Democritus first proposed the idea of atoms in 400 BC as the smallest indivisible particles of matter. In the early 1800s, John Dalton developed the first modern atomic theory, proposing that all matter is made of atoms that are identical for a given element. In the late 1800s and early 1900s, scientists like J.J. Thomson, Ernest Rutherford, and others discovered subatomic particles like electrons and the nucleus through experiments such as cathode ray tubes and bombarding atoms with alpha particles. These discoveries led to the modern understanding of atoms as a dense nucleus of protons and neutrons surrounded by electrons in orbits.

1. THE ATOMTHE ATOM

2. D e v e l o p i n g o u rD e v e l o p i n g o u r
m o d e l o f am o d e l o f a
n a t o mn a t o m

3. D e m o c r i t u sD e m o c r i t u s
 A Greek philosopher - in the year 400A Greek philosopher - in the year 400
B.C. described the atom as the smallestB.C. described the atom as the smallest
particle of a substance.particle of a substance.
 He used the wordHe used the word atomos (which meansatomos (which means
not to cut)not to cut) to describe the smallestto describe the smallest
possible particle of matter.possible particle of matter.
 Neither Plato nor Aristotle accepted theNeither Plato nor Aristotle accepted the
atomic concept.atomic concept.

4. J o h n D a l t o nJ o h n D a l t o n
Developed theDeveloped the
first modernfirst modern
atomic theory inatomic theory in
1803. Dalton is1803. Dalton is
referred to as thereferred to as the
father of modernfather of modern
atomic theory.atomic theory.

5. Dalton’s Atomic TheoryDalton’s Atomic Theory
 1) All matter is made of atoms. Atoms are1) All matter is made of atoms. Atoms are
indivisible and indestructible.indivisible and indestructible.
 2) All atoms of a given element are identical in2) All atoms of a given element are identical in
mass and propertiesmass and properties
 3) Compounds are formed by a combination of3) Compounds are formed by a combination of
two or more different kinds of atoms, whichtwo or more different kinds of atoms, which
combine in whole number ratios.combine in whole number ratios.
 4) Atoms can neither be created or destroyed.4) Atoms can neither be created or destroyed.

6. Dalton’s Atomic TheoryDalton’s Atomic Theory
 Modern atomic theory is, of course, a little moreModern atomic theory is, of course, a little more
involved than Dalton's theory but the basics ofinvolved than Dalton's theory but the basics of
Dalton's atomic concept remains valid.Dalton's atomic concept remains valid.
 We cetainly know today that atoms can be destroyedWe cetainly know today that atoms can be destroyed
by nuclear reactions and by bombarding the nuclei ofby nuclear reactions and by bombarding the nuclei of
atoms in high speed atomic accelerators (that bust theatoms in high speed atomic accelerators (that bust the
nuclei apart), but not by chemical reactions.nuclei apart), but not by chemical reactions.
 There are different kinds of atoms (that haveThere are different kinds of atoms (that have
different masses) for a particular element that aredifferent masses) for a particular element that are
known asknown as isotopes,isotopes, but isotopes of an element havebut isotopes of an element have
the same chemical properties.the same chemical properties.
 Many heretofore unexplained chemical phenomenaMany heretofore unexplained chemical phenomena
were quickly explained by Dalton with his theory.were quickly explained by Dalton with his theory.
Dalton's theory quickly became the theoreticalDalton's theory quickly became the theoretical
foundation in chemistry.foundation in chemistry.

7. CATHODE RAY TUBECATHODE RAY TUBE
DISCOVERED IN LATE 1800sDISCOVERED IN LATE 1800s
Cathode-ray tubes contain a pair of metal plates sealed into a
glass tube that has been partially evacuated. If the residual
pressure of the gas is small enough, the glass at the end of the
tube across from the cathode will glow when the tube is
connected to a series of batteries.

