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Atom
Atomic number
Atomic theory
Electromagnetic spectrum
Electron
Electron cloud
Emission spectrum
Energy level
Hypothesis
Isotope
Law of definite proportions
Lewis dot diagram
Mass number
Neutron
Nucleus
Proton
Scientific law
Theory
Valence electron
What do you know about atoms?
Early Atomic Theory
Philosophically based from ethical, religious, or other
schools of thought, not modern scientific methodology
Greek
Democritus – proposed atomic theory to account for
philosophical disagreements on matter versus void
Indian
Multi-faceted between scientific elemental forms and religious
theories concerning souls and knowledge
Islamic
Atomism reconciles the intent of the divine with the existence
of the physical
Early Scientific Work
France, 1789
Antoine Lavoisier develops the Law of Conservation of Mass
through highly accurate measurements
France, 1799
Joseph Proust develops the Law of Definite Proportions, which
states that when broken down, compounds always have the
same ratio of elemental ingredients
England, 1802
John Dalton builds on previous work to develop the Law of
Multiple Proportions; fixed amounts of element A will always
combine with element B in proportions that reduce to whole
numbers
Dalton’s Atomic Theory (1803)
All matter is made up of atoms.
Atoms are indestructible and cannot be divided into
smaller particles.
All atoms of an element are exactly alike, but are different
from atoms of other elements.
Dalton’s Work Published
Though
brilliantly
and
empirically
derived, his
model was
flawed and
incomplete.
Why?
Questions Raised -
Why are there so many different elements?
What makes one atom different from another?
How do atoms combine to form compounds?
Review Slide
Early work on Atomic Theory was based in philosophy,
not science.
Real science-based work started in the late 1700’s in
France and England; Lavoisier and Proust.
Laws of; Conservation of Mass, Definite and Multiple
Proportions led to Dalton formulating his Atomic Theory. All matter is made up of atoms.
Atoms are indestructible and cannot be divided into smaller particles.
All atoms of an element are exactly alike, but are different from atoms of other elements.
Dalton’s Model; a solid ball of the ?element? that is tiny.
Questions were left unanswered!
Cracking into the Mystery of the Atom!
Additional refinement was done throughout the 1800’s to
further find atomic weights and relative masses of
elements (Avagadro)
The inner secrets of the atom were not divulged until
very recently.
J. J. Thompson, England (1897)
British physicist experimenting with electricity and
vacuum tubes.
Constructed a vacuum tube with positive and negative
terminals at either end, and a phosphorescent (glows
when energized) strip.
Thompson’s Thoughts
Something is passing between the two metal nodes.
Is it light?
Is it something else?
How could he know?
Thompson’s Conclusions
The stream is NOT light; magnets do not bend light.
Therefore the stream is made up of charged particles.
The stream can NOT be something already in the tube, it
is a vacuum.
Therefore the stream MUST be coming from the metal
nodes at either end.
Therefore atoms must have particles that can be separated
and shot as a ray, particles with an electric charge!
More Information and Conclusions
The magnet bends the stream certain ways.
Therefore the charged particles had a NEGATIVE charge.
The particles were EXTREMELY small, 1/1387 the mass of
the smallest atom.
Therefore the atom must have other stuff inside it!
Atoms normally have a NEUTRAL charge.
Therefore the other stuff is POSITIVELY charged.
Thompson’s Model
Thompson described the atom as a ball of positive
charges with electrons drifting around somehow.
Though including new subatomic particles, his model
maintained the ‘solid ball/clump’ structure of Dalton’s
earlier model.
Complications Plague Thompson
It was discovered that the positive terminal also emits a
ray.
Thompson proposed a positive particle so this was expected.
The masses of some elements were found to be slightly
variable – sometimes they weighed more, sometimes a
little less
The rational explanation is a third particle, a neutral one called
the neutron.
Atoms that are chemically identical but of different weights are
called isotopes.
