ATOMIC THEORY

Democritus

“Could matter be divided into smaller and smaller pieces forever?”

His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece “atomos” meaning “not to be cut”.

Democritus

Greek philosopher No experiments to support idea No protons, electrons, or neutrons Small, solid particles that were made

of the same material but were different shapes and sizes

Democritus

Atoms are infinite in number, always moving and capable of joining together to form different objects.

The characteristics of an object are determined by the shape of its atoms.

Democritus (400 BC)

Hard substances have rough atoms that stick together

Smooth atoms are in liquids and they slide over each other

Sleep is caused by atoms escaping the brain

Dalton

1803 proposed that elements consist of individual particles called atoms

Four parts of the theory He had the first atomic theory that

had evidence to support it Called elements “pure” Atom looked like a marble

Dalton

All matter is made of atoms Atoms are indivisible and indestructible All atoms of a given element are identical

in Mass and properties Compounds are formed by a combination

of two or more different kinds of atoms A chemical reaction is a rearrangement of

atoms Atoms can be neither created nor

destroyed

Dalton

The essence of Dalton’s theory remains valid.

His ideas were basis for chemistry

Dalton

Developed the Law of Multiple Proportions When elements form multiple

compounds, the proportions can be expressed as a whole number ratio.

Mendeleev

He noticed a pattern in various elements according to their masses.

He later arranged the elements into a table—the periodic table.

Sir William Crookes

First person to confirm the existence of cathode rays by displaying them with his tube invention.

He was able to show that cathode rays are negatively charged by studying the direction in which cathode rays are deflected by a magnetic field.

Sir William Crookes

Cathode rays—streams of electrons observed in vacuum tubes.

Crookes tube—an early experimental electrical discharge tube in which cathode rays, streams of electrons, were discovered

A cathode ray tube is a vacuum tube that produces images when its phosphorescent surface is struck by electron beams.

Becquerel

Contributed to the knowledge of radiation and radioactive decay

His photographic plate didn’t need a source of energy to make an image—discovered radioactivity! (He was using sunlight and artificial light previously)

JJ Thomson

Plum Pudding Model (Chocolate Chip Cookie)

Totally changed the view of an atom by discovering the electron

Thomson’s atomic theory suggested that the atom is not indivisible because it is made of smaller pieces—electrons and protons

An atom consists of a sphere of positive charge with negatively charged electrons embedded in it

Thomson

The positive and negative charges in an atom are equal in magnitude due to which an atom is electrically neutral.

It has no overall negative or positive charge.

Thomson

Discovered the electron when he was experimenting with gas tubes

He noticed a movement and called the movement cathode rays

The rays travel from negative to positive He concluded that atoms do contain

subatomic particles—they are divisible This contradicted Dalton’s theory and

was not widely accepted by fellow physicists and chemists

Thomson continued….

Cathode rays originate at the cathode (negative) and move to the anode (positive)

The rays bent toward the positive pole, indicating the negative charge

Thomson continued….

Thomson proved that atoms of any element can be made to emit tiny negative particles—therefore, all atoms must contain these negative particles

Rutherford

Learned physics in Thomson’s lab Conducted gold foil experiment He took a thin sheet of gold foil and

used special equipment to shoot alpha particles (positive) at the gold foil

Rutherford continued….

Most particles passed straight through the foil like the foil wasn’t even there…

Some particles went straight back or were deflected as if they had hit something…

Rutherford continued…

The experiment shows: Atoms are made of a small, positive

nucleus; the positive nucleus repels (pushes away) positive alpha particles

Atoms are mostly empty space

Niels Bohr (early 1900s)

Danish physicist He proposed a model of the atom

that is similar to the model of the solar system.

The electrons go around the nucleus like planets orbit around the sun.

Bohr continued…

All electrons have energy levels—a certain distance from the nucleus

Each energy level can hold a certain number of electrons