Grade 11 Pre-AP Chemistry - Atomic History, Experiments

Aristotle's Ideology

only 4 elements: earth, water, wind, fire

believed that matter was continuous and not made up of smaller particles

Democritus' Ideology

believed if you kept cutting a substance in half you would reach a particle that couldn't be cut further

origin of modern idea of atom

Democritus' Errors

-believed all substances had their own particles (e.g. tree particle)

-didn't recognize similarities between different particles

Aristotle vs Democritus

Aristotle's ideology was more believed and popular, even though Democritus' was more correct

John Dalton

-elements are made of indivisible atoms

-atoms of the same element are identical

-atoms can be neither created nor destroyed

(atomic theory)

Dalton's atomic model

plain timbit

J.J. Thomson

cathode ray tube experiment

discovered electron

cathode ray tube experiment

(+) and (-) plates were placed above and below the CRT

the ray deflected away from the (-) plate, hinting that the particles were (-) charged

when magnetic plates were placed above and below the CRT the ray was again deflected; the particles have mass

applications of the cathode ray tube experiment

old TVs

computer monitors

J.J. Thomson atomic model

plum pudding model

electrons just scattered throughout and rest of atom was positive

Ernest Rutherford

gold foil experiment

discovered nucleus

gold foil experiment

positive particle was shot at gold foil

rays were expected to pass through

most did but some deflected away and some even reflected back towards the origin

Rutherford atomic model

gold foil experiment discoveries

-most of the atom is empty space (most positive particles went through)

-mass/positive charge concentrated in dense core called nucleus (which is why the positive particle deflected)

-electrons orbit the nucleus like planets orbit the sun

problem with electron orbiting the nucleus

-an object that moves in a circular orbit is constantly accelerating

-an accelerating object releases energy

-object will eventually slow down

-electron will crash into the nucleus

Eugene Goldstein

canal ray tube experiment (same as CRT experiment)

discovered proton

James Chadwick

discovered the neutron

Neils Bohr

discovered that electrons are in energy levels

spectroscopy experiment

spectroscopy experiment

-Bohr studied hydrogen gas

-looked at light produced when H was burned/attached to electricity

-split the light with spectroscope

-saw a set of 4 colored lines

-called it the emission spectrum of H

-speculated that electrons can only exist at certain distances (stationary states/E levels)

-electrons absorb energy when supplied with electricity/heat and move to a higher orbit (excited state)

-electron not stable in excited state so drops back down (ground state)

-relaxes by emitting energy in the form of light (4 lines observed)

mass spectrometer

particles are separated based on mass by a magnetic field

sample gets heated to turn into gas

hit by an electron beam, knocking off some electrons (+)

1st electric plate is (+), 2nd is (-), pulling the sample forward and accelerate

sample hits magnetic field where they deflect based on their mass

lightest particles deflect most

detection plate coated in phosphorescent material (lights up when particles hit it)

Timeline of Atomic History

Dalton -> Thomson -> Rutherford -> Bohr -> Chadwick