Review Slide
More work on the Atomic Theory provided very important additional details.
Thompson’s experiments with vacuum tubes, cathodes, and magnets demonstrated;
That atoms were made of smaller particles.
The particles were negative, positive, or neutrally charged.
Named the Electron, Proton, and Neutron.
Elements sometimes came in different weights (isotopes) – this was proposed to be because of having more or less neutrons inside.
Important question; How is everything arranged?
Rutherford, 1908
Another British physicist interested in working with
positively charged particles, called α particles (alpha).
He was working on scattering, or finding out how these
particles bounced for complex physics reasons.
Rutherford’s Experimentation
After constructing an
alpha particle gun, it was
aimed at a sheet of gold
foil.
Gold can be easily
flattened until it is a few
atoms thick.
Source of fast moving,
positively charged
particles (α particles)
ZnS fluorescent screen
Gold foil
Deflected
particles
Undeflected
particles
Exercise
1. What kinds of particles are emitted by the radioactive source? What is their charge?
2. Toward what are the alpha particles being directed?
3. What happens when the charged particles strike the surface of the gold foil?
4. What was the purpose of the fluorescent screen?
5. What did Rutherford conclude from the gold foil experiment regarding the amount of empty space in the atom? Why was he able to conclude this?
6. How does the path of a charged particle that strikes the center of a gold atom differ from the path of a particle that passes near the center?
7. Based on this experiment, where is most of the positive charge and mass of an atom found?
Rutherford’s Model
Rutherford or Nuclear model
In the early twentieth century, Rutherford
showed that most of an atom's mass is
concentrated in a small, positively charged
region called the nucleus.
Questions post Rutherford
How are the electrons arranged?
What size are these atoms?
How does energy figure into this?
Niels Bohr
Another physicist, but Danish!
His idea was that electrons orbited the nucleus – but
with quantum energy states keeping them in place instead
of gravity! (This is complicated, more on it later.)
Electrons ‘orbit’ the nucleus in layers, or electron shells,
which depend on the number of electrons and size of the
nucleus.
Each shell can only hold a certain number of electrons
Electron Shells
Each element has a different number of electrons, and so
they have different electron shell arrangements;
Hydrogen for example has one electron:
Oxygen has 8 electrons:
There can be many different layers!
Bohr
Bohr model
After Rutherford's discovery, Bohr proposed
that electrons travel in definite orbits, arranged in shells or levels
Around the nucleus.
Review Slide
Rutherford discovered that the mass of an atom is
concentrated in the center, or nucleus of an atom.
He was unable to determine the exact arrangement of
other particles.
Bohr calculated the locations of electrons using high-
energy equations and complex math.
Bohr demonstrated that electrons orbit in energy layers,
or shells
Atomic Model Review
Rutherford or Nuclear model
In the early twentieth century, Rutherford showed that
most of an atom's mass is concentrated in a small,
positively charged
region called the nucleus.
Bohr model
After Rutherford's discovery, Bohr proposed
that electrons travel in definite orbits, arranged
in shells or levels around the nucleus.
Thompson model
After working with cathode ray tubes, Thompson
created the ‘plum pudding’ model with electrons
Embedded in an unknown positively charged
substrate.
Dalton model
Not much of a model, but Dalton’s understanding was
of a discrete little ball of whatever made atoms different
from one another.
?
Atomic Makeup
Nucleus
Contains protons
Contains neutrons
Electron Shells
Holds all the electrons in orbits
How many subatomic particles?
Every element has different numbers of subatomic
particles making up its nucleus and electron shells.
The number of protons is unique for each element and
does not change.
The number of electrons can vary.
The number of neutrons can vary.
Reading the Periodic Table
The periodic table gives us all the information we need;
An examples is below.
Breaking it down
Every element has a unique atomic number.
The # of protons = atomic number.
The mass of an atom is concentrated in the nucleus so the atomic mass = # protons + # nuetrons.
