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The Atom
Mr. Sackman South Dade Senior High
2010
History
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The Greeks
Dalton
J.J. Thompson
Millikan
Rutherford
Chadwick
Bohr
History
• The Greeks were the first to attempt to describe matter and
atoms
• Philosophers were intellectual thinkers of the time and
anything that they said many believed without argument
• The Greeks first classified matter as Earth, Wind, Water,
and Fire
• Their ideas were creative, however, there was no way to
test their theories at the time; another reason many just
accepted what philosophers said
• They believed that matter could endlessly, meaning
infinitely, be divided into smaller and smaller pieces with
no end
History
• Democritus
– First to propose that matter isn’t infinitely divisible
– Believed matter was made of tiny particles called
atomos (atoms)
– Believed atoms could not be created, destroyed,
or further divided
– Matter is composed of empty space through
which atoms move
History
– Atoms are solid, homogenous, and indivisible
– Different kinds of atoms have different shapes and
sizes
– The differing properties of matter are due to the
size, shape, and movement of atoms
– Changes in matter can only be caused by changes
in grouping of atoms and not from changes in the
atoms themselves
– His thinking was way ahead of his time and some
of his ideas still hold
History
• Aristotle (384 B.C.-322B.C.)
– One of the most influential minds of his time
– Gained wide acceptance for his view on nature
– What ever he stated most accepted, or believed,
to be fact or true
– He rejected atomic theory all together simply
because of his own ideas didn’t agree
– His major argument was that matter isn't empty
space through which atoms move, he didn’t
believe nothingness could exist
History
– Democritus was unable to answers challenges to
his ideas paving the way for Aristotle's beliefs
– Democritus' ideas were then eventually thrown
out
– Aristotle’s theory was accepted and he threw out
the existence of atoms altogether
– Because of Aristotle’s influence the answer to the
question of the acceptance, or denial, of atoms
went unchallenged for ~2000 years
History
• John Dalton (1766-1844)
– Finally thousands of years later someone
attempted to describe the atom
– He marked the beginning of modern atomic
theory
– Science now allowed for the study of matter and
attempted to prove the existence of atoms
– Proposed new atomic theory in 1803
– Some of his theories were the same as
Democritus
History
-Dalton’s Atomic Theory states the following
-All matter is composed of extremely small
particles called atoms
-All atoms of the same element are identical and atoms of different elements differ
completely from others
-Atoms cannot be created, destroyed, or divided
-Different numbers of atoms combine in simple whole number ratios to form
compounds and in chemical reactions atoms are separated, combined, or
rearranged
History
• Recall the law of conservation of mass? Dalton's
theory easily explains this law by stating that atoms
are only separated or rearranged in reactions which
would neither create or destroy atoms
• The law of definite proportions states that no matter
how large the sample is a compound is always
composed of the same elements in the same
proportion by mass. For example water is always 11%
H and 89% O no matter how large or small the
sample
• Dalton had used some form of technology and
the new aged science to refine Democritus'
theory
• Dalton observed and recorded numerous
reactions making careful observations, and
measurements, as he performed his
experiments, does this process sound
familiar?
History
– Using Lavoisier's, and Proust’s, ideas he had come
up with his own
– Is his theory completely correct?
– How did his theory differ from that of the Greeks?
– What do you think gave him the advantage of
acceptance of his theory at that time?
Defining the Atom
• What is the actual definition of an atom?
• What does an atom look like?
• Can you picture something that is so small you
can’t see?
Defining the Atom
• Suppose you decide one day you want to grind
your pure silver necklace down, how far could
you grind?
• Could you eventually grind down far enough
that you reach something that is not divisible
or visible?
• Does every smaller and smaller piece you
grind retain the properties of what you are
grinding down?
Defining the Atom
• Exactly how small is an atom?
– The book gives a very good example and it is
• Consider the size of the population in the world which
in 2000 was about 6 billion or 6,000,000,000 now
compare that to how many Cu atoms are in a penny
which is 29,000,000,000,000,000,000,000
• This is almost 5 billion times more copper atoms than
people
Defining the Atom
• Now take the diameter of that same penny,
1.20X10-10m
• If one were to place six billion copper atoms
side by side, this is the same as the world
population, the line of copper atoms would be
less than the length of a meter stick
Defining the Atom
• Can one actually see an atom with technology
these days?
• Now can you begin to see, or picture, the size
and existence of atoms?
The electron
• Everyone here knows what an electron is, or
do you?
• How do we prove the existence of an electron
• Did we set out to prove there was such a
particle called the electron or was it accident?
• Curiosity sparked the investigation between
electrical charge and matter
• By accident, one day, Henry Crookes noticed a
flash of light from one of his tubes he created
while working in a dark laboratory
The electron
• These flashes were the result of something striking a
light producing coating applied at one end of a cathode
tube
• Further investigation showed that that a stream
appeared to flow from the cathode to the anode
• This device led to the one of the most important social
developments of all time, T.V. (old school ones) and
computer monitors. (also old school ones) Pictures on
these screens are just formed when radiation from the
cathode strikes light producing chemicals that coat the
backside of a screen producing an image
The electron
• Research showed this stream of light was
actually a ray of particles not just some
invisible rays
• The particles were shown to carry a negative
charge when a magnetic either deflected or
attracted the stream. How could you prove
this with only a magnet?
