1. Models of the Atom A Historical Perspective
CHEMISTRY
J.P. HANCOCK, B.S., M.Ed

2. EVOLUTION OF ATOM

3. Early Greek Theories fire air water earth
Democritus
400 B.C. - Democritus thought matter could not be divided indefinitely.
Derived from the greek word atomos=uncuttable
fire
air
water
earth
Aristotle
350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air.
Aristotle was wrong. However, his theory persisted for 2000 years.

4. DALTON’S ATOMIC THEORY
1800 –John Dalton proposed a modern atomic model
based on experimentation not on pure reason.
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

5. Size of an Atom
A pure copper coin the size of a penny has 2.4x1022 atoms
Earth’s population is 6x109 people
4x1012 times as many atoms in the coin as people on Earth
Radii of most atoms
5x10-11m to 2x10-10m

6. History of Current Atomic Model
John Dalton’s Model(1803)
English chemist and school teacher ( )
Studied ratios of atoms in chemical reactions
Atom was solid and indivisible
“Billiard Ball” model
Eugen Goldstein 1886
Discovered protons
J.J.Thomson’s Model (1897)
New Zealand physicist
Atom is positive sphere with negative particles embedded throughout.
Plum Pudding Model-like a plum pudding or chocolate chip cookie

7. JJ Thomson’s Experiment
Passed an electric current through low pressure gases sealed in glass tubes with electrodes at both ends
Anode Cathode -
Caused a glowing beam(cathode ray)
Electrons are discovered
Opposites attract!!

8. Atom has central nucleus
Hantaro Nagaoka(1904)
Japanese physicist
Atom has central nucleus
Electrons move around in orbits like rings around Saturn
Rutherford Model (1911)
called the Nuclear Atom
Most of the atom’s mass was in a positive central sphere surrounded by negative electrons
Protons and neutrons are located in nucleus
Electrons are around the nucleus and occupy almost all volume of atom

9. Ernest Rutherford (movie: 10 min.)
Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance
path of invisible -particles
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass.

10. Bohr’s Model-1913 Niels Bohr Danish Physicist & student of Rutherford
Electrons orbit the nucleus in “shells”
Electrons can be bumped up to a higher shell if hit by an electron or a photon of light called a Quantum of Energy
Robert A. Millikan-1916
U.S. Physicist
Calculated mass of electron
Louis de Broglie-1923
French Physicist
Electrons have some properties of waves as well as particles

11. Quantum Mechanical Model 1926
Erwin SchrÖdinger
Austrian physicist( )
Used mathematical equations to determine where electrons are in the atom
Electron Cloud Model- “probability cloud”
Determined the energies of electrons and how likely to find their location around nucleus
James Chadwick-1932
English physicist
Confirmed existence neutrons
No charge with a mass near the proton’s

12. THE ATOM

13. THE ATOM Basic building block of all matter
Elements on periodic table are represented as one neutral atom
Chemical symbols are abbreviated ways of representing elements
C, H, N, O
Na, Cl, Ne, Cu
NaCl, H2O, CO2

14. Parts of the Atom Protons Neutrons Electrons
Quarks= smallest particles arranged in
groups of threes to make p+, e-, or n0

15. 6 Types of Quarks

16. Properties of Subatomic Particles
Particle Symbol Charge Relative Actual Mass
Mass
Electron e / x10-26 g
Proton p x10-24 g
Neutron n x10-24 g

17. Location of Subatomic Particles
10-13 cm
electrons
protons
neutrons
10-8 cm
nucleus
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18. 1st Periodic Table
Dmitri Mendeleev- a Russian chemist who arranged elements in order of their increasing atomic masses

20. Henry G.J.Moseley determined the atomic number for each known element
Modern Periodic Table
Father of Modern Periodic Table
1913 British Physicists
Henry G.J.Moseley determined the atomic number for each known element
Arranged the periodic table according to increasing atomic numbers

21. The Periodic Table

22. Periodic Table
Arrangement of atoms in increasing order of atomic numbers
Represents physical and chemical behavior of elements
Collects atoms with similar properties in vertical columns
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23. Properties of elements in any vertical column do differ
Properties of elements in any vertical column do differ. The properties are similar but not identical in chemical behavior.
Adjacent elements in each horizontal row(period) differ in both chemical and physical properties but the properties change in a regular way across each period.

24. Vertical columns = families or groups
1-18
Have similar properties due to similar electron arrangements
Horizontal rows= periods or series
Elements to the left of the “stair step” line are metals except hydrogen
Elements to the right of the “stair step” line are non metals

25. Why do elements in a group have similar properties?
Similar electron arrangements
While atoms of different elements have diff. # of e-. The atoms of diff. e- may have the same # of e- in their outer energy levels.

26. The number of e- in the outer energy level determines the chemical properties of the element i.e. different elements w/ the same # of e- in the outer energy levels will have similar chemical properties!

27. Electron Cloud Structure
The goal of every atom is stability.
OCTET RULE
An atom is stable when it has 8 valence electrons(a full outer energy level)
Hydrogen and Helium are the exceptions to the rule
The valence electrons will determine the reactivity of the element
Valence electrons are electrons in the outermost energy level

28. PERIODIC TABLE

29. Atomic Symbols
Atomic Number
Symbol
Element Name
Atomic Mass Number

30. Atomic Number
The number of protons in the nucleus of an atom All atoms of an element have the same number of protons
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31. Number of Electrons An atom is neutral The net charge is zero
Number of protons = Number of electrons
Atomic number can be used to infer the Number of electrons
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32. Mass Number
(protons + neutrons)
Approx. equal to the mass of the nucleus in atomic mass units(amu).
Whole number of the amu

33. Number of Neutrons (mass number – atomic number)
In a neutral atom the number of e- and number of p+ are equal
Every atom of the same element has the same number of protons
MASS NUMBER =ATOMIC MASS

34. Atomic Mass Units
Do we now know how much atoms really weigh, in pounds or grams?
Yes, but we don't normally use those units for measuring the mass of an atom. It's much more convenient to use something called the atomic mass unit, or amu.

35. Using AMU is simplier!
One gram is about 600,000,000,000,000,000,000,000 amu (that's 600 sextillion, or a 6 followed by 23 zeros)
A pound is just shy of 300 septillion amu
that’s, 300,000,000,000,000,000,000,000,000 (3 followed by 26 zeros)

36. Na 22.99 Atomic Mass Unit Listed on the periodic table
Gives the mass of “average” atom of each element compared to 12C
Average atom based on all the isotopes and their abundance %
Atomic mass is not a whole number
1amu= x grams
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37. RECAP

38. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
atomic number is the same
mass number is different
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37
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39. Protium Deuterium Tritium
ISOTOPES
Protium Deuterium Tritium

40. Calculating # of Neutrons
Protons + Neutrons = Mass Number
6p+ + 6no = 12, the mass number of the first isotope of carbon 6p+ + 7no = 13, the mass number of the second isotope of carbon 6p+ + 8no = 14, the mass number of the third isotope of carbon

43. Chapter 4 Vocabulary page 121-(15 words plus the 11 below) Quantum
Quantum Mechanical Model
Bohr Model
Electron Cloud Model
Thomson Model
Dalton Model
Rutherford Model
Stability
Octet Rule
Orbital
Energy Levels