Unit 3: Atomic Theory
History of the Atom
• Democritus Greek Philosopher
around 400B.C. was the first to
consider the atom; he said that all
matter would never disappear so
these particles had to exist He named
them Atomos
• Atoms are tiny individual particles
that have their own characteristics.
The Dalton Model/Postulates
• Proposed the 1st Atomic theory in 1800’s
• 1. All elements are made up of tiny indivisible
particles called atoms.
• 2. Atoms of the same element are identical. The
atoms of one element are different from the atoms
of another element.
• 3. Atoms of different elements chemically
combine to form chemical compounds.
• 4. During chemical reactions, atoms are
rearranged. Atoms of one element cannot be
changed into atoms of a different element as a
result of a chemical change.
The new theory leads to
discovery of new laws.
• Law of Conservation of Mass: which
states that mass is neither destroyed nor
created during ordinary chemical reaction or
physical change.
• Law of definite proportions: A chemical
compound contains the same elements in
the same proportions by mass regardless of
the size of the sample.
Laws continued
• Law of multiple proportions: If two or
more different compounds are composed of
the same two elements, then the weight of
the first is in proportion to the second
element.
• Example: CO
•
1 to 1
CO2
CO3
1 to 2
1 to 3
Cathode Rays and
Electrons
• J.J. Thomson
• (Thomson was awarded the Nobel
Prize in 1906 for his "discovery" of the
first sub-atomic particle; the electron.)
Robert Millikan’s Oil-Drop
Experiment (1909)
Discovered the mass to charge ratio of the electron
( how much negative charge there is for the mass of
the electron)
Ernest Rutherford (1911)
• 1st one to challenge
Dalton’s Model
• Proposed that atoms are
not dense, but rather
have a dense nucleus and
mostly empty space
• Designed the gold foil
experiment
• Called his model the
Plum-pudding model
Gold Foil Experiment
Rutherford’s Revised model
• Prediction Model
before
experiment
• Model after
experiment
James Chadwick
• He discovered the
neutral particle we
know as the neutron
in 1932.
• Won Noble Prize for
it in Physics in 1935.
Niels Bohr
• Proposed the orbital
model for the atom in
1913.
• He said that the atom
has individual
particles in the
nucleus (nucleons)
• Also, electrons roam
around the nucleus
in specific patterns
called orbitals or
shells
Energy Levels and Electrons
• According to Niels Bohr, each energy level
has a certain number of electrons
• #e- = 2n2 ; where n = energy level
–
–
–
–
–
1st Level – 2 electrons
2nd Level – 8 electrons
3rd Level – 18 electrons
4th Level – 32 electrons
*5th Level and on… -- 32 electrons per level
The Modern View of
Atomic Structure
Erwin Rudolf Josef Alexander Schrödinger
was an Austrian physicist that wrote a number of
fundamental equations that results in the field of
quantum theory, which formed the basis of wave
mechanics and the modern view of the atom. Father
of Modern Atomic Theory. He worked along with
Heisenberg
• Atomic mass unit (amu)
– 1/12 of the mass of a 12C
– 1.6605410-24 g
• Size: 1-5 Å (10-10 m)
Electron
Cloud
Model
Atoms are made up from:
• Proton -A positively charged particle (+)
(Rutherford)
• Neutron - A particle that has no charge (0)
(Chadwick)
• Nucleus - Contains protons and neutrons
(Rutherford)
• Electron - A negatively charged particle that
rotates around the nucleus in an area called
the electron cloud. Orbitals are referred to
as shells.
(Thomson, Millikan, Bohr)
Atomic Number and Mass
•The ATOMIC NUMBER is the number that each
element is given and where you find it on the periodic
table.
•It is also the number of protons in the element and the
number of electrons (when the element is neutral).
•The MASS NUMBER is determined by the number of
protons and neutrons. In AMU’s ( Atomic Mass Units)
•Note: You can identify the atomic number in any
periodic table as it is the smallest number of the two
listed.
Atomic Mass (Average)
• The Atomic mass is the average weight of
each element.
• It is determined by the sum of the mass of
each isotope of that element that have
been multiplied by their relative
abundance.
• Relative abundance is the percent
abundance divided by 100. (decimal form
of the percent)
Atomic Mass Continued
• Ex. 50.54% of the naturally occurring isotopes of
bromine have an atomic mass of 78.92 (Br-79)
while 49.46% of the rest naturally occurring
isotopes of bromine have an atomic mass of
80.92 (Br-81).
• Calculate the average atomic mass of bromine,
showing all work:
• Ave. At. Mass =
[(0.5054)x(78.92)]+ [(0.4946)x(80.92)]
= 79.91
amu
Atomic Mass Continued
*Notice that the percent gets turned into relative
abundance by dividing by 100, also all of the
percents from all of the isotopes add up to 100%
or 1 for relative abundances
Atomic Symbol
Atomic mass = Neutrons +
Protons
4
He
2
Element
symbol
Atomic Number = Protons
and also electrons in a
neutral element
Isotopes
• Isotopes are formed from the same element ,
but different atomic weight. Each contain a
different number of neutrons
• Example: Iodine-131 has 53 protons and 78
neutrons. The original element has 53 protons
and 74 neutrons.
Isotopes (continued)
• Isotope notation
– Two forms:
• Name – Mass
• Symbol – Mass
• Ex.
– Potassium – 40 (19 protons, 19
electrons, 21 neutrons)
– O – 17 (8 protons, 8 electrons,
__ neutrons)
Ions
• Ions are formed when electrons are
added or removed from a neutral
element.
• Positive ions are formed by removing
electrons
• Negative ions are formed by adding
electrons
Ions (continued)
• Ex.
– O-2 means 2 electrons have been
added to a neutral oxygen atom.
(8 protons, 8 neutrons, 10 electrons)
– K+1 means 1 electron has been
removed from a neutral Potassium
atom. (19 protons, 20 neutrons,
__ electrons)
A and Z
• The Atomic number (Z) is equal to the
number of protons in that element.
• The number of neutrons plus the number of
protons equal the mass number. (A)
• Find the mass number of Oxygen
• Oxygen is element 8 that means 8 p +8n=
16 amu’s.
How atoms are composed
Atom Strucuture
Atom
Outside the atom
you
Have
Elecrons (-)
Nucleus
Center of the atom
Protons (+)
Neutrons (0)
Located in the
center of the nucleus
Protons and neutrons
determine
mass of each element
mass in (AMU) atomiic mass units
How an Atom Looks
Electron(-)
Proton(+)
neutron(0)
nucleus