1. Models of the AtomModels of the Atom
September 24September 24thth
& 25& 25thth
, 2009, 2009

2. Ancient GreeceAncient Greece
300 BC in Greece300 BC in Greece
School of thought that matter is made upSchool of thought that matter is made up
of tiny indivisible, invisible, indestructible,of tiny indivisible, invisible, indestructible,
fundamental units of matter called atomsfundamental units of matter called atoms
(means “that which cannot be divided”)(means “that which cannot be divided”)
Democritus of Abdera most well knownDemocritus of Abdera most well known
atomist.atomist.
Did no experimentsDid no experiments
No proofNo proof
PhilosopherPhilosopher

3. 1770s1770s
Antoine Lavosier made observations and didAntoine Lavosier made observations and did
experiments resulting in the Law ofexperiments resulting in the Law of
Conservation of MassConservation of Mass
Law of Conservation of MassLaw of Conservation of Mass
Matter cannot be createdMatter cannot be created
or destroyedor destroyed
Mass of materials before theMass of materials before the
chemical reaction is the samechemical reaction is the same
mass after the reactionmass after the reaction

4. 1799- Joseph Proust1799- Joseph Proust
Showed that the proportion by mass of theShowed that the proportion by mass of the
elements in a compound is always the same (COelements in a compound is always the same (CO
vs. COvs. CO22))
Now called the Law of Definite ProportionsNow called the Law of Definite Proportions
Part of our definition of a chemical - any substancePart of our definition of a chemical - any substance
with a definite compositionwith a definite composition

5. 1808- John Dalton1808- John Dalton
English School TeacherEnglish School Teacher
Wanted to learn in what ratios differentWanted to learn in what ratios different
elements combineelements combine
Based on the Greek idea of the atomBased on the Greek idea of the atom
Atoms were tiny indestructible particlesAtoms were tiny indestructible particles
Which clay model is this?Which clay model is this?

6. Dalton’s Atomic TheoryDalton’s Atomic Theory
1.1. All matter is made of indivisible, invisible, andAll matter is made of indivisible, invisible, and
indestructible particles called atomsindestructible particles called atoms
2.2. Atoms of the same element are identicalAtoms of the same element are identical
3.3. Atoms of different elements differ in their physical andAtoms of different elements differ in their physical and
chemical propertieschemical properties
4.4. Atoms of different elements combine in simple wholeAtoms of different elements combine in simple whole
number ratios to form compoundsnumber ratios to form compounds
5.5. Chemical Reactions occur when atoms are separated,Chemical Reactions occur when atoms are separated,
joined or rearranged. Atoms of one element are NEVERjoined or rearranged. Atoms of one element are NEVER
changed into atoms of another element.changed into atoms of another element.

7. Problems with Dalton’s ModelProblems with Dalton’s Model
Atoms are not indestructible, they are made ofAtoms are not indestructible, they are made of
smaller parts like protons, neutrons, electrons andsmaller parts like protons, neutrons, electrons and
quarksquarks
Atoms of the same element are notAtoms of the same element are not completelycompletely
identical, isotope exist with different massesidentical, isotope exist with different masses
Atoms of the same element can combine with eachAtoms of the same element can combine with each
otherother
Atoms can “turn into” other elements throughAtoms can “turn into” other elements through
nuclear decay (though there are byproducts)nuclear decay (though there are byproducts)

8. 18971897
Sir J.J.ThomsonSir J.J.Thomson
Proposed that the cathodeProposed that the cathode
ray formed in the tube is aray formed in the tube is a
stream of negativelystream of negatively
charged particlescharged particles
Atoms have parts that areAtoms have parts that are
negatively chargednegatively charged
Since atoms are neutral,Since atoms are neutral,
they must also havethey must also have
positively charged areaspositively charged areas
Gave rise to the plumGave rise to the plum
pudding model of the atompudding model of the atom

9. 1909 -Ernst Rutherford1909 -Ernst Rutherford
Experimented with the positively charged particleExperimented with the positively charged particle
He figuredHe figured
Atoms have no chargeAtoms have no charge
Electrical charges are properties of matterElectrical charges are properties of matter
Electrical charges exist in single whole numberElectrical charges exist in single whole number
units + or - (no fractions)units + or - (no fractions)
Electrical Charges cancel each other, thereforeElectrical Charges cancel each other, therefore
a + and a - together = neutral atoma + and a - together = neutral atom

10. Rutherford’s modelRutherford’s model
Key Idea -NUCLEAR ATOMKey Idea -NUCLEAR ATOM
All of the mass of the positive particles (protons)All of the mass of the positive particles (protons)
is at the center region of the atomis at the center region of the atom
Center region called the NUCLEUSCenter region called the NUCLEUS
Electrons surround the nucleus in a “cloud”Electrons surround the nucleus in a “cloud”

11. Plum PuddingPlum Pudding vsvs NuclearNuclear
Which clay models are these?Which clay models are these?

12. Bohr’s Atom- 1913Bohr’s Atom- 1913
There are certain energy levels thatThere are certain energy levels that
mathematically allow the electron to stay inmathematically allow the electron to stay in
the cloud (not crash into the nucleus)the cloud (not crash into the nucleus)
Key idea - There are certain circular ORBITS inKey idea - There are certain circular ORBITS in
which an electron can travel around thewhich an electron can travel around the
nucleus without losing energynucleus without losing energy
The farther away from the nucleus, the higherThe farther away from the nucleus, the higher
the energy levelthe energy level

13. What do weWhat do we
know now??know now??
All models are not exactly correct, but they lead toAll models are not exactly correct, but they lead to
further understanding and discoveryfurther understanding and discovery
All have important key ideasAll have important key ideas
Atoms are made up of subparticlesAtoms are made up of subparticles
Atoms are divisible, but not by ordinary chemical meansAtoms are divisible, but not by ordinary chemical means
Atoms of elements can vary (isotopes)Atoms of elements can vary (isotopes)
Electrons reside in ORBITALS, not orbitsElectrons reside in ORBITALS, not orbits

14. What happens when you change…What happens when you change…
Protons: Since the number of protons is the same as theProtons: Since the number of protons is the same as the
atomic number, protons are what define an element.atomic number, protons are what define an element.
If you change the protons you change the elementIf you change the protons you change the element
Neutrons: The neutrons add to the mass of the element.Neutrons: The neutrons add to the mass of the element.
If you change the neutrons you change the mass ofIf you change the neutrons you change the mass of
the element (make an isotope)the element (make an isotope)
Electrons: The electrons balance the charge of theElectrons: The electrons balance the charge of the
protons.protons.
If you change the electrons you change the charge ofIf you change the electrons you change the charge of
the element (make an ion)the element (make an ion)

15. Quotes about atomsQuotes about atoms
FromFrom A Short History of Nearly EverythingA Short History of Nearly Everything
by Bill Brysonby Bill Bryson
““protons give an atom itsprotons give an atom its
identity, electrons itsidentity, electrons its
personality” p. 140personality” p. 140
““if an atom were expandedif an atom were expanded
to the size of a cathedral,to the size of a cathedral,
the nucleus would be onlythe nucleus would be only
about the size of a fly- but aabout the size of a fly- but a
fly many thousands of timesfly many thousands of times
heavier than the cathedral”heavier than the cathedral”
p. 141p. 141