Download Atomic Theory

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Studying Atoms
* How is it possible for us to study something we
can’t see?
* Need indirect evidence
* Atoms are so small that even with the most
powerful microscopes we can’t see them
*
*Democritus believed that all matter
consisted of small particles that
couldn’t be divided. He called these
atoms from the Greek word atmos
which means uncut or indivisible
*Democritus thought there were
different types of atoms with different
types of properties: atoms in liquids
were round and smooth, atoms in solids
were rough and prickly
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*Aristotle didn’t think there was any
limit to the number of times matter
could be divided
*Aristotle thought all matter was built
up from four basic substances; earth,
air, fire, and water.
*This idea was accepted up until the
1800’s (over 2000 years)
*By the 1800 evidence to support an
atomic model began to appear
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*
* Dalton was interested in predicting the
weather so he studied the behavior of gases in
air
* Predicted that a gas consisted of individual
particles because of the pressure they exert
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* When elements combine the ratio of the
masses of the elements in the compounds is
always the same
* Compounds have a fixed composition
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* All matter is made up of individual particles
called atoms, which cannot be divided.
* All elements are composed of atoms
* All atoms of the same element have the same
mass, and atoms of different elements have
different masses.
* Compounds contain atoms of more than one
element (is this always true?)
* In a particular compound, atoms of different
elements always combine in the same way
*
*His model of the atom pictured the atom
as a solid sphere with different masses
*Reminder: A theory explains the results
from many experiments
*Was widely accepted for a long time
*Eventually scientists found that not all of
Dalton’s ideas were completely correct
*This lead to a revision of the theory
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*
*Used an electric current to learn more
about atoms
*Vacuum tube attached to an electric
current: a glowing beam appears
*Beam contained charged particles
*Used charged plates on either side of
the beam: beam attracted to positive
plate and repelled from the negative
plate
*
* Particles in the beam had a negative charge
* His experiments showed that atoms are made
of smaller particles (Contrary to Dalton’s idea
of a solid ball)
*
* Atoms are basically neutral – no positive or
negative charge
* Negative charges were evenly scattered
throughout an atom filled with a positively
charged mass of matter..
* Plum pudding model (Chocolate Chip Ice Cream
model)
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*
* Discovered that uranium emitted fast-moving
particles that had a positive charge (He called
them alpha particles)
* Gold Foil Experiment
* Aimed the alpha (positive particles) at a thin
gold foil
* They did not behave as Rutherford expected
*
*
* Rutherford proposed that the positive charge
was concentrated in a small area which he
called the nucleus
* Nucleus: a dense, positively charged mass
located in the center of the atom
*
* All of an atoms positive charge is concentrated
in the nucleus
* Through experimentation they discovered that
the total volume of an atom is about a trillion
(1,000,000,000,000) times the volume of the
nucleus.
*
*
* Rutherford saw the existence of two subatomic
particles and predicted a third.
* Protons
* Electrons
* Neutrons
*
*Rutherford concluded the the amount
of positive charges varied from element
to element
*Each nucleus must contain at least one
positive particle which he called a
proton
*Has a +1 charge
*Some nuclei contain more than 100
protons
*
* An electron is a negatively charged subatomic
particle found in the space outside the nucleus
* Electrons have a charge of -1
*
*Neutrons were shown to exist by James
Chadwick in 1932
*Charged objects did not deflect their paths
*A neutron is a neutral subatomic particle
that is found in the nucleus of an atom
*It has a mass almost exactly equal to that
of a proton (See page 109 in your textbook)
*
* All three subatomic particles can be
distinguished by mass, charge, and their
location in an atom
* All of this information is based on how particles
behave. We still don’t have any instrument
that can see inside an atom.
*
* Atomic Number is the number of protons in the
nucleus of an atom
* Atoms of different elements have different
atomic numbers
* The atomic number also equals the number of
electrons. Why?
*
*The mass number is the sum of the
number of protons plus the number of
neutrons
*You can figure out the number of
neutrons by subtracting the atomic
number from the mass number
*Al = 27 How many neutrons?
*Carbon = 12 How many neutrons?
*Argon = 40 How many neutrons?
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*Every atom of the same element does
not have the same number of neutrons
*Isotopes are atoms of the same element
that have different numbers of neutrons
and therefore different mass numbers
*Radioactive elements typically are
isotopes
*Some others are oxygen and carbon
*All of the elements have isotopes
*
*
*Nanotechnology
*Using atoms and molecules as construction
materials to build very small objects
*1/1,000,000,000 of a meter = 1 nanometer
*Human hair = 80,000 nm
*Can build from the top down or from the
bottom up
*Nanogears
*Nanorobots
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* Model looks like planets orbiting around a sun
in a solar system
* Bohr agreed with Rutherford about the nucleus
* Focused on the electrons
* A description of the arrangement of the
electrons is the main part of modern atomic
theory
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*
*Electrons move with constant speed in a
fixed orbit around the nucleus
*Each electron has a fixed amount of energy
*Electrons can gain or lose energy
*Energy levels are the possible energies that
an electron can have
*Like steps – you can’t be between – you
must be on one level or the other – each
step would represent an energy level
*No two elements have the same set of
energy levels
*
* Electrons move from one energy level to
another when they gain or lose energy
* Can move more than one energy level
* Evidence: the light given off by fireworks
* Raising energy levels and light given off when
they return to their original energy level
* Each element has a unique color spectrum they
give off because each element has a different
set of energy levels
* Move from the ground state – get excited –
return to the ground state and give off unique
light signature
*
* An improvement on Bohr’s model
* Electrons move in a less predictable way
* Electron Cloud: a visual model of the most
likely locations for the electrons of an atom to
be around the nucleus
* Cloud is denser where the electrons are most
likely to be – based on probability
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*
*An orbital is a region of space around the
nucleus where an electron is likely to be
found
*These orbitals have unique shapes and can
hold 2 electrons each
* s, p, d, and f
* s orbitals (there are 1) – can hold 2 electrons
* p orbitals (there are 3) – can hold 6 electrons
* d orbitals (there are 5) – can hold 10 electrons
* f orbitals (there are 7) - can hold 14 electrons
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Energy Level
Maximum # of
Electrons
1
Number of
Orbitals
Possible
1
2
4
8
3
9
18
4
16
32
2
*Electron
Configuration: the arrangement
of electrons in the orbitals of an atom
*The most stable electron configuration is
the one in which the electrons are in
orbitals with the lowest possible energies
(we call this the ground state)
*If an electron absorbs energy it is said to
be in an excited state which is less stable
(light emission)
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