8. J.J. THOMSON AND THEJ.J. THOMSON AND THE
DISCOVERY OF THE ELECTRONDISCOVERY OF THE ELECTRON
 In 1897, J. J. Thomson foundIn 1897, J. J. Thomson found
that the cathode rays can bethat the cathode rays can be
deflected by an electric field,deflected by an electric field,
as shown at right. Byas shown at right. By
balancing the effect of abalancing the effect of a
magnetic field on a cathode-magnetic field on a cathode-
ray beam with an electricray beam with an electric
field, Thomson was able tofield, Thomson was able to
show that cathode "rays" areshow that cathode "rays" are
actually composed ofactually composed of
particles. This experimentparticles. This experiment
also provided an estimate ofalso provided an estimate of
the ratio of the charge to thethe ratio of the charge to the
mass of these particles.mass of these particles.
The cathode rays also can be deflected by
an electric field in a direction which
suggests they are negatively charged.
From: http://chemed.chem.purdue.edu/genchem/history/electron.html

9. J.J. THOMSON AND THEJ.J. THOMSON AND THE
DISCOVERY OF THE ELECTRONDISCOVERY OF THE ELECTRON
 Thomson found the sameThomson found the same charge-to-mass ratiocharge-to-mass ratio
regardless of the metal used to make the cathode andregardless of the metal used to make the cathode and
the anode. He also found the same charge-to-mass ratiothe anode. He also found the same charge-to-mass ratio
regardless of the gas used to fill the tube. He thereforeregardless of the gas used to fill the tube. He therefore
concluded that the particles given off by the cathode inconcluded that the particles given off by the cathode in
this experiment are a universal component of matter.this experiment are a universal component of matter.
Although Thomson called these particlesAlthough Thomson called these particles corpusclescorpuscles,,
the namethe name electronelectron, which had been proposed by, which had been proposed by
George StoneyGeorge Stoney several years earlier for theseveral years earlier for the
fundamental unit of negative electricity, was soonfundamental unit of negative electricity, was soon
accepted.accepted.
From: http://chemed.chem.purdue.edu/genchem/history/electron.html

10. THOMSON’S PLUM PUDDINGTHOMSON’S PLUM PUDDING
MODEL OF THE ATOMMODEL OF THE ATOM
 Since it was known at the time that atoms are neutral,Since it was known at the time that atoms are neutral,
there must be a positive charge to offset the negativethere must be a positive charge to offset the negative
charge of the electrons. Thecharge of the electrons. The plum pudding modelplum pudding model ofof
the atom was proposed by J. J. Thomson (thethe atom was proposed by J. J. Thomson (the
discoverer of the electron in 1897).discoverer of the electron in 1897). The plumThe plum
pudding model was proposed in 1906pudding model was proposed in 1906 by Thomsonby Thomson
before the discovery of the atomic nucleus. In thisbefore the discovery of the atomic nucleus. In this
model, the atom is composed of electrons (whichmodel, the atom is composed of electrons (which
Thomson still called corpuscles), surrounded by a soupThomson still called corpuscles), surrounded by a soup
of positive charge to balance the electron's negativeof positive charge to balance the electron's negative
charge, like plums surrounded by pudding. Thecharge, like plums surrounded by pudding. The
positive pudding was supposedly massless, thus sincepositive pudding was supposedly massless, thus since
the electron had a mass of 1/1840 of a hydrogen atom,the electron had a mass of 1/1840 of a hydrogen atom,
there would have to be 1840 electrons embedded in thethere would have to be 1840 electrons embedded in the
positive pudding to make up a hydrogen atom.positive pudding to make up a hydrogen atom.

