Presentation on theme: "History of Atomic Theory"— Presentation transcript:
1 History of Atomic Theory DemocritusB.C.ancient Greek philosopherbelieved all matter consisted of extremely small particles that could not be dividedatoms, from Greek word atomos, means “uncut” or “indivisible”Aristotlebelieved all matter came from only four elements—earth, air, fire and water
2 Who Was Right? Greek society was slave based No experiments It was all a thought gameSettled disagreements by argumentAristotle was more famous so he wonHis ideas carried through to the middle ages.
3 John Dalton (Late 1700’s) School teacher in England Based his conclusions on experimentation and observations.Combined ideas of elements with that of atoms
4 Dalton’s Atomic Theory All elements are composed of submicroscopic indivisible parts called atoms.Atoms of the same element are identical, those of different atoms are different.Atoms of different elements combine in whole number ratios to form compounds.Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.
5 Parts of Atoms Most of Dalton’s theory is accepted today. Except the part about atoms being indivisible
6 J.J. Thomson and the Cathode Ray Tube 1897 English physicistProvided the first evidence that atoms are made of even smaller particlesDescription of a cathode ray tube and a short video of how it works:
10 Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative to the positive end.
11 Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative to the positive end.
12 Thomson’s ExperimentBy adding an electric field
13 Thompson’s Experiment By adding an electric field
14 Thompson’s Experiment By adding an electric field
15 Thompson’s Experiment By adding an electric field he found the moving particles were negative
16 Thompson’s Model Found the electron 1 unit of negative chargeMass 1/2000 of hydrogen atomLater refined by Millikan to 1/1840Concluded that there must be a positive charge since atom was neutralAtom was like plum puddingA bunch of positive stuff, with electrons able to be removed.
17 Other PiecesProton – positively charged pieces 1,840 times heavier than the electronNeutron – no charge but the same mass as a proton.
18 Ernest Rutherford Former student of J.J. Thomson Believed in plum puddingWanted to find out how big they areFired positively charged alpha particles at a piece of gold foil, which can be made a few atoms thick
19 Rutherford’s Experiment When alpha particles hit a flourescent screen it will glow.Here’s what it looked like (pg. 90)
25 How he explained it Atom is mostly empty Small dense, positive piece at the centerAlpha particles are deflected if they get close enough to positive center
26 Niels Bohr ( )Electrons have orbits about the nucleus (planetary theory)Electrons could only exist at given energy levelsAn energy level is where an electron is likely to be movingEnergy levels were like steps on a ladderAn electron can only be at any given step at any given time
27 Modern Atomic TheoryBohr Model—shows electrons in orbit around protons and neutronsQuantum-mechanical model—doesn’t show exact location of electrons, just probable place
28 Structure of the Atom There are two regions The nucleus Electron cloud Protons and neutronsPositive chargeAlmost all of the massElectron cloudMost of the volume of an atomRegion where electron can be found
30 Counting the pieces Atomic number = number of protons Same as the number of electrons in a neutral atomMass number = the number of protons + neutrons
31 Atomic Mass Unit AMU Mass of a proton = 1.67 x 10 -27g A pretty inconvenient numberNew unit referenced to mass of an isotope of carbon: carbon -12Carbon-12 has 6 protons and 6 neutronsHas a mass of amu – an atomic mass unitTherefore 1 proton and 1 neutron has a mass of 1 amu.
32 So why not whole numbers for atomic masses in periodic table? Reported numbers are average atomic mass units, reflecting the abundance of isotopes for any given number.In nature most elements occur as a mixture of two or more isotopes
33 IsotopesAtoms of the same element can have different numbers of neutronsDifferent mass numbersCalled isotopes