The History of the Atom

What is Everything made from?

During the middle ages, they believed that everything was made from the 4 elements, ‘earth, fire, water and air.’

They had forgotten a theory posed by the Greek Democritus (approx. 400B.C). He believed everything was made of particles that could not be split up, he called these atoms.

Between 1803-1807, Dalton proposed that the atoms that make up an element were identical. He also predicted that they were hard and spherical in structure.

The Discovery of the Electron

o In 1897, J.J.Thomson discovered that atoms contain a negative particle called an electron.

o He passed electricity through a chamber with a positive and negative end. The electricity split apart the outside of the atom. This split apart bit of the atom was attracted to the positive end of the chamber, which told him it must be negative.

o Because it split apart so easily, he also concluded that electrons were small and very light.

The Discovery of the Protono In 1911, Geiger and Marsden

discovered that the middle (nucleus) of an atom contains a positive particle called a proton.

o In their experiment, they fired small positive particles at a thin sheet of gold atoms.

o Almost all the particles passed straight through. However some were repelled and therefore bounced back.

o The scientists concluded that most of the atom must be empty space, but the middle of the atom must contain a positive charge because it repelled the positive particles.

o They called these positive particles protons. M produced electrons. F is

the gold atoms.

The Nucleus

Also in 1911, a British Scientist by the name of Ernest Rutherford carried out a number of experiments and discovered that an atom contained a central bit called the nucleus.

Discovery of the Neutron

o Finally, in 1932 a British scientist called Chadwick discovered the final particle that made up the atom.

o He discovered that this particle has no charge (i.e. was neutral) so he called it the neutron.

Putting it all Together!!