11. Millikan’s 1909 Oil Drop ExperimentMillikan’s 1909 Oil Drop Experiment

12. MILLIKAN’S OIL DROP EXPERIMENTMILLIKAN’S OIL DROP EXPERIMENT
AND THE CHARGE AND MASS OF ANAND THE CHARGE AND MASS OF AN
ELECTRONELECTRON
 Millikan determined the fundamental charge ofMillikan determined the fundamental charge of
an electron to be -1.60 x 10an electron to be -1.60 x 10-19-19
Coulombs.Coulombs.
 He also determined the mass of an electron toHe also determined the mass of an electron to
be 9.11 x 10be 9.11 x 10-31-31
Kg (about 1/1840 the mass of aKg (about 1/1840 the mass of a
hydrogen atom).hydrogen atom).

13. ERNEST RUTHERFORD AND THEERNEST RUTHERFORD AND THE
DISCOVERY OF THE ATOMICDISCOVERY OF THE ATOMIC
NUCLEUSNUCLEUS
 Ernest Rtherford is the BritishErnest Rtherford is the British
physicist who inphysicist who in 1911 determined1911 determined
that the atom had a small densethat the atom had a small dense
nucleus.nucleus. Starting sometimeStarting sometime
around 1909, Rutherford beganaround 1909, Rutherford began
to notice that alpha particlesto notice that alpha particles
would not always behave inwould not always behave in
accordance to theaccordance to the plum puddingplum pudding
model of an atom when fired atmodel of an atom when fired at
a piece of gold foil. Thesea piece of gold foil. These
observations stimulated furtherobservations stimulated further
research that was eventuallyresearch that was eventually
published in 1911 and has beenpublished in 1911 and has been
known ever since asknown ever since as
Rutherford's Gold FoilRutherford's Gold Foil
ExperimentExperiment
((http://myweb.usf.edu/~mhight/goldfoil.htmlhttp://myweb.usf.edu/~mhight/goldfoil.html).).

14. RUTHERFORD (cont.)RUTHERFORD (cont.)
 Rutherford bombarded gold foil with radioactiveRutherford bombarded gold foil with radioactive
particles (alpha particles-positively charged). Heparticles (alpha particles-positively charged). He
expected the particles to pass right through (and mostexpected the particles to pass right through (and most
of them did) and was surprised that a few of the alphaof them did) and was surprised that a few of the alpha
particles were scattered at high angles.particles were scattered at high angles.
 From this work Rutherford determined that the alphaFrom this work Rutherford determined that the alpha
particles were deflected by a positively charged denseparticles were deflected by a positively charged dense
nucleus within a gold atom. So atoms must have anucleus within a gold atom. So atoms must have a
small, dense, positively charged nucleus.small, dense, positively charged nucleus.

15. RUTHERFORD’S GOLD FOILRUTHERFORD’S GOLD FOIL
EXPERIMENTEXPERIMENT

16. RUTHERFORD’SRUTHERFORD’S
INTERPRETATION OF RESULTSINTERPRETATION OF RESULTS
Expected based on Plum Pudding Model.
Actual Results.

17. RUTHERFORD (CONT.)RUTHERFORD (CONT.)
 Based on the scattering of alpha particles, RutherfordBased on the scattering of alpha particles, Rutherford
estimated that the positive nucleus must be orbited byestimated that the positive nucleus must be orbited by
electrons at a distance of 100,000 times the radius ofelectrons at a distance of 100,000 times the radius of
the nucleus (so,the nucleus (so, atoms are composed mostly ofatoms are composed mostly of
empty spaceempty space).).
 Within a few years Rutherford was able to isolate andWithin a few years Rutherford was able to isolate and
define the positive particles in the nucleus, which wedefine the positive particles in the nucleus, which we
now callnow call protonsprotons. He speculated on the existence of. He speculated on the existence of
neutral particles in the nucleus,neutral particles in the nucleus, neutronsneutrons..
 James ChadwickJames Chadwick isolated and identified the neutron inisolated and identified the neutron in
1932.1932.

