Atom’s History:

In 450 BC, Democritus brought up “atoms”

• Matter is made up of atoms

Atoms → Extremely small, indivisible particles

Democritus was rejected for being unable to explain how they stick together

Ben Franklin - 1740s

He studied static electricity ⚡️

• (Idea) lighting “big static”

He used a Leyden Jar for experiment

→ He concluded that two positives repel and a positive and a negative attract

1700’s

Lavoisier

• Burned magnesium and oxygen producing MgO in a sealed container

• Mass of reactants = Mass of products

• Proved law of Conservation of Mass

Proust

• Proves law of def in the proportions, H20

Dalton - 1803

| School teacher who developed the 1st atomic theory

1. Matter is made up of indivisible particles called atoms

2. All atoms of a given element are identical but differ from atoms of another element

3. Atoms are not created or destroyed in chemical reactions

4. A given compound has the same relative number and types of atoms

Mid 1800’s

Crookes

He created the “cathode Ray-tube”

• Glass tube coated with fluorescent material

The material flows across the tube, this material is radiation ☢

• Connects two electrodes (connected to a battery to the tube)

Concludes about Radiation from battery

1. It can move a paddle (has mass)

2. Has magnetic properties

1890’s

JJ Thomson

British scientists who used an apparatus to probe particles are “electron”

• Develops “mass to charge ratio”

He was eating dessert when he developed the “plum pudding model”

1909

Robert Millikan

| American scientist famous for “oil drop experiment”

• Used mass to charge ratio to determine mass and charge of electrons

Charge of electron 1.6 × 10^-19

Mass of electron 9.11 × 10^-28g

Late 1800 - Henry Becquerel

• Discovers radioactive uranium

• Exposed photograph plate to uranium

• First ever x-ray was of a rock 🪨

• Discovery of Radiation Phenomenon

Early 1900’s - Ernest Rutherford

Discovers Alpha (a), Beta (B), and Gamma (Y) radiation

Performs the “gold foil” experiment

1. Discovery of the nucleus (positively charged center of an atom)

2. Atoms are mostly empty space (everything is made of atoms)

1932 - James Chadwick

British scientist who discovered the neutron

• Neutrally charged subatomic particles

Early 1900’s - Moseley

Discovered positively charged subatomic particles (protons)

• discovered atomic number