Scientists Behind Atomic Theory

Democritus

Greek philosopher

Thought that you could eventually end up with a particle that could not be cut

Called this particle atomos, a Greek word meaning "not able to be divided"

Aristotle

Greek philosopher

disagreed with Democritus

Did not believe that suck a particle could make up all substances found in nature

Did not do experiments to test his theory

400 BCE

John Dalton

Theory based on evidence from experiments

Dalton's theory

1808, British chemist

all matter is made up of atoms

Atoms cannot be created, divided, or destroyed

all atoms of a certain element are identical, but different from atoms of all other elements

Every substance is made up of atoms combined in certain ways

J.J. Thomson

experiments provided evidence that atoms are made up of even smaller particles

Found particles within the atom that have a negative charge

These negatively charged particles late known as electrons

1897

Thought that an atom was a positive sphere with the electrons mixed through it

Ernest Rutherford

1909

conducted an experiment to study the parts of the atom

Experiment suggested that atoms have a nucleus

Nucleus: small dense center that have a positive charge and is surrounded by moving electrons

Later discovered that the nucleus is made up of smaller particles

Called positively charged particles in nucleus: protons

Niels Bohr

1913

made observations that led to a new theory of how the electrons in the atom behaved

Agreed that an atom has a positive nucleus surrounded by electrons

In his model, electrons move around the nucleus in circular paths

His model helped scientists predict the chemical properties of elements

Each path is a certain distance from the nucleus

Scientists have since made observations that cannot be explained by Bohr's model

James Chadwick

Rutherford's student

1932

discovered that nucleus contains uncharged particles called neutrons

current atomic theory- electrons do not move in circular paths around the nucleus

instead, electrons move within an area around the nucleus called the electron cloud