The electron
• J.J. Thompson (1856-1940)
– Began a series of cathode ray tube experiments
when using Crooke’s technology
– He calculated both the magnetic and electrical
fields and found the mass to charge ratio
– When comparing this ratio to other known ratios
he concluded that this charged particle was
actually lighter than a hydrogen atom
– What did the mass of this particle being less than
that of the smallest atom prove?
The electron
– When changing the matter that filled the tube the
results were the same, what does this mean?
– His theory went unaccepted for some time as
many still believed the atom is indivisible, Dalton’s
theory
– What did Thompson just prove and demonstrate,
how important was this at this time?
The Electron
• Millikan (1856-1940)
– Determined, and proved, the charge of an electron to
be negative in 1909
– His technique, and set up, was so accurate that his
value found in 1909 still only has an error of ~1.0%
– Charge was determined to be that of a single charge,
meaning ,-1
– Now knowing the charge, and the mass to charge
ratio discovered my Thompson, he calculated the
mass to be 9.1 X10-28g which is 1/1840 the mass of a
hydrogen atom; what does this mean?
The Proton
• Matter is in a electrically neutral state most of
the time so how are atoms neutral if they
carry a particle with a negative charge?
– There must be a positive charge as well
– J.J. Thompson proposes plum pudding model
describes the atom to be a spherical shape of
uniformly distributed positive charge spherically
around the atom with the electrons packed inside.
The Proton
• The plum pudding model doesn’t hold for long
when Rutherford comes around
• In 1911, Ernest Rutherford, simply interested
on how alpha particles interacted with matter,
began a series of experiments
• He conducted experiments to see if alpha
particles were deflected if passed through a
thing sheet of gold foil, this experiment was a
breakthrough in atomic theory
The Proton
• Knowing Thompsons model Rutherford expected
only minor deflections of alpha particles
• Believed that if there was any deflection it would
be due to the collision, or near collision, of a
negatively charged electron
• He also believed that the positive charge was so
uniform throughout it wouldn't deflect the
massive alpha particles
• His results were stunning and opposite of what
he expected
• As you saw in the pictures of this gold foil
experiment some of the rays went directly through
the foil, some were deflected a little, and some
deflected straight back to the source, what did this
indicate?
• The ones that passed right trough were not
interacting with the atom as they passed through
empty space
• The rays deflected were due to the interaction of the
positively charge nucleus of atoms, the closer the ray
to the nucleus the great the deflection
• He proposed that the atom was mostly empty
space with a central, and extremely, densely
packed nucleus which contained all the
positive charge
The Nucleus
• An atom is mostly empty space so how big, or
small is an atom?
• We already studied the size of an atom, but these
particles are smaller than the atom itself, aren't
they?
• If the nucleus were the size of a dot on an
exclamation point than the mass of that nucleus
would be that of ~70 automobiles
• If the atom had the diameter of a football field
the nucleus would be the size of only a nickel
• What does this mean?
Particle
Symbol
Location
Relative
Relative
The
Neutron
electrical
Mass (amu)
Actual Mass
charge
• Now putting all the results together we discover the
-28
Electron
e
In
the
space
11/1840
9.11X10
mass of a electron,
and that of a proton, did not
around
account for the entire
mass of the lightest known
some place
the
atom, what doesaround
this mean?
nucleus
• The answer
came with the discovery of the neutron -24
+
Proton
P
Nucleus
1+
1
1.673X10
• James Chadwick, a student of Rutherford discovered
another particle in the nucleus with no charge that had
a mass equal to that of a proton both ~1.675X10-24g, or
0
-24
Neutron
Nucleus
0 mass of1an electron
1.675X10
1 amu,Nmuch
larger
than the
which
is nearly equal mass to the mass of a proton, both
subatomic particles are given a mass of 1amu
How Atoms Differ
• Not long after Rutherford a man named, Henry
Moseley, demonstrated that atoms of each different
element had a unique positive charge in the nucleus,
what does this mean?
• The number of protons defines, or identifies, the
element and is called the atomic number(Z), each
proton has a charge of +1, the opposite of the electron,
and has a mass of 1amu
• Since the mass of a neutron, and proton, are equal and
more massive than an electron both particles added
together for the mass number(A) and the electron isn't
How Atoms Differ
• How could on calculate the number of neutrons
from these two numbers?
• If the mass number is equal to the number of
protons plus neutrons, and the atomic number is
the number of protons, all we have to do is
subtract the atomic number from the mass
number, A-Z
• What are the letters for each element on the
periodic table, why is the first always capitalized
and the second always lower case?
Isotopes
• If the mass of a neutron, and a proton, both equal 1
amu why are the masses on the periodic table not
whole numbers?
