History of the Atomic Model

Democritus

Coined the term Atom and believed atom meant indivisible

Democritus

Did no experiments

Democritus

All elements have different shapes/sizes (no visual model)

Democritus

First to come up with the idea of atoms

Aristotle

Did not believe in atoms (ie, disagreed with Democritus)

Aristotle

Believed that all matter was continuous - idea lasted 2000 years

Dalton helped verify this

Law of conservation of mass - mass can’t be created or destroyed by ordinary chemical or physical reactions

Law of definite proportions

A compound contains the same proportions of mass regardless of size or source

Law of multiple proportions

If two or more compounds of the same two elements exists, then the ratio of moles are ratios of whole numbers

Dalton

All matter is made of atoms

Dalton

Atoms are an element are all the same, atoms of a different element will differ - was later proven wrong

Dalton

Atoms can’t be subdivided, created, or destroyed - was later proven wrong

Dalton

Atoms of different elements form compounds in whole number ratios

Dalton

In reactions, atoms are combined, seperated, or rearranged

Dalton’s theory

Isotopes and ions proved this theory wrong

Dalton’s theory

Nuclear reactions can split an atom proved this theory wrong

Dalton’s atomic model

Whose atomic model is this?

Thomson

Cathode ray experiment

Thomson

Measured the charge/mass ratio of cathode ray particles and found that this ratio is the same for all metals he tested as a cathode and any gas used. 

Thomson

Concluded that cathode rays were made of identical negatively charged particles

Thomson

Discovered the electron

Thomson

“Plums” were negative because the cathode ray deflected towards the positive end

“Pudding” was mostly positively charged because the overall charge had to be neutral

Rutherford

Shot alpha particles (positvely charged particles) through gold foil

Rutherford

Came to conclusion that there is a positive, densely packed center which repels the electrons

Rutherford

Discovered the nucleus

Rutherford

Which atomic model is this?

Bohr

Used Planck’s and Einstien’s work on energy

Borh

Determined that different electrons have different energy 

Bohr

Showed the difference in energy using energy levels or orbitals

First to use energy orbits

Bohr

Lowest allowable energy state is the “ground state”

Bohr

When energy is added, electron moves to “excited state”

Bohr

When electron moves back to its normal energy level, it releases electromagnetic energy

Bohr

Which model is this?

Electron Cloud model

Nucleus: contains protons and neutrons

Electrons: shown as a “cloud” indicating a “high probability of finding electrons”

Thomson

Determined the charge on electrons

Thomson

First model to show electrons