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CHAPTER 5 Electrons in Atoms
From Bohr to Today -- Background Research
A. The de Broglie Hypothesis
• if photons are waves behaving like particles, then perhaps particles can have some of the
properties of waves
B. The Apparent Contradiction
• Are e- waves or particles?
BOTH!!, the wave-particle duality of nature allows one to focus on the properties most
important for their studies.
•
Heisenberg's Uncertainty Principle
when considering an e- as a particle, you can never know the exact position and velocity
of electron at the same time
Think Ceiling Fan
•
Schrödinger's Equations
treating the e- as a wave, he could make a mathematical graph of the function of the
electron's wave pattern
Max Born
showed that by squaring a portion of Schrödinger's equation, one could have a numerical
probability of an electron's position
•
III. Quantum Mechanical Model - (current theory)
A. Based on mathematical equation (previous models were basically physical)
B. Concerned with predicting the probable location of electrons
1. when all the possible mathematical solutions are graphed, a 3-D shape results (a "cloud" of
probability) (orbital)
2. although drawn spherical, atom is not necessarily spherical
C. Similarity to Bohr: based on quantized energy levels of eD. Unlike Bohr: Does not define the exact path of an e-, just probability of finding an e- in a
certain position
E. The contemporary model of the atom assigns quantum numbers that indicate the relative sizes
and energies of atomic orbitals. There are four quantum numbers. We will only discuss the first
two.
1
1. principal quantum number (n) specifies the atom's major energy levels
average distance from the nucleus
called energy levels, or shells
1 to 7 (correspond to Bohr's model)
level 1, the closest to the nucleus and has the lowest energy level called ground state
level 7 is the furthest from the nucleus and has the highest level of energy
the energy of the electron depends primarily on this number
n
1
2
3
4
5
6
7
Max # of
2
8
18
32
50
72
98
electrons
to calculate the maximum number of electrons use the equation 2n 2
the period (rows) in the periodic table indicate the energy level for the elements in that
row.
Within each n energy level, there are sublevels each with distinctive shape
2. Subshells of the Orbital
nomenclature: s,p,d,f
a. s: spherical
b. p: dumbbell shape
c. d:
d. f:
the number of sublevels possible on an energy level equals the "n" value of the energy level
Energy Level (n)
# Sublevels Possible
Identity of sublevels
1
1
s
2
2
s,p
3
3
s,p,d
4
4
s,p,d,f
o the number of orbitals per sublevel and the maximum # of electrons possible
are shown in the table below
2
Sublevel
# of orbitals
Maximum # of electrons possible
s
1
2 [1 pair]
p
3
6 [3 pair]
d
5
10 [5 pair]
f
7
14 [7 pair]
[note: memorize this chart it is important!]
F. Electron Distributions
A. Rules for Describing Electron Distributions
1. Aufbau Principle
Electrons enter orbitals of the lowest energy first, s<p<d<f
2. Pauli's Exclusion Principle
maximum of two electrons may occupy a single atomic orbital, but only if the electrons have
opposite spins
3. Hund's Rule
When electrons occupy orbitals of the same shape on the same energy level, one e- enters
each orbital until all orbitals contain an e- of the same spin direction
B. Electron Configuration Notations (1s22s2) and Orbital Notation
1. Electron Configuration is a method of using the quantum mechanical model of the atom to
predict the probable location of electrons in every type of atom
a. Steps to Writing Electron Configuration
1. First determine the # of electrons in the atom
2. Use the diagonal rule or hotel to write the configuration
3. When finished make sure the superscripts add up to the number of electrons
2. Orbital Notation - shows the position of each electron in the orbitals
a. