Atomic Structure

1
Atomic Structure
2
Chemical Discoveries

• Democritus - a Greek philosopher that proposed
  that matter is composed of small indivisible
  particles called atoms.

3
Alchemy

• Alchemy dominated the next 2000 years of chemical
  history.
• Alchemists were often mystics obsessed with
  turning cheap metals into gold.
• Several elements were discovered during this
  period.

4
Robert Boyle (1627-1691)

• The first chemist to perform truly quantitative
  experiments.
• He measured the relationship between the pressure
  and volume of air.
• Established the accepted
  definition of an element.
• An Englishman.

5
Joseph Priestly (1733-1804)

• An English clergyman and scientist.
• His interest in combustion reactions lead to the
  discovery and identity of oxygen and several
  other gases.

6
Antonie Lavoisier (1743-1794)

• Known as the Father of Chemistry.
• He brought chemistry out of the misconceptions of
  the 18th century into more modern ideas such as
  the importance of accurate measurement and the
  composition of the atmosphere.
• He formulated the Law of Conservation of Mass.
• He was a Frenchman.

7
John Dalton (1766-1844)

• An English teacher,
  meteorologist, and chemist.
• His interest lead to a study of gases and the Law
  of Partial Pressure.
• He developed the Law of Multiple Proportions
• He proposed the first meaningful atomic theory.

8
Daltons Atomic Theory

• John Dalton performed experiments to study the
  ratio in which elements combine in chemical
  reactions
• He then formulated a hypotheses and theories to
  explain his observations

9
Daltons Atomic Theory

• All elements are composed of indivisible
  particles called ATOMS
• Atoms of the same ELEMENT are identical
• ATOMS of any one ELEMENT are different from those
  of any other element
• Atoms of different elements can physically mix
  together or chemically combine to form compounds
• Chemical reactions occur when atoms are joined,
  separated or rearranged
• Atoms of one element are never changed into atoms
  of another by a chemical reaction

10
What did Dalton base his theory on?

• Law of Conservation of Mass (Antoine Lavosier,
  1789)
• Atoms are neither created or destroyed (under
  normal chemical reactions)
• Law of Definite Proportion, (Proust's Law)
• Atom ratio is fixed, so mass must be constant.
• Law of Multiple Proportions (Dalton)
• Applies where two elements, A and B, form more
  than one compound.

11
How Big is an Atom

• Imagine a penny, made up of copper (Cu)
• If we ground up the penny into dust, the dust
  would still have the same properties as copper

If you placed atoms of copper in a row, 100 000
000 atoms would make a line 1 centimeter long
12
How Big is an Atom?

• A penny contains around 2.4 x 1022 atoms of
  copper (Cu)
• Can we see atoms?
• To see atoms we need a scanning tunneling
  electron microscope or an ion beam microscope

13

• A micrograph of a Nickel-Molybdenum (Ni4Mo)
  bi-metallic compound, each dot is a single atom
• The specimen is in the form of a needle that is
  over 1000 times sharper than an ordinary
  household sewing needle. In fact, the end of the
  needle is so sharp that it cannot be seen by the
  naked eye or even a standard optical microscope

http//www.ornl.gov
14
Atomic Diameter Units

• The typical atom has a diameter on the order of 1
  - 3 x 10-10 m
• Angstrom (Å) 10-10 m
• (non-standard)
• Nanometers (nm) 10-9 m
• Picometers (pm) 10-12 m
• Typical atom diameter 1 - 3 Å, 0.1 0.3 nm or
  100 300 pm

15
Whats Wrong with DAT?

• Most of Daltons theory is accepted today
• However, We now know that atoms ARE divisible
• Atoms contain sub-atomic particles
• ELECTRONS
• PROTONS
• NEUTRONS

16
Electrons

• Discovered in 1887 by J. J. Thomson
• Thompson initially called electrons CATHODE RAYS

Negative
Positive
Vacuum
High Voltage
17

• Thomson then added electrical plates above and
  below the tube
• He found that positively charged plates ATTRACTED
  the ray, negatively charged plates repel
• Cathode ray made up of tiny negatively charged
  particles

18

• 1900 Electron defined as a small charged
  particle approximately 1/2000 the mass of a
  Hydrogen atom
• 1916 Robert A. Millikan determined the charge
  carried by an electron and the charge to mass
  ratio
• Look up Milikans Oil Drop Experiment

19
The Electron Today

• Symbol e-
• Relative mass 1/1840
• (compared to a proton)
• Actual mass 9.11 x 10-28g
• Relative electrical charge 1-
• Actual electrical charge -1.602 x 10-19 C

20
Protons

• 1866 E. Goldstein found evidence of positively
  charged particles traveling in the opposite
  direction to electrons in a cathode ray tube
• He named them CANAL RAYS
• He deduced that canal rays were positively
  charged particles we now know as PROTONS

21
Proton Summary

• Symbol p
• Relative mass 1
• Actual mass 1.67 x 10-24g
• (1840 x me)
• Relative electrical charge 1
• Actual electrical charge 1.602 x 10-19 C

22
Neutrons

• 1932 James Chadwick discovers the neutron
• Symbol n
• Relative mass 1
• Actual mass 1.67 x 10-24g
• (1840 x me)
• Relative electrical charge 0
• Actual electrical charge 0C

23
Rutherfords Experiment

• 1911, University of Manchester, England
• A beam of a-particles (a Helium nucleus He 2e-)
  aimed at a gold foil
• Foil surrounded by fluorescent screen which
  flashed when hit by an a-particle
• Experiment done in a vacuum

24
Vacuum
Flourescent Screen
Au Foil
a-source
c. Rutherford Appleton Laboratories
25
Rutherfords Results
26
Conclusions

• Rutherford concluded that the atom is mostly
  empty space
• However, the atom contains a nucleus where the
  mass and positive charge of the atom are
  concentrated
• We now know that the nucleus contains PROTONS and
  NEUTRONS
• Electrons ORBIT the nucleus but are not part of
  it

27
Review

• What do you think of these statements?
• All atoms are identical
• Chemical Reactions occur when atoms of one
  element change into atoms of another
• An atom has protons, neutrons and electrons in
  the nucleus