Chapter
11 Notes: “Modern Atomic Theory” Per. #
Review & Remember
1.
2. J. J. Thompson:
Atoms contain negative particles called electrons.
Used the cathode ray.
3.
4. Robert
Millikan:
Electrons have a negative charge with extremely small
mass.
5.
Problem: What are the
electrons doing? How were they arranged around the nucleus? Why didn’t they
just crash into the nucleus?
In the early 20th century, a new model
evolved as a result of investigations into the absorption and emission of light
by matter. They revealed an intimate relationship
between light and an atom’s electrons.
Let us look at the properties of waves and light
Energy can be transmitted from one place to another
by Electromagnetic Radiation.
This energy moves about by waves.
What factors define waves?
·
Velocity v or c this is normally equal to the speed of light in a vacuum.
(Note: Speeds of waves are different through different media)
·
·
Wavelength
λ the distance
between two consecutive peaks
(Note:
color and pitch are related to frequency and
wavelength)
·
Amplitude height
of the wave from resting position to the crest
·
Energy E
·
Big
Idea!!! Knowing the color,
frequency, wavelength and frequency, you can
determine any of the others. Using the following formulas:
c = λ ν, E = h ν, E = hc/ λ
(Note: h = Planck’s
constant; 6.626 x 10-34 J s)
What do waves do?
· reflect
·
· form standing waves….
reflection and interference (like a vibrating string)
· refract … bend when changing media
·
For
many years’ scientists thought light behaved solely as a wave. This changed
when they discovered “The Photoelectric Effect”.
This is the emission on electrons from a
metal when light of sufficient frequency shines on it and thus creating an
electric current. (Like a soda machine)
This simple observation would mean that light has
particle-like properties.
· The light “particles” are
called photons
· So is it a wave
or a particle? both!!!
· Called wave-particle
nature of light
When an atom receives energy from some source it becomes excited. Excited State.
An atom can then release this energy by emitting (giving off) light, which is carried away by a
photon. Thus returning an atom to its Ground
State.
The energy they give off is the exact amount of
Energy they received.
Remember: Different wavelengths carry different
amounts of Energy per photon…
When hydrogen absorbs energy from an outside source
it becomes excited and releases this energy. Interestingly, only certain colors
of visible light are emitted, that is, only certain photons at specific
wavelengths and frequencies.
This suggests that the electron of a hydrogen atom can
exist only in very specific energy states. The spectra
display what these discreet energies are. We also say that the energies are
quantized.
Problem: Need a model to
explain the electron of a hydrogen atom and its quantized energy levels.
Why don’t the electrons fall into
the nucleus? Move like planets around the sun in circular
orbits at different levels. Amounts of energy separate one level from another.
Increasing Energy · Further away from the nucleus means more energy. · There is no “in between” energy · Energy Levels } First Second Third Fourth Fifth
Problems with Bohr’s Model
Schrödinger, Heisenberg and
De Broglie devised some math equations to explain the “wavicle” properties of
electrons. They explained it like this:
·
The
atom is found inside a blurry “electron cloud”.
·
The
cloud is an area where there is a 90% chance of finding an electron.
The cloud or probability map
is called an orbital. The set of
math equations that describes the electrons is called wave mechanics or quantum
mechanics.
These equations contain
variables we call quantum numbers: n, l, m, and s.
Think of them as addresses for electrons.
The letter n stands
for the energy levels (principal energy
levels).
Within the energy level the
complex math of Schrödinger’s equations describes several shapes. These shapes are called atomic
orbitals. These are regions where there is a high probability of finding an
electron.
The letter l
designates the overall shape of the orbital shell.
The letter m
determines the orientation of orbital within a subshell.
The letter s
designates the spin. Pauli Exclusion Principle: an
orbital can only hold 2 electrons and they must have opposite spin.
The arrangement of
electrons in an atom is called its electron configuration.
It is a list showing how many
electrons are in each orbital or subshell in an atom or ion.
A subshell notation will list
the subshells of increasing energy, with number of electrons in each subshell
as a superscript. The subshells are s, p, d and f
Example:
1s22s22p6
means 2 electrons in the 1s subshell, 2 electrons in the 2s subshell and 6
electrons in the 2p subshell. This atom
has 10 electrons and represents the element Neon or some other ion with 9
electrons, like F-.
The d subshells (The 3dxz orbital)
The f subshells
You are responsible
for knowing up to the 4th level and only the shapes of s, p, and
d orbitals (s and p in detail)
Other points
Valence electrons are those on the outermost
principal energy level
Core electrons are the ones inside the
outermost