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Unit II –Atomic Theory
Democritus
• 460 B.C. – 370 B.C.
• 400 B.C. – First to
hypothesize that
matter was composed
of atoms
Antoine-Laurent de Lavoisier
• 1743-1794
• Father of Chemistry
• Performed experiment
leading to the Law of
Conservation of Mass
Joseph Louis Proust
• 1754-1826
• Performed experiment
leading to the Law of
Definite Proportions
John Dalton
• 1766-1844
• 1803-Responsible for the
Billiard Ball model of the
atom
• Father of the Atomic
Theory which he based
on experimental evidence
Atomic Theory
• All Matter is composed of atoms which are
indivisible
• All atoms of the same element are
identical
• Atoms of different elements are different
• Atomic units are in definite proportions to
form compounds
Joseph John “J.J.” Thomson
• 1856-1940
• 1897-Responsible for the
Plum Pudding model of
the atom
• Discovered the electron
•
(1st subatomic particle
discovered)
Late 1800’s - Electrons
were embedded in a
positive dough
Eugen Goldstein
• 1850-1930
• 1886-Discovered the
proton
• Worked with anode
rays of H+ was
apparently the first
observation of the
proton
James Chadwick
• 1891-1974
• 1932 – Discovered
the neutron
Ernest Rutherford
• 1871-1937
• Responsible for the Nuclear
Model of the atom
• 1909 – Performed the Gold
Foil Experiment
• Discovered the atom had a
•
•
•
nucleus
Protons were in the nucleus
The atom was mostly space
Electrons were around the
nucleus
Niels Bohr
• 1885-1962
• 1913 – Proposed the Planetary
Model of the atom
• Electrons had a definite path
around the nucleus called an
orbit
• Electrons were arranged around
the nucleus according to their
energy
• Electrons with the lowest energy
are near the nucleus
Werner Heisenberg
• 1901-1976
• Responsible for the
Heisenberg Uncertainty
Principle (1927)
• Stated that the exact
position of electrons
cannot be pinpointed
Erwin Schrödinger
• 1887-1961
• 1926- Formulated a
mathematical formula
that describes the
probable area (called
orbitals) where
electrons are most
likely located
Many scientists contributed to our
most current understanding of the
atom
• 1923 - Quantum
Mechanical Model
• Modern Language Electron Cloud
• Electrons are located
anywhere within an
area 95% of the time
Subatomic Particles
• Nucleons – particles located in the nucleus
Name
Symbol
Relative
Mass
Charge
Position
Proton
1H
p+
1 amu
1
Nucleus
Electron
e-
0 amu
(see below)
-1
Outside
nucleus
Neutron
n
1 amu
0
Nucleus
•
•
•
•
•
, H+, or
Actual mass of an electron = 9.1x10-31 Kg
Impractical number to use
Protons and neutrons are 1836 times heavier
amu - atomic mass unit
Based on carbon-12; which is 1/12 the mass of carbon-12 (C-12)
Nuclear Symbol
Atomic Mass 
Atomic Number 
14
7
N
Can also be written as nitrogen - 14
• Volume of the atom – determined by the
electrons
• Atomic mass or Mass Number
• protons + neutrons
• In an atom (which are neutral)
• #protons = #electrons
• Ion – charged particle (formed when atoms
which have lost or gained electrons)
• #protons ≠ #electrons
• Ca+2 has 20 protons and 18 electrons
• N-3 has 7 protons and 10 electrons
Isotopes
• Atoms of the same element (same number of
•
•
•
protons (atomic number)) with a different
number of neutrons and therefore a different
mass.
Similar to small, medium and large shirts.
All shirts (same element) just different sizes.
Ex. Isotopes of hydrogen
• Protium
• Deuterium
• Tritium
1 proton, 0 neutrons
1 proton, 1 neutron
1 proton, 2 neutrons
Atomic Mass = 1 amu
Atomic Mass = 2 amu
Atomic Mass = 3 amu
• Give 2 ways to write the isotopes of hydrogen
Average Atomic Mass
• If the mass of a proton and a neutron is 1
amu and the mass of an electron is too
small to consider the why is the mass of
hydrogen on the periodic table 1.00794?
• Answer – It is an average of all the
isotopes found in the earth’s crust.
Calculating Average Atomic Mass
• Imagine three isotopes of the element
Cycreekium exist
220Cc
• 200Cc
• In a sample
230Cc
• 10% of atoms weigh 200 amu
• 20% of atoms weigh 220 amu
• 70% of atoms weigh 230 amu
• Find the average atomic mass for Cc
• Total of (%) X (mass)
Calculating Average Atomic Mass
• 10% X 200 = .10 X 200 =
• 20% X 220 = .20 X 220 =
• 70% X 230 = .70 X 230 =
Total
20
44
161
225
amu
amu
amu
amu
• Average of naturally occurring isotopes explains
why the atomic mass is usually not an even
number on the periodic table.