History of the Atom

Scientists who shaped the modern atomic theory

4th Century Greece

Democritus first suggested

matter was made of atoms

there was no way to test these theories

Aristotle suggested matter

was continuous, not made of atoms

this was believed

Alchemists

Transmutation: tried to turn all metals into gold

brought observation and experimentation to science

Antoine Lavoisier

In ordinary chemical reactions in a closed system, mass remained constant.

Law of conservation of mass• matter can not be created or

destroyed

John Dalton and the Atomic Theory

In the early 1800s, Dalton wrote explanations of the

atom based on experiments by Lavoisier and Proust.

Dalton’s Atomic Theory Atoms are indivisible. Elements are made of atoms. Atoms of the same element are

identical. Atoms of different elements are different.

Atoms combine in simple, whole number ratios.

Reactions occur when atoms are separated or combined.

J.J. Thomson

• 1897• cathode rays are composed of electrons• determined the charge of an electron• “plum pudding model”

In the early 1900s...

Robert Millikan (American)• oil drop experiment• accurately measured the charge of an

electron

Then a ray of light??

1896 Henri Becquerel

discovered that uranium exposes photographic film.

Radioactivity

Early 1900s Marie and Pierre

Curie discovered that rays are given off by uranium and radium.

Atoms will split into smaller units

Albert Einstein

Explained the origin of the energy produced in nuclear reactions with E = mc2.

Then the modern theory began to develop

Lord Rutherford 1913

Proved Thomsonwrong

Rutherford Model

Also called the planetary model

Most of the atom is empty space

Consists of a central nucleus surrounded by electrons in orbits

The “final” particle is discovered...

1932 James Chadwick

• found particle with the same mass as a proton, but no charge

• now known as a neutron

So what did this all mean to the atomic theory?

Dalton’s theory had to be revised to include subatomic particles!

Bohr Model

Calculated specific energy levels for electrons

Quantum mechanical model(Schrodinger, de Broglie, &

Heisenberg) Current model Electron cloud

model Electrons do not

have fixed positions, but exist somewhere in the electron cloud