• The answer is because of isotopes
• When you take a sample of an element the sample
contains the element plus any of its isotopes
• Isotopes are the same element but have different
number of neutrons causing the masses to be different
• Why couldn’t there be isotopes of the same element
with different numbers of protons?
Isotopes
• JJ Thompson discovered one day that he two
separate samples of neon gas, the same
element, but they had different masses, how
could this be?
• The answer again is isotopes, if the number of
neutrons is different the mass will be as well.
Isotopes
• The reason the masses on the periodic table are
not whole numbers is also because of isotopes
• As stated before a sample of any given element
will contain its isotopes as but one of them will
be more abundant than the others and expressed
in a percent of all isotopes called % abundance
• If you take the % abundance, for each isotope,
and multiply it by the isotopes mass then sum the
values for all isotopes you will get the weighted
atomic mass which is what is expressed on the
periodic table.
Isotopes
• Lets demonstrate calculation of the weighted atomic
mass
• Element X has 4 different isotopes what is its weighted
atomic mass using the data below (this is not a real
element, remember the % abundance must always be
changed to a decimal when multiplying
• Isotope 1, 67.0% abundance, mass = 1.03 amu
• Isotope 2, 3.00% abundance, mass = 1.23 amu
• Isotope 3, 17.00% abundance, mass = 1.09 amu
• Isotope 4, 13% abundance, mass =1.10 amu
• (.6700)(1.03) + (.0300)(1.23) + (.1700)(1.09) +
(.1300)(1.03) = you figure it out and ask me
• Isotope X-6 has a mass of 6.015 amu and a
percent abundance of 7.5%, isotope X-7 has a
mass of 92.5%, and a % abundance of 92.5
what element is this on the periodic table
using method just shown? Again try it on your
own then ask me if you are correct
Atomic Models
• Planetary
• Bohr Model
• Which model is correct?
• As we already know the atom has a centrally
located nucleus with all the protons and
neutrons inside the nucleus
• And the atom has mostly empty space with
electrons orbiting in some place around the
nucleus at any given time but is the end of the
description of the atomic model?
Atomic Model
• How many electrons can actually orbit an
atom in the same area, or orbit?
• Is there a certain way electrons are placed, or
found, to be around the atom at any given
time?
Atomic Model
• The next model proposed, after the plum
pudding model, is called the planetary model,
as its name suggest you expect to find the
electrons orbiting the nucleus but all electrons
have the same orbit
• This means if I were to take sodium (Na), I
would draw all the protons and neutrons in
the nucleus and all 11 electrons in one orbit
around the nucleus, is this accurate?
• Instead of saying we have electrons around
the nucleus in an orbit we can say in an
energy level
• Energy levels are where the electrons have the
highest probability of being found when
attempting to locate them.
Atomic Model
• Niels Bohr, a student of Rutherford, proposed
his own model in 1913 his model was
quantized ,which you will learn for now very
basics, you will learn the reasoning behind the
Bohr Model in more detail later when we
study light and quantized energy
Atomic Structure
• Bohr proposed that electrons can only be
placed in certain energy levels that orbit in
certain circular orbits around an atom and
that each of these energy levels can only hold
a certain number of electrons
• The first energy level is the closest to the
nucleus, and every energy level added to the
atom causes the atomic radius to increase
Atomic Structure
• Now know that the first 5 energy levels can hold
the following number of electrons: 2,8,18,32,50
• To find the number of electrons use the following
formula 2n2, where n is the number of the energy
level
• Now take sodium (Na) and draw the Bohr Model
for an atom.
• You will need 3 energy levels for this one,
meaning, three orbits around the nucleus each
one successively larger in diameter going away
from the nucleus, draw the model now
• The electrons cannot choose any orbit they
wish. They are restricted to orbits with only
certain energies.
• Electrons can jump from one energy level to a
higher one, or from a higher one to a lower
one, only when a specific amount of energy is
absorbed or emitted
• There is no in between energy levels, the electrons
can only be in the ground state or an excited state.
• For example think of a ladder and the rungs, or steps.
The steps of the ladder are energy levels of an atom
and your feet are the electrons. The lowest step is
the ground state and the higher steps are excited
states
• Can you stand in between, the rungs, or steps, of the
ladder,? This is the same concepts are electrons and
energy levels?
• That exact, specific, or required, amount of
energy, enough to place them in a higher
energy level, or lower energy level is called a
quantum, plural quanta
• Electrons can be promoted from the lowest
energy state to a higher energy state as long
as they have absorbed the same quantum of
energy that the higher state possesses
• When an electron goes down from that
excited state, back to the ground state, it has
to give off that same amount, quanta, of
energy because energy can not be destroyed
• When that electron drops back down to a
lower energy state it gives off that energy as a
photon which can be seen as light
• The amount of energy that the photon
contains is just the difference in energy
between the excited state and the lower state
Atomic Structure
• Draw the following atoms according to the
planetary model and the Bohr Model
• Na, Mg, Br, H, He, Ne, Hg, Cu, Fe, Xe, C, Rb