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CH 7 NOTES
Review: What you already know!
Modern Atomic Theory
 Electron behavior has been interpreted through studying
each elements’ emission spectra
DEMO: Emission Spectra of Elements
 Energy levels are regions of space in the atom where
there is a high probability of finding electrons.
 Because the emission spectra show closely spaced lines
of similar, but unequal energy, we have evidence of the
existence of sublevels within each energy level
DEMO: How is light created?
 As electrons are excited by energy, they move to a
higher E level.
 When the electron falls back down to the lower E level,
it releases (emits) the same amount of energy (in the
form of light) as it absorbed.
 Higher E light comes from “bigger jumps”
Valence Electrons
 Electrons located in the outermost E level
 How do we determine how many?
Group 1
Group2
Group 13-18
 What do they tell us?
Heisenberg's Uncertainty Principle
 It is impossible to measure exactly both the position and
momentum (mass & speed) of an object (electron) at the
same time
Ex. moving fan, time-lapse photo of cars on a highway
DEMO: Uncertainty
Electron Cloud
 Because we cannot pinpoint exactly where an electron is
on the “surface” of an atom, we refer to its position as an
electron cloud.
 The chemical behavior and properties of any 2
substances are determined by the # and arrangement of
these electrons around the nucleus.
HOW DO WE DESCRIBE THE ELECTRON CLOUD?
 Quantum #s – represent different E states of the
electron; there are 4 quantum numbers, n, l, m and s
 The difference in E states is represented by the
different spectral lines of an emission spectrum.
PRINCIPAL QUANTUM #, n
1. describes the general size of the electron cloud
2. numbered levels low to high, 1, 2, 3, 4….(integers)
3. electrons may be found in each
4. the maximum # of electron possible in any one level is
2n2
2nd QUANTUM #, l
1. it represents/describes the sublevels w/in an E level
2. the value of l is 0 to (n – 1)
3. the # of sublevels is equal to the value of n
4. the lowest sublevel is s, then p, d and f
5. Each sublevel can hold up to a specific # of electrons:
 s sublevel can hold 1 pair
 p can hold 3 pair
 d can hold 5 pair
 f can hold 7 pair
6. Each pair has a different place in space, this space is
called an orbital.
7. The sum of all the electron clouds in any sublevel is a
spherical cloud.
8. Electrons are repelled by each other and attracted to
the positive nucleus.
THESE FACTORS, IN ADDITION TO n, GOVERN
THE SIZE AND SHAPE OF THE CLOUD
3RD QUANTUM #, m
 Describe/designate the orbitals direction in space
 The value of m is +l or - l
4th QUANTUM #, s
 describes the spin of the electron w/in an orbital
 the value is +1/2 (clockwise) or –1/2 (counterclockwise)
 if there are 2 electrons in an orbital, they must spin in
opposite directions
ELECTRON CONFIGURATION
 We describe where electrons are (and how much energy
they have) by using our 4 quantum numbers
 The PAULI EXCLUSION PRINCIPLE states that no 2
electron can have the same set of all 4 quantum numbers
 As atomic number goes up, so do the number of electrons
 Where do they go?
ELECTRONS WILL ALWAYS ARRANGE THEMSELVES
IN THE E LEVEL AND SUBLEVEL THAT WILL
PRODUCE THE LOWEST ENERGY.
One way of remembering the order that electrons will
arrange themselves is called the AUFBAU SEQUENCE:
1s
2s
3s
4s
5s
6s
7s
2p
3p
4p
5p
6p
7p
3d
4d
5d
6d
4f
5f
6f
When we draw the downward, diagonal arrow through these
sublevels, it indicates the order of the sublevels that
electrons will go into…for example, the electron
configuration for rhodium, Rh (atomic number 45) is:
1s22s22p63s23p64s23d104p65s24d7
Notice how all the superscripts (your electrons) add up
to 45! The WEIRD thing (I know) is why do the
electrons go into 4s before 3d? REMEMBER: Electrons
want to arrange themselves into the lowest energy
configuration…4s is a little lower in energy than 3d.
SHORTCUTS: NOBLE GAS, INNER-CORE
ABBREVIATIONS
Because the electrons are filling in the same order every
time, we have a way of shortening the configuration! Use
the symbol for the noble gas that occurs before the
element that you’re working on, then write the
remainder of the electron configuration. For example:
Here’s what rhodium, Rh looks like now:
[Kr]5s24d7 (nice, huh?)
Kr, krypton has 36 electrons…add that to 9, and you’ve
got 45!
Complete the Section Review problems (1 through 5) on
p. 251 in your notes…you may work together on these.
The Half-Filled Orbital Anomaly
The expected electron configuration for chromium is
[Ar] 4s23d4, but instead the observed configuration is
[Ar] 4s13d5. Why do you think this is?