The # of neutrons = atomic mass – atomic number.
Atoms have no charge (ground state) so the positive charges of the protons must be canceled by the negative charges of the electrons.
The # of electrons = # of protons.
For Example;
Aluminum
Protons = 13 (Atomic Number)
Neutrons = 13 (26[atomic mass]-13[atomic number])
Electrons = 13 (To cancel the positively charged protons.)
Phosphorus
Protons = 15 (Atomic Number)
Neutrons = 15 (30[atomic mass]-15[atomic number])
Electrons = 15 (To cancel the positively charged protons.)
Zinc
Protons = 30 (Atomic Number)
Neutrons = 35 (65[atomic mass]-30[atomic number])
Electrons = 30 (To cancel the positively charged protons.)
Electron numbers can change?
Yes!
Atoms can lose or gain electrons, which means they
gain/lose a charge that is either positive or negative.
As electrons are lost or gained the +/- balance shifts.
More electrons makes the atom become negatively charged.
Fewer electrons makes the atom positively charged.
The newly charged atoms are called ions.
Positive ions are cations.
Negative ions are anions.
Notation and Ions
Al+2
Protons = 13 (Atomic Number)
Neutrons = 13 (26[atomic mass]-13[atomic number])
Electrons = 11 (Two protons are not being cancelled.)
P-1
Protons = 15 (Atomic Number)
Neutrons = 15 (30[atomic mass]-15[atomic number])
Electrons = 16 (There is an extra negative charge.)
Zn+4
Protons = 30 (Atomic Number)
Neutrons = 35 (65[atomic mass]-30[atomic number])
Electrons = 26 (Four protons are not being cancelled.)
Neutron numbers shift as well? Yes.
Atoms naturally exist with variable numbers of neutrons, which can change their overall atomic mass.
An atom with the same atomic number but a different atomic mass is called an isotope.
One of the most common ways to identify an isotope is to write the symbol then the mass; Carbon -14
Neon – 21
Oxygen -16
The atomic mass on the periodic table is a weighted average of all isotopes of that element.
Isotope Examples
Aluminum - 27
Protons = 13 (Atomic Number)
Neutrons = 14 (27[atomic mass]-13[atomic number])
Electrons = 13 (To cancel the positively charged protons.)
Phosphorus - 31
Protons = 15 (Atomic Number)
Neutrons = 16 (31[atomic mass]-15[atomic number])
Electrons = 15 (To cancel the positively charged protons.)
Zinc - 64
Protons = 30 (Atomic Number)
Neutrons = 34 (64[atomic mass]-30[atomic number])
Electrons = 30 (To cancel the positively charged protons.)
Atomic Mass
The atomic mass of an element represents the
average mass of all the isotopes found in nature.
No element exists with only one possible isotope.
Hydrogen has the smallest number of isotopes:
H-1 protium, H-2 deuterium, H-3 tritium.
Calculating Atomic Mass
If you look at your periodic table for hydrogen, the
atomic mass is 1.0079 amu (atomic mass units).
The atomic mass is calculated by adding the % of H-
1 mass found in nature to the % of H-2 mass found
in nature plus the % of H-3 mass.
% H-1 + % H-2 + % H-3 = average mass (atomic
mass)
Generally the formula used is:
% X + % Y + % Z… = atomic mass.
Sample Problem
Silver is found to have two stable isotopes, one has an atomic mass of 106.904 amu and the other weighs 108.905 amu.
The first isotope represents 51.82 % of the mass of the element and the second represents 48.18 %.
What is the atomic mass of the element silver?
Solution
The equation to use is: %X + % Y = average atomic mass And remember to convert your percentage amounts
into fractions (by dividing by 100) before you begin
anything! (0.5182) 106.904 amu + (0.4818) 108.905 amu =
mass 55.398 amu + 52.470 amu = 107.868 amu
Now look at the periodic table to verify the answer.
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