18. MODERN MODEL OF THE ATOMMODERN MODEL OF THE ATOM
Protons – Red.
Positive Charge.
Neutrons – Green.
Neutral Charge.
Electrons – Tan.
Negative Charge.
Nucleus
Electron
Orbits

19. MODERN UNDERSTANDINGMODERN UNDERSTANDING
OF THE ATOMOF THE ATOM
 ATOMIC NUMBERATOMIC NUMBER
 ISOTOPESISOTOPES (EXAMPLES H, C, Cl)(EXAMPLES H, C, Cl)
 CARBON-12 AND ATOMIC MASS UNITSCARBON-12 AND ATOMIC MASS UNITS
 ATOMIC MASSATOMIC MASS OF A PARTICULAROF A PARTICULAR
ISOTOPE (IN ATOMIC MASS UNITS)ISOTOPE (IN ATOMIC MASS UNITS)
 ATOMIC MASS NUMBERATOMIC MASS NUMBER
 ATOMIC WEIGHTATOMIC WEIGHT (IN ATOMIC MASS(IN ATOMIC MASS
UNITS)UNITS)

20. ATOMIC SPECTRAATOMIC SPECTRA
 During the 1800s and into the early 1900s scientistsDuring the 1800s and into the early 1900s scientists
(such as Max Planck) were studying the nature of(such as Max Planck) were studying the nature of
electromagnetic radiation and its relationship to matter.electromagnetic radiation and its relationship to matter.
Around 1900, Max Planck was studying theAround 1900, Max Planck was studying the
electromagnetic radiation given off by hot solids. Thiselectromagnetic radiation given off by hot solids. This
radiation is referred to asradiation is referred to as black body radiationblack body radiation and isand is
dependent on the temperature of the body emittingdependent on the temperature of the body emitting
electromagnetic radiation.electromagnetic radiation.
 Black body radiation emitted from a hot solid, hotBlack body radiation emitted from a hot solid, hot
liquid, or hot dense gas and viewed through a prism orliquid, or hot dense gas and viewed through a prism or
spectroscopespectroscope (or(or spectrographspectrograph) forms a) forms a continuouscontinuous
spectrumspectrum in the visible light region of thein the visible light region of the
electromagnetic spectrumelectromagnetic spectrum..

21. ELECTROMAGNETICELECTROMAGNETIC
SPECTRUMSPECTRUM

22. The Electromagnetic SpectrumThe Electromagnetic Spectrum

23. DISPERSION OF LIGHT INTODISPERSION OF LIGHT INTO
DISCRETE WAVELENGTHS THROUGHDISCRETE WAVELENGTHS THROUGH
A PRISM OR SPECTROSCOPE (ORA PRISM OR SPECTROSCOPE (OR
SPECTROGRAPH)SPECTROGRAPH)

24. CONTINUOUS SPECTRUMCONTINUOUS SPECTRUM

25. EMISSION (BRIGHT LINEEMISSION (BRIGHT LINE
SPECTRA)SPECTRA)
 However, light from a low density hot gas isHowever, light from a low density hot gas is
dispersed into an emission (line) spectrum atdispersed into an emission (line) spectrum at
discrete wavelengths. The wavelengths of lightdiscrete wavelengths. The wavelengths of light
that are emitted are characteristic for thethat are emitted are characteristic for the
particular elements that are present in the gas.particular elements that are present in the gas.
Thus the emission (bright line) spectrum is aThus the emission (bright line) spectrum is a
fingerprint of the gas (or gases) that is (are)fingerprint of the gas (or gases) that is (are)
emitting the light.emitting the light.

