Sec.
4.1
Ch 4:
History of Atomic Structure
I. Our concept of the nature of the atom has undergone ∆ and
will probably continue to do so.
A.
Empedokles (490-430 B.C.) http://en.wikipedia.org/wiki/Empedocles
1. A Greek, pre-Socratic philosopher that attempted to
describe the things around us.
2. Established the idea of the four roots (elements):
Fire, Water, Air, Earth
a. Fire: associated with active and enthusiastic
b. Water: associated with sad and brooding
c. Air: associated with irritable and changeable
d. Earth: associated with apathetic and sluggish
3. Believed that everything contained different ratios
of these four elements.
a. Examples:
i. Stone  contains high amount of earth
ii. Rabbit  contains higher ratio of water
and fire
4. Problems:
a. No matter how many times you break a stone
in half, the pieces never resemble any of the
core elements
5. Importance:
a. First to suggest that some substances looked
like pure materials, while other materials such
as a rabbit, are actually made up of a
combination of different “elements”
B.
Democritus (400-370 B.C.) http://en.wikipedia.org/wiki/Democritus
1. A Greek, pre-Socratic philosopher who formulated
the atomic theory for the cosmos
a. Cosmos: describe the natural order of the
universe
2. Democritus’ ideas were based on reasoning rather
than science
3. Many believe he is the “father of modern science”
4. Believed that
a. everything is composed of “atomos” or
“atoms”, which are infinitesimally small
pieces of matter.
b. between atoms lies empty space
c. atoms are indestructible
d. atoms have been and always will be in motion
e. there are an infinite number of atoms as well
as kinds of atoms that differ in size and shape
5. Limitations
a. No tools to help confirm his ideas
b. His ideas were based on his beliefs and
reasoning rather than observation
Stone
Fur
C.
Aristotle (384 – 322 B.C.) http://galileoandeinstein.physics.virginia.edu/lectures/aristot2.html
1. Accepted the theory of Empedokles and added his
own (incorrect) idea that the four core elements
could be transformed into one another
2. Because of his great influence Democritus’ theory
would have to wait 2,000 years before being
rediscovered
D.
Dalton and Dalton’s Modern Atomic Theory (1800s)
http://www.visionlearning.com/library/module_viewer.php?mid=49
1. All matter is composed of extremely small,
indivisible particles called “atoms”
(Democritus)
a. Atoms resemble tiny billiard-ball-like
particles
2. Atoms of a given element are identical; atoms of
different elements differ in size, mass, and
chemical properties
3. Law of Definite Proportions: compounds are
formed by chemical combinations of different
atoms in whole-number ratios; the ratio by mass of
elements in a compound is constant no matter the
size of the sample (Joseph Proust)
Ex:
H2O is always H2O
it cannot be: H2.5O or H2O2
or any other ratio except a 2:1 ratio of H to O
4.Law of Multiple Proportions: Can form more
than one kind of compound with the same two
elements by varying their ratios
Ex: H2O and H2O2
Note that these are two DIFFERENT compounds
(water and hydrogen peroxide) yet they are made with
the same kinds of atoms; what differs is the ratio of
atoms
5. Law of Conservation of Matter: In chemical
reactions, atoms are combined, separated, or
rearranged but never created or destroyed; mass
in all reactions remains constant
(Antoine Lavoisier)
Ex: Given the reaction: A + B  C + D
If I react 10.0g of reactant A with 20.0g of
reactant B and form 25.0 g of product C, how
much of product D must I have made?
II.
Corrections to Dalton’s Atomic Theory
A. Atoms are the smallest particle of matter that still
retains the identity of that matter
B. Atoms ARE divisible
1. have a central nucleus which contains
2.
a.
protons
b.
neutrons
have electrons orbiting outside the nucleus
C.
Isotopes exist
Atoms of the same element may contain different
numbers of neutrons, which causes the mass to vary
for a given atom type (or element)
Ex: C-12 (“normal” carbon atoms)
and
C-14 (heavier, “radioactive” carbon atoms)
III. How did we come to know that atoms were made of
smaller, subatomic particles?
A.
JJThomson (1897) “plum pudding model”
http://www.chem.uiuc.edu/clcwebsite/cathode.html
1.
worked with gases in cathode ray tubes.
2.
deflected the glowing gases by using electrically
charged metal plates (magnets)
a. noticed that the gases always bent away
from the negatively charged plate
3.
knowing that opposite charges attract and like
charges repel, he proposed that a cathode ray is
a stream of negatively charged particles which he
called corpuscles, which are now called
electrons.
