ChemistryChemistryScience 10Science 10

History of the AtomHistory of the Atom

Atomic TheoryAtomic Theory Answer the following questionsAnswer the following questions

• How long has the Atomic Theory been How long has the Atomic Theory been around?around?

• Is the Atomic Theory the work of one Is the Atomic Theory the work of one scientist?scientist?

• Have there been many changes to the Have there been many changes to the Atomic Theory?Atomic Theory?

• Do you think we fully understand atomic Do you think we fully understand atomic behaviour?behaviour?

History of the AtomHistory of the Atom Began with Began with

DemocritusDemocritus

Theory continually Theory continually tested and refined tested and refined through history through history until we have the until we have the current modelcurrent model

DemocritusDemocritus 460-370BC460-370BC Greek PhilosopherGreek Philosopher Pictured the atom Pictured the atom

as a solid sphere – as a solid sphere – hypothesised that hypothesised that there was a piece there was a piece of matter that of matter that could no longer be could no longer be divided in twodivided in two

No real scienceNo real science

John DaltonJohn Dalton 1766-18441766-1844 Hypothesised that Hypothesised that

there was a building there was a building block of matterblock of matter

Gave it the name of an Gave it the name of an atomatom

Basically, he restated Basically, he restated Democritus’s Theory Democritus’s Theory based on scientific based on scientific observationobservation

John Dalton’s TheoryJohn Dalton’s Theory Matter was composed of atomsMatter was composed of atoms Atoms of the same type all had Atoms of the same type all had

similar masssimilar mass A pure substance would contain the A pure substance would contain the

same type of atomssame type of atoms Compounds were different types of Compounds were different types of

atoms combined togetheratoms combined together Chemical reactions were the Chemical reactions were the

rearranging of compoundsrearranging of compounds

JJ ThompsonJJ Thompson 1856-19401856-1940 Used electron beam Used electron beam

directed at the atom to directed at the atom to test behaviourtest behaviour• Discovered that cathode Discovered that cathode

rays were streams of rays were streams of electrons (negative electrons (negative particles)particles)

Discovered there are Discovered there are both positive and both positive and negative particlesnegative particles

JJ Thompson’s TheoryJJ Thompson’s Theory ““Raisin-Bread” Model of Raisin-Bread” Model of

the Atomthe Atom• Solid mass of positive Solid mass of positive

charge (bread)charge (bread)• Random electrons with Random electrons with

negative charge negative charge throughout (raisins)throughout (raisins)

Attempted to predict Attempted to predict number of electrons number of electrons using electrostatic using electrostatic forcesforces

Ernest RutherfordErnest Rutherford 1871-19371871-1937 Set out to test Set out to test

Thompson’s ModelThompson’s Model Used alpha Used alpha

particles particles (radioactive) (radioactive) directed at Gold Foildirected at Gold Foil

Proposed the Proposed the Nuclear Model of Nuclear Model of the atomthe atom

Ernest Rutherford’s ModelErnest Rutherford’s Model All atoms would have All atoms would have

a positive nucleusa positive nucleus• 99.9% of atom’s mass99.9% of atom’s mass• Only .1% of atom’s Only .1% of atom’s

sizesize Atoms would have Atoms would have

orbiting electrons orbiting electrons (like planets)(like planets)

Majority of the atom Majority of the atom is empty spaceis empty space

Neils BohrNeils Bohr 1885-19621885-1962 Tested planetary orbital model of atomTested planetary orbital model of atom Used Hydrogen spectra for analysisUsed Hydrogen spectra for analysis

Neils Bohr’s ModelNeils Bohr’s Model Discrete energy Discrete energy

levels in which levels in which electron can existelectron can exist

Electrons can move Electrons can move between levels but between levels but never have a non-never have a non-whole number whole number energy levelenergy level

Quantization of Quantization of energyenergy

Quantum Mechanical ModelQuantum Mechanical Model No longer No longer

individual scienceindividual science Teams work Teams work

together to solve together to solve problemsproblems

Current model Current model explains why the explains why the periodic table periodic table worksworks

Quarks & LeptonsQuarks & Leptons Newest theoryNewest theory Quarks make up Quarks make up

protons and protons and neutronsneutrons

Electrons are Electrons are another type of another type of matter called matter called leptonsleptons

Bosons create Bosons create forcesforces