Discovery of the Atom

DemocritusDemocritus was an ancient Greek who had a philosophical idea of an atom.

His approach was not based on the scientific method.

Dalton

Dalton transformed the philosophical concept of an atom into a scientific theory.

A.) All elements are composed of indivisible atoms. What does indivisible mean?

Cannot be divided.

B.) Atoms of the same element are identical. Atoms of different elements are different.

C.) Atoms combine in fixed ratios to form compounds. 

D.) Chemical reactions occur when atoms are rearranged.

What does subatomic mean?Smaller than atomic

What are the three kinds of subatomic particles?Protons, Neutrons and Electrons

Who discovered the electron?

J.J. Thomson

What did the cathode-ray tube experiment allow to be concluded?

Electrons are in ALL atoms.

Millikan

Millikan found the quantity of charge carried by an electron.This allowed the calculation of the mass of the electron. It’s mass is 1/1840 the mass of a hydrogen atom.

Atoms are electrically neutral.

Positively charged subatomic particles are called protons.

Subatomic particles with no charge are called neutrons.

RutherfordRutherford’s gold-foil experiment gave evidence of the atomic nucleus.

Alpha particles were aimed at a sheet of gold foil.

Rutherford’s Gold Foil Experiment

Most of the particles passed through.

A few of the particles bounced back.

It was concluded that an atom is mostly empty space.

The mass and positive charge are concentrated in the nucleus.

Atom Make-Up

The protons and neutrons are located in the nucleus.

The electrons are distributed around the nucleus and occupy

almost all of the volume of the atom.