Download File

Models of the Atom
Ancient Greece
300 BC in Greece
School of thought that matter is made up
of tiny indivisible, invisible, indestructible,
fundamental units of matter called atoms
(means “that which cannot be divided”)
Democritus of Abdera most well known
atomist.
Did no experiments
No proof
Philosopher
1770s
Antoine Lavosier made observations and did
experiments resulting in the Law of
Conservation of Mass
Law of Conservation of Mass
Matter cannot be created
or destroyed
Mass of materials before the
chemical reaction is the same
mass after the reaction
1799- Joseph Proust
Law of Definite Proportions: that the proportion
by mass of the elements in a compound is always
the same (CO vs. CO2)
Part of our definition of a chemical - any substance
with a definite composition
1808- John Dalton
English School Teacher
Theory based on the Greek idea of the atom
Atoms were tiny indestructible particles
Dalton’s Atomic Theory
1. All matter is made of invisible and indestructible
particles (atoms)
2. Atoms of same element are identical
3. Atoms of different elements differ in physical and
chemical properties
4. Atoms of different elements combine in simple whole
number ratios to form compounds
5. Chemical Reactions occur when atoms separate, join or
rearrange. Atoms of one element NEVER change into
atoms of another element.
Problems with Dalton’s Model
Atoms are destructible (protons, electrons, neutrons)
Atoms of the same element are not completely
identical, isotopes exist
Atoms of the same element combine with each other
(O2)
Atoms can “turn into” other elements through
nuclear decay
1897
Sir J.J.Thomson
British physicist
Theory: cathode ray is a
stream of negatively
charged particles
Experiment:
cathode ray experiment
Experiment
Cathode ray: a tube filled with gas that would glow
when an electric current was passed through tube
(like a neon sign)
Thomson held the “+” pole of magnet next to tube,
and the light bent towards magnet
Therefore he concluded that there must be some
negative charge in atom!! (discovered electron)
Also hypothesized that there must be some positive
charge in atom (as atoms are neutral)
Plum Pudding Model
Electrons are
embedded in
atom
Positive charge
floats around
rest of atom
Thomson
Misconceptions?
Placement of electron
Location of positive charge
1909 -Ernst Rutherford
Physicist and Chemist from New Zealand
Proposed:
that the theory of Thomson was actually correct
Any charge that occurs in atom must occur in
whole number ratios
Ex: +1 or -1, not + 1.5
Rutherford's Gold Foil Experiment
Fired small radioactive particles (basically a helium nucleus)
at a piece of gold foil
Expected that most of these particles would pass right
through the gold foil, deflecting only a little bit
* what actually happened was that while that majority did
pass right through, those that deflected deflected at many
different angles
Think of throwing a grain of salt at a chain link fence!
Rutherford’s Experiment
Rutherford’s Conclusion
Key Idea -NUCLEAR ATOM
All of the mass of the positive particles (protons)
is at the center region of the atom
Center region called the NUCLEUS
Electrons surround the nucleus in a “cloud”
Atom is mostly empty space
Rutherford’s
Misconceptions
Still could not place electrons correctly
Was not aware of the neutron
Niels Bohr
Danish Physicist
Proposed his model in 1915 (this is the one most of you know)
He coined the term, “Planetary Model” of atom
Electrons orbit the nucleus of atom like planets around the sun
Different orbits exist, each having a specified level of energy
Suggested that outermost energy levels can hold more electrons
than inner energy levels
Bohr’s Experiment
Studied line emission spectrum of hydrogen
Noticed that when electrons were excited (basically
electrified) they would “jump”.
Further, he noticed that these jumps were similar in
energy
This helped to solidify his theory of electrons
orbiting the nucleus in specific energy levels
Bohr’s Atom
Conclusions:
There are certain circular ORBITS in which an
electron can travel around the nucleus
The farther away from the nucleus, the
higher the energy level
Problems with Bohr
Electrons do not exist in orbits!
Quantum
Model/Schrodinger 1925
Theory:
states that electron can act as a particle and a wave
electrons can exist in any of an unlimited number of
energy levels
Experiment: Schrodinger wave equation
Quantum
Model/Schrodinger
Conclusions:
Electrons do not orbit nucleus, they exist in orbitals
Orbitals are probable locations of the electron
There is an unlimited number of energy levels in which
electrons can exist
Energy increases as you get farther from nucleus
Misconceptions?
Quantum
Model/Schrodinger
What do we
know now??
All models are not exactly correct, but they lead to
further understanding and discovery
All have important key ideas
Atoms are made up of subparticles
Atoms are divisible, but not by ordinary chemical means
Atoms of elements can vary (isotopes)
Electrons reside in ORBITALS, not orbits
Parts of Atom
Particle
Location
Relative Mass & Size
Charge
Protons
Nucleus
Heavy mass
1.673 E - 24 g
+
Neutron
Nucleus
Heavy mass
1.675 E - 24 g
No charge
Electron
Orbitals outside of
nucleus
1.1 E -29 g
mass is 1/1836 that of
proton
_
What happens when you change…
Protons: Since the number of protons is the same as the
atomic number, protons are what define an element.
If you change the protons you change the element
Neutrons: The neutrons add to the mass of the element.
If you change the neutrons you change the mass of
the element (make an isotope)
Electrons: The electrons balance the charge of the
protons.
If you change the electrons you change the charge of
the element (make an ion)
Quotes about atoms
From A Short History of Nearly Everything
by Bill Bryson
“protons give an atom its
identity, electrons its
personality” p. 140
“if an atom were expanded
to the size of a cathedral,
the nucleus would be only
about the size of a fly- but a
fly many thousands of times
heavier than the cathedral”
p. 141