.The positive electrode is called the anode.
In 1897, JJ Thomson showed that the cathode rays are attracted by positive plate in an electric filed which ment that they had a negative charge. These negatively charged particles were called electrons by an Irish scientist called George Johnstone Stoney.
In 1910, Robert Millikan conducted the oil drop experiment which allowed the charge on the electron to be measured accurately.
Thomson’s “Plum Pudding” model of the atom
n 1898, JJ Thomson proposed a structure for the atom which had a positively charges spheres (+) in which negatively charged electrons (-) were embedded. However, this model was soon shown to be inadequate.
Ernest Rutherford fired alpha particles at a very thin gold foil. Most of the particles went straight through which passed through empty space. Some particles were deflected in small angles. A small number of particles deflected at angles greater than 90 degrees and a very small number bounced back toward their source.
He proposed that all atoms are densely concentrated in a tiny positively charged central region called the nucleus, and the rest of the atom is mostly empty space.
Protons and neutrons
Rutherford found that protons made up the positive part of all atoms. He also suggested that the nucleus also contained other particles of equal mass to protons but with no charge. In 1932, James Chadwick produced the evidence for these particles, which he called neutrons.