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Democritus was a greek philosopher who 

developed the idea of an atom. He believed that 

matter could only be broken down so far before 

it was in its smallest form. He called the 

simplest form of matter, atoms.

John Dalton was an English chemist in 

the 1800s. He did experiments to prove 

that matter was made up of small 

atoms. He didn't know their structure, 

but he confirmed that they were there.

J.J. Thompson 

discovered the electron 

and came up with a 

model for the structure 

of an atom. He figured 

out that electrons had a 

negative charge, so he 

thought matter must 

have a positive charge. 

His model looked like 

raisins stuck in pudding, 

so they called it the 

"Plum Pudding Model."

Ernest Rutherford knew that 

atoms contained a positively 

charged nucleus and there were 

negatively charged electrons 

outside of it. His experiment 

involved shining particles onto 

the atoms in gold foil and 

observing the particles. His 

discovery was that most of the 

particles went through the foil, 

so he came up with idea that 

electrons must orbit the nucleus 

like the planets in a solar 

system. Using particles, he 

knocked nuclei out 6 elements, 

and named them protons.

James Chadwick discovered the particle in the 

nucleus of an atom that became known as the 

neutron because it has no electric charge. 

Neils Bohr came up with a model showing that 

electrons orbit the nucleus in the same way 

that the planets orbit the sun. It became 

known as the Solar System Model.

The Electron Cloud 

Model was created 

to show about 

where each 

electron was 

located around the 

nucleus. If you 

looked at an atom 

from above, it 

would look like a 

cloud of electrons 

orbiting the 

nucleus.

Glossary

Atom: The smallest particle of a substance 

that can exist by itself or be combined with 

other atoms to form a molecule.

Electron: A very small particle of matter that 

has a negative charge of electricity and that 

travels around the nucleus of an atom.

Model: A small copy of something to be 

shown as an example.

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