26. EXAMPLES OF EMISSIONEXAMPLES OF EMISSION
(LINE) SPECTRA(LINE) SPECTRA
Hydrogen:
Helium:
Carbon:

27. ABSORPTION (DARK LINEABSORPTION (DARK LINE
SPECTRA)SPECTRA)
 If light from a dense hot gas, hot liquid, or hot solidIf light from a dense hot gas, hot liquid, or hot solid
passes through a region that contains a cool, diffusepasses through a region that contains a cool, diffuse
gas, then energy from the continuous spectrum will begas, then energy from the continuous spectrum will be
absorbed by the atoms in the cool gas. Theabsorbed by the atoms in the cool gas. The
wavelengths that are absorbed by the gas (or gases)wavelengths that are absorbed by the gas (or gases)
present are exactly the same as the wavelengths thesepresent are exactly the same as the wavelengths these
gases give off when they emit light. Since light is beinggases give off when they emit light. Since light is being
absorbed from the continuous spectrum, dark linesabsorbed from the continuous spectrum, dark lines
appear superimposed on the continusous spectrum.appear superimposed on the continusous spectrum.
This is called anThis is called an absorption (dark line) spectrum).absorption (dark line) spectrum).

28. ABSORPTION (DARK LINE)ABSORPTION (DARK LINE)
SPECTRASPECTRA

29. SUMMARY – THREE TYPES OFSUMMARY – THREE TYPES OF
SPECTRASPECTRA

30. THE BOHR MODEL OF THETHE BOHR MODEL OF THE
HYDROGEN ATOMHYDROGEN ATOM
 In 1913, the Danish physicistIn 1913, the Danish physicist
Niels Bohr (1885 - 1962)Niels Bohr (1885 - 1962)
managed to explain themanaged to explain the
spectrum of atomic hydrogenspectrum of atomic hydrogen
by an extension ofby an extension of
Rutherford's description ofRutherford's description of
the atom. In that model, thethe atom. In that model, the
negatively charged electronsnegatively charged electrons
revolve about the positivelyrevolve about the positively
charged atomic nucleuscharged atomic nucleus
because of the attractivebecause of the attractive
electrostatic force accordingelectrostatic force according
to Coulomb's law.to Coulomb's law.

31. THE BOHR MODEL OF THETHE BOHR MODEL OF THE
HYDROGEN ATOMHYDROGEN ATOM
 The Bohr model consists of four principles:The Bohr model consists of four principles:
 1)Electrons assume only certain orbits around the nucleus. These1)Electrons assume only certain orbits around the nucleus. These
orbits are stable and called "stationary" orbits.orbits are stable and called "stationary" orbits.
 2)Each orbit has an energy associated with it. For example the2)Each orbit has an energy associated with it. For example the
orbit closest to the nucleus has the lowest energy (theorbit closest to the nucleus has the lowest energy (the groundground
statestate), the next closest orbit (the), the next closest orbit (the first exicted statefirst exicted state) has a) has a
certain higher energy, and so on. (the electron is defined to havecertain higher energy, and so on. (the electron is defined to have
zero energy when it escapes from the hydrogen atom, thus thezero energy when it escapes from the hydrogen atom, thus the
energies of the electron positions in the hydrogen atom areenergies of the electron positions in the hydrogen atom are
negative energies)negative energies)
 3)Light is emitted as a photon when an electron jumps from a3)Light is emitted as a photon when an electron jumps from a
higher orbit to a lower orbit and absorbed (as a photon) when ithigher orbit to a lower orbit and absorbed (as a photon) when it
jumps from a lower to higher orbit.jumps from a lower to higher orbit.
 4)The energy and frequency of light emitted or absorbed is given4)The energy and frequency of light emitted or absorbed is given
by the difference between the two orbit energies.by the difference between the two orbit energies.

32. Atomic Excitation and De-excitationAtomic Excitation and De-excitation
 Atoms can make transitions between the orbitsAtoms can make transitions between the orbits
allowed byallowed by Bohr ModelBohr Model (also by the(also by the QuantumQuantum
Mechanics Model) by absorbing or emittingMechanics Model) by absorbing or emitting
exactly the energy difference between the orbits.exactly the energy difference between the orbits.
The following figure shows an atomic excitationThe following figure shows an atomic excitation
caused by absorption of a photon and an atomiccaused by absorption of a photon and an atomic
de-excitation caused by emission of a photon.de-excitation caused by emission of a photon.

