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The Atom
Chapters 4-5
Atomic Theories
Democritus ~ 400 BC believed that atoms were indivisible and
indestructible
Dalton ~ 1800’s Developed through experiments First Atomic Model
Dalton’s Atomic Model
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
Dalton’s Atomic Model (cont)
Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
Discovery of Electron
1897 – JJ Thomson, using cathode ray tube, discovered negatively charged particles called electrons
1909 – Robert Millikan - Oil Drop Experiment Determined charge on an electron.
“Plum Pudding” Model
Uniform positive sphere with negatively charged electrons embedded within.
Radiation
Late 1800’s – discovery of radiation
Three Types Alpha Beta Gamma
Rutherford Gold Foil Experiment - 1909
Shot alpha particles at gold foil Most went through foil with little or no
deflection. Some were deflected at large angle and
some straight back.
A.K.A. Geiger Marsden Experiment
Rutherford Gold Foil Experiment - 1909
Rutherford Model
Conclusions from Gold Foil Experiment Atom is Mostly Empty Space Dense positive nucleus Electrons moving randomly around nucleus
Subatomic Particles
Electron Discovered in 1897 by JJ Thomson Negative charge (-1) Mass = 9.109389*10-28g Approx mass ~ 0 Found outside of nucleus
Subatomic Particles
Proton Discovered in 1919 by Rutherford Positive charge (+1) Mass = 1.672623*10-24g Approx mass ~ 1 atomic mass unit (u) Found inside nucleus
Subatomic Particles
Neutron Discovered in 1932 by James Chadwick No charge (0) Mass = 1.6749286*10-24g Approx mass ~ 1 atomic mass unit (u) Just slightly larger than a proton Found inside nucleus
Atomic Structure
Atoms have no net charge
# of electrons = # of protons
# of electrons around nucleus = # of protons in nucleus
Atomic Structure
Atomic Number Number of protons in an element All atoms of the same element have the same
number of protons
Mass Number Number of protons and neutrons in an atom
Atomic Structure
# of Neutrons = Mass Number – Atomic Number
Atoms of the same elements can have different numbers of neutrons
Isotope – atoms of the same element with different number of neutrons
Chemical Symbols
Cl-35 Chlorine-35
Cl3517
Mass Number
Atomic Number
Ion
Atom or group of atoms that have gained or lost one or more electrons Have a charge
Example: H+, Ca2+, Cl-, OH-
Ions
H+ 1 proton 0 electrons
Ca2+ 20 protons 18 electrons
Cl- 17 protons 18 electrons
OH- 9 protons 10 electrons
Atomic Theories
Rutherford’s model could not explain the chemical properties of elements
Niels Bohr believed Rutherford’s model needed to be improved
Bohr proposed that electrons are found only in circular paths around the nucleus
Bohr Model
Dense positive nucleus
Electrons in specified circular paths, called energy levels
These energy levels gave results in agreement with experiments for the hydrogen atom.
Bohr Model
Bohr Model
Each energy level can only hold up to a certain number of electrons
Level 1 2 electrons Level 2 8 electrons Level 3 18 electrons Level 4 32 electrons
Electron Configuration
The way in which electrons are arranged in the atom
Example: Na 2-8-1
Valence Electrons Electrons in the outermost energy level
Wave Mechanical Model
More detailed view of the Bohr Model
Schrödinger Wave Equation and Heisenberg Uncertainty provides region of high probability where electron COULD be. Orbital
Modern Model AKA Quantum Mechanical Model, Electron Cloud Model
Wave Mechanical Model
Orbital Regions of space where there is a high probability
of finding an electron
M&M’s Demo
What colors are found in a regular M&M’s bag? Green Yellow Orange Blue Red Brown
M&M’s Demo
Do you get an equal amount of each color in each bag?
If we opened up all the regular M&M bags in the world would we get an equal number of each color?
Are you supposed to?
M&M’s Demo
Color 1 bag World
Blue % 24%
Green % 16%
Yellow % 14%
Orange % 20%
Red % 13%
Brown % 13%
M&M’s Demo
M&M’s come in certain abundances (percentages)
So do isotopes of each element
Relative Abundance Percent of each naturally occurring isotope found
in nature
Average Atomic Mass
Atomic Mass Weighted average based on the relative
abundance and mass number for all naturally occurring isotopes
Example C-12 98.9% 12.011u C-13 1.1%
Atomic Mass
C-12 98.9% C-13 1.1%
Carbon = 0.989*12 + 0.011*13 = 12.011u