4.
experimented with many types of gases and
observed the same phenomenon
5.
B.
plum pudding model: knowing that all
atoms are neutral, this led to idea that Dalton’s
“solid spheres” are perhaps “solid, positively
charged spheres with dots of negative charges
embedded in it”
Robert Millikan(1909) found the exact charge and
mass of an electron by using an oil drop apparatus
http://www.whfreeman.com/modphysics/PDF/3-1bw.pdf
1.
2.
charge is -1
mass is 1/1836 of a proton or neutron
C.
Eugen Goldstein made an amendment to JJ
Thompsons plum pudding model:
1 . Found rays travelling in the opposite direction to
that of the cathode ray; called these rays canal
rays
2.
3.
D.
Concluded that canal rays were composed of
positively charged particles called “protons”
individual protons were calculated to weigh 1836
times the mass of an electron; also calculated to
have an equal but opposite charge as electrons
(+1)
Ernest Rutherford’s Gold Foil Experiment(1911)
1. procedure: bombarded thin sheets of metal
Au (foil) with radioactive, positively-charged
alpha particles
2. according to Thompsons theory the alpha
particles should have passed easily through
the gold with every particle experiencing a
slight deflection due to the positive charge
thought to be spread out in the gold atoms
3. surprising observations:
a. most of the alpha particles traveled
without deflection through the thin sheet
of gold metal foil
b. some alpha particles traveled through
the gold foil but deflected from their
straight-line path
c.
3.
a lesser amount of alpha particles were
deflected straight back from the Au-foil
conclusion:
a. atoms contain mostly space
b. there must be a tiny, positively-charged,
dense central region to an atom; he called
this central region a “nucleus”
c. nucleus contains protons and neutrons.
Neutrons were discovered later by James
Chadwick. This will be discussed later.
IV.
Subatomic Particles
amu = atomic mass unit = 1/12 carbon-12 atom
V.
Subatomic Particles On Periodic Table
(in a sneaky way…)
A.
Atomic Number
1. Equals:
a. the number of protons in the
nucleus
b. the nuclear charge
Ex.
Nuclear charge of Nitrogen is +7
c. the number of electrons because
elements are always neutral
2.
3.
B.
Mass number
1. Equals:
a. the number of protons and neutrons
2. will always be a whole number
3. used to help calculate number of
neutrons present in an atom
Ex.
Carbon’s mass number = 12
Carbon’s atomic number = 6
12 – 6 = 6 neutrons
4.
C.
D.
defines an element
never changes
can be represented in many different
ways
Ex.
12C or C-12 or Carbon-12
Atomic Mass
1. also known as average atomic mass
2. is the weighted average of all
NATURALLY occurring isotopes
3. is shown as a decimal on the periodic
table
4. indicates the mass number of the most
abundant isotope
This will be discussed later.
Ion
1. is a charged atom created by gaining or
losing electrons
2.
3.
4.
if an atom loses an electron  positive
ion
if an atom gains an electron  negative
ion
how or why would an atom gain or lose
electrons?
a. you’ll just have to wait until we get
to the periodic table and bonding
units
VI. Isotopes (Iso = “same”) (back to Dalton’s theory)
A.
B.
C.
D.
Atoms of the SAME element that have
different numbers of neutrons (same # of
protons & electrons)
Isotopes differ only in mass, not their
chemical properties
Explains why the atomic masses are not
whole numbers; the atomic masses on the
periodic table are the AVERAGE of all of the
naturally occurring isotopes that exist for an
element
Example: hydrogen (H) naturally contains
3 isotopes:
1. protium =
2.
deuterium =
3.
tritium =
E.
Example calculation
How many neutrons are in the following
isotopes of this element?
8X
Ques:
F.
8X
8X
What is the name of the element
above?
Can calculate the average atomic mass of an
element’s atoms if percentages of its
different isotope abundances are known
Example:
What is the average atomic mass of an element if
20.0% of the atoms have a mass of 176amu, 30.0%
have a mass of 177amu, and 50.0% have a mass
of 178amu?
Answer:
(%)(isotope’s amu) + (%)(isotope’s amu) + (%)(isotope’s
amu)
(.20 x 176amu) + (.30 x 177amu) + (.50 x 178amu)
177.3 amu (no sig digs required!!)