33. Atomic Excitation and De-excitation (Decay)Atomic Excitation and De-excitation (Decay)

34. Atomic Excitation and De-excitationAtomic Excitation and De-excitation
 In each case the wavelength of the emitted orIn each case the wavelength of the emitted or
absorbed light is exactly such that the photonabsorbed light is exactly such that the photon
carries the energy difference between the twocarries the energy difference between the two
orbits. This energy may be calculated by dividingorbits. This energy may be calculated by dividing
the product of the Planck constant and thethe product of the Planck constant and the
speed of lightspeed of light hchc by the wavelength of the light).by the wavelength of the light).
Thus, an atom can absorb or emit only certainThus, an atom can absorb or emit only certain
discrete wavelengths (or equivalently,discrete wavelengths (or equivalently,
frequencies or energies).frequencies or energies).

35. THE BALMER SERIESTHE BALMER SERIES
 Bohr determined that the line spectrum fromBohr determined that the line spectrum from
hydrogen gas gave four lines in the visiblehydrogen gas gave four lines in the visible
portion of the spectrum that corresponded toportion of the spectrum that corresponded to
jumps from n=3 to n=2, n = 4 to n = 2, n =5 tojumps from n=3 to n=2, n = 4 to n = 2, n =5 to
n = 2, and n=6 to n =2 (corresponding to then = 2, and n=6 to n =2 (corresponding to the
emission lines red, blue-green, violet, and violet,emission lines red, blue-green, violet, and violet,
respectively). Remember n = 1 is the groundrespectively). Remember n = 1 is the ground
state orbital, n = 2 is the first excited statestate orbital, n = 2 is the first excited state
orbital, etc. This sequence of lines is called theorbital, etc. This sequence of lines is called the
Balmer SeriesBalmer Series (named for J. J. Balmer who first(named for J. J. Balmer who first
studied the hydrogen spectrum).studied the hydrogen spectrum).

36. THE BALMER SERIES – ALSOTHE BALMER SERIES – ALSO
See figure 8.11 in textSee figure 8.11 in text

37. BOHR’S MODEL INCOMPLETEBOHR’S MODEL INCOMPLETE
 Although Bohr’s solar system model of the atomAlthough Bohr’s solar system model of the atom
made excellent predictions for the energy ofmade excellent predictions for the energy of
light emitted and absorbed from the hydrogenlight emitted and absorbed from the hydrogen
atom, it did not work well for higher atomicatom, it did not work well for higher atomic
numbered elements. Although Bohr’s modelnumbered elements. Although Bohr’s model
did predict correctly that photons for otherdid predict correctly that photons for other
elements would be absorbed or emitted aselements would be absorbed or emitted as
electrons jumped to higher or lower orbits.electrons jumped to higher or lower orbits.
 Bohr’s model considered the electron as aBohr’s model considered the electron as a
discrete particle at a certain distance from thediscrete particle at a certain distance from the
nucleus and that it was contained in anucleus and that it was contained in a
radiationless orbit. He could not demonstrateradiationless orbit. He could not demonstrate
that this was the case.that this was the case.

38. Quantum Mechanical ModelQuantum Mechanical Model
 Soon after Bohr’s work, a more complex and completeSoon after Bohr’s work, a more complex and complete
model of the atom emerged that considered both themodel of the atom emerged that considered both the
particle nature and wave nature of the electron (andparticle nature and wave nature of the electron (and
also of light). Electrons were demonstrated to behavealso of light). Electrons were demonstrated to behave
as standing waves in allowed orbits. This moreas standing waves in allowed orbits. This more
complex and more correct model of the atom is calledcomplex and more correct model of the atom is called
thethe Quantum Mechanical ModelQuantum Mechanical Model..
 However, for the work that we will do in this class andHowever, for the work that we will do in this class and
for many problems in chemistry, the simple solarfor many problems in chemistry, the simple solar
system model works fine and makes valid predictions.system model works fine and makes valid predictions.