Atomic Theory Subatomic Particles Regions of the Atom Back to Notes
Reading Assignment 1: KTU & Read Ch. 4.1 & 4.2 p101-109. The current model of the atom shows
the nucleus to be 1/10,000 times smaller than the atom itself. Create a model, like this one, that relates the
nucleus to the atom.
I. A History of the Atomic Theory- “How Small is Small”
A. Democritus- Greek- "atomos" (means: indivisible) "All matter is composed of indivisible units."
Atoms of Salt, Water & Iron. The chemical properties were conserved in the structure of the atoms
B.
John
Dalton- English Physicist (1766-1844)
1. All elements are composed of atoms and the atom is the smallest piece of that element
2.All atoms of the same element are the same. These atoms then have the same mass.
Law of Definite Proportions ( Law of Constant Composition) - All compounds are composed of specific ratios of elements.
Resource: Law of Definite Proportions
-This was actually coined by Joseph Proust, which Dalton used to develop his theory
4. Chemical reactions occur when atoms are separated, joined or rearranged.
on-your-own practice. Quiz over Dalton's Atomic Theory
II. Composition of the Atom & Historical Atomic Models
Model: Simplified View of the Subatomic Particles.
Video: How small is the atom- TedEd
A. Electron- negative particles, very small
1. William Crookes (1832-1919)- Cathode Ray Tube- (Crookes Tube)
-cathode rays:
a. travel from cathode to anode in straight lines
b. heated metals placed in path of rays
c. deflected by electric & magnetic charges- shows rays are made of negative particles.
2.
J.J.
Thomson (1856-1940)- 1897 England-
Using
Crookes Tube
Thomson's experiment lead to identification of the electron.
Video: The discovery of the electron
Resource: Results of Thomson's experiment.
-measured deflection of electrons in magnetic & electric fields-
Image- Thomson's plum pudding model
Video: finding the ratio of charge to mass (e/m)
-determined the ratio was constant regardless of the gas used.
Thomson's presentation for the Nobel Prize, 1906
3. R.A. Millikan- 1909- using X-ray induced charged oil droplets
-found the charge and mass of electrons- mass = 0.00055 a.m.u (1/1837 amu)
Movie: Oil drop experiment, the charge of an electron
B. Proton- positive particles- more massive than electrons
1. Eugen Goldstein- 1886- Using modified Crookes Tube.
- observes that a cathode-ray tube produces, in addition to the cathode ray, radiation that travels in the opposite direction
- away from the anode; these rays are called canal rays because of holes (canals) bored in the cathode; later these will be
found to be ions that have had electrons stripped in producing the cathode ray.
2. Wilhelm Wien- 1889- found charge to mass ratio
-ratio
is smaller than that of the electron but varied with kind of gas
-Hydrogen has smallest ratio- mass of proton = 1.0073 a.m.u
Video: Canal Ray tube experiment-- Goldstein & Wein
Video: Canal Ray tube
C. Neutron- neutral in charge- mass is comparable to proton
1. James Chadwick- (1891-1974)- 1932, finding the neutron
-bombarded beryllium atoms with high velocity alpha particles and emitted uncharged particles:
The reaction of the experiment can
be written in the form:
42He
+ 94Be -----> 126C + 10n
-unstable when not in atom- decay into proton and electron
- Here is an outline of Chadwick's experiment
Video: Discovery of the neutron
D. Elementary Particles- particles that make up the subatomic particles
The Sub-Atomic Zoo. A comprehensive outline of elementary particles
The Particle Adventure- a trip into defining elementary/fundamental particles
Assignment 1: Lesson Check questions 1-15
Reading Assignment 2: KTU & Read Ch. 4.3 pp.112-119. Answer questions p. 121: 26-34
A. Nucleus- central dense region containing proton/neutrons.
1. Ernest Rutherford Gold Foil Experiment (1871-1937)- 1911- (more on Rutherford)
-Bombarded gold foil with high velocity alpha particles. Some particles were deflected, scattered, showing a dense
region within atoms containing positive region.
Rutherford's paper on a - particle scattering.
Video: Discovering the nucleus
Movie: Scattering radioactive particles
Movie: Rutherford's experiment
Image: Disproving plum-pudding
2. Henry Moseley- (1887-1915)- 1913- Moseley's Experiment
-Using an X-ray tube (modified cathode ray tube) to identify the number of protons in varying elements.
a. Atomic Number (Z)- The number of protons found in nucleus
b. Mass Number- The number of protons + neutrons in an atoms
-a.m.u.- atomic mass unit -
1/12 of Carbon atom ~ 1.67 x 10-24 g
-written at upper left of chemical symbol.
c. Isotopes- elements with varying number of neutrons
-to determine # of neutrons = mass number – atomic number
Resource: Isotopes of the Elements. Allows you to search for specific isotopes
Mass Spectrometers are tools used to identify isotope concentrations within an elemental sample.
resource: How are mass spectrometers used to find atomic mass? Here.
d. Atomic Mass- the average of the known masses of all isotopes and their occurrences in nature.
|
Calculating Average Atomic Mass |
|
Atomic Mass = S (Ai * mi)n Ai = isotopes abundance mi = isotopes mass n = number of isotopes to averaged. |
Assignment 2: Calculating Atomic Mass.
in-class Practice: Finding Subatomic Particles in atoms/isotopes
on your own practice: Finding Subatomic Particles. Problems 1-29. (#13 is incorrect)
Assignment 3: Ch. 4 Review questions, 54, 55, 60, 64, & 71 and Standard test prep- questions 1- 9
Additional Links
Complete Review of The History of the Atomic Model and a preview of The Modern Atomic Model by Jim Walker
Highly recommended link for you to visit.
Timeline of Atomic Theory advancements
