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Unit 2 – Atomic Theory
Chapter 4 & 11.2
Unit Test:
Democritus
Lived around 400 B.C.
Came up with the concept of the atom
John Dalton - 1807
Known for Billiard Ball Model
Called the father of Atomic Theory
Dalton’s Atomic Theory - 1807
All matter is composed of atoms which are indivisible
All atoms of the same element are identical
Atoms of different elements are different
Atoms bond in different proportions to form compounds (Law of Definite Proportions)
J. J. Thomson - 1903
Discovered the electron (1st subatomic particle) through experiments with cathode ray tube
Plum Pudding model (or Chocolate Chip Cookie model)
Thomson’s Model
“Pudding” or “Cookie” is the positive charge and most of the mass of the atom
“Plums” or “Chocolate Chips” are the scattered electrons
POSITIVE CHARGE ELECTRON
S
Ernest Rutherford - 1911
Nuclear Model (atom contains a nucleus)
Gold foil Experiment
Rutherford’s Experiment
Rutherford’s Model
Atoms have: A nucleus Protons (positive
charge) in nucleus Mostly open space Electrons found
somewhere around the nucleus
Niels Bohr - 1913
Planetary Model Electrons (e-) have
definite path around the nucleus (orbit)
e- arranged around the nucleus according to energy level
e- with lowest energy level are closest to nucleus
Bohr’s Model
Quantum Mechanical Model - 1923
Electron Cloud (modern theory) Calculates the probability of finding the
electron within a given space Electrons, instead of traveling in
defined orbits, travel in diffuse clouds around the nucleus
Quantum Mechanical Model
Present Model of the Hydrogen Atom
Stepwise Timeline of Atomic Theory
Dalton 1803
Thomson1897
Rutherford1909
Bohr1913
Modern Theory
Subatomic Particles
amu – atomic mass unit; based on carbon-12
1 amu = 1/12 mass of C-12 = mass H Impractical to use actual mass of subatomic
particles
Name Symbol Relative Mass
Charge Position
Proton 1H or p+ 1 amu 1 Nucleus
Electron e- 0 amu -1 Outside
Neutron 1n 1 amu 0 Nucleus
Elements of the Periodic Table
Atomic Number Identifies element # protons # electrons in neutral
atom Atomic Mass
a.k.a. mass number # protons + # neutrons
Atomic Number
Atomic Symbol
Atomic Mass
Isotope Notation
Element Symbol with mass number and atomic number
Can also be the element name dash mass number
XMass
Number
Atomic Number
or Element- Mass
Practice
Symbol # Protons
# neutrons
# electrons
Atomic Number
Mass Number
9 10 9 9 19
28 59
150 94
30 65
F199
C146
Isotopes
Atoms of the same element with different numbers of neutrons
Think of it as different sized shirts!6 neutrons Carbon-12
7 neutrons Carbon-13
8 neutrons Carbon-14C
C
C
146
136
126
Isotopes
Average Atomic Mass
Mass listed on the periodic table Determined by averaging the masses
of all naturally occurring isotopes of that element
Explains why the atomic mass is usually not a whole number
Finding Average Atomic Mass
You are told there are 3 isotopes of Cycreekium and their percent abundance: Cycreekium-20; 10% Cycreekium-22; 20% Cycreekium-23; 70%
Find their average atomic mass!
Finding Average Atomic Mass
Formula: % abundance as decimal × atomic mass for each isotope; sum results for avg. atomic mass.
10% × 20amu = 0.10 × 20amu = 2.0amu
20% × 22amu = 0.20 × 22amu = 4.4amu
70% × 23amu = 0.70 × 23amu = 16.1amu
Total: 22.5amu
Percent Abundance
If you are given information about an elements isotope you can estimate the most abundant isotope!
Example: Carbon-12, Carbon-13, Carbon-14. Look at the atomic mass on the periodic table. Which isotope is the mass closest to?
End of Unit 2 Notes!
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