1. The Nuclear Atom
Atomic Scientists’ Song 2:52

2. You need to understand and be able to explain the “evolution” of the atom. No names or dates are necessary

3. Aristotle (460 B.C. – 370 B.C.)
emphasized that nature consisted of four elements: air, earth, fire, and water
did not believe in discontinuous or separate atoms, but felt that matter was continuous

4. Democritus (460 B.C. – 370 B.C.)
first to suggest the existence of “atoms” (Greek word “atomos” = indivisible)
atoms are indivisible and indestructible
no experimental support

5. John Dalton ( )
used scientific method to test Democritus’s ideas
Dalton’s atomic theory
elements composed of atoms
atoms of the same element are alike
different atoms can combine in ratios to form compounds
chemical reactions can occur when atoms are separated, joined, or rearranged (but atoms are not created nor destroyed)

6. J.J. Thompson (1856-1940) discovered the electron
thought atom was negative charges stuck in a positive charged lump
referred to as the “plum pudding model”

7. Robert A. Millikan ( )
found the quantity of charge carried by an electron (one unit of negative charge)
calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

8. Ernest Rutherford ( )
“Like howitzer shells bouncing off of tissue paper!”
proposed that the atom is mostly empty space
positive charges and almost all of the mass are in a small, centralized region called the nucleus

9. Rutherford Flash Animation

11. Try it Yourself!
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? ?

12. The Answers
Target #1
Target #2

13. Niels Bohr ( )
electrons found only in specific circular paths (orbits) around the nucleus
based on information about how the energy of an atom changes when it absorbs and emits light
called these fixed energies “energy levels”

15. Erwin Schrodinger (1926) quantum mechanical model
probability of electron locations around the nucleus
not an exact orbit
eventually became the electron cloud model

16. Schrödinger's Cat video 1:41

17. Werner Heisenberg (1927)
Heisenberg Uncertainty Principle – impossible to know the exact position and momentum of an electron at the same time
“the observer affects the observed”

23. Structure of the Atom

25. (considered negligible)
Particle
Charge
Mass
(atomic mass units)
Location
Proton
+ 1 1
nucleus
Neutron Ø
Electron
- 1
5.0 x 10-4
(considered negligible)
orbit, level, cloud

26. Carbon- 12 as a standard carbon- 12
ALL masses on the periodic table are based on their relationship to carbon-12
the carbon- 12 isotope has been given the atomic weight of exactly and is used as the basis upon which the atomic weight of other isotopes is determined

27. Even smaller particles
quarks
make up protons & neutrons
6 types He

28. Isotopes

31. Learning Check An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is
C. The element is
1) Si 2) Ca 3) Se
D. The number of electrons in a neutral atom is
1) ) ) 20

32. same element but differ in their number of neutrons
the atomic mass on periodic table is the WEIGHTED AVERAGE MASS of “all” the isotopes of that element
this is based on an isotope’s natural abundance
the percentage of each isotope of an element that occurs in nature
isotopes have the same chemical properties (reactivity) but different physical properties (density, melting/boiling point…)

36. 2.3

38. Chemical symbols for isotopes
two different ways to write isotopes
example for sodium
sodium- 23
only shows mass number (23) of the sodium isotope
23 Na
shows the mass number (23) and the atomic # (11) of the sodium isotope 11
Mass Number X A
Element Symbol
Atomic Number Z

39. Isotopes?
Which of the following represent isotopes of the same element? Which element?
234 X X 235 X 238 X
92 is the element uranium

40. Do You Understand Isotopes?
How many protons, neutrons, and electrons are in-- C 14 6
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in-- C 11 6
6 protons, 5 (11 - 6) neutrons, 6 electrons

41. Radioactive Isotopes (don’t need to know this)
unstable isotopes that break down over time
uses:
cobalt 60
radiation treatment for cancer
carbon 14
used to date objects up to 60,000
years old
iodine 125 and iodine 131
ingested and used for
medical imaging
Dangerous, but worth the risk

42. The Mass Spectrometer

44. Mass spectrometer video (2:26)
has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element
the relative atomic mass can be calculated from the data from the mass spectrometer
Mass spectrometer video (2:26)

46. magnesium results from the mass spectrometer:
Calculate the relative atomic mass of magnesium with the provided data.
magnesium results from the mass spectrometer:
80% 24Mg
10% 25Mg
10% 26Mg
just a simple weighted mean
.80(24) + .10(25) + .10(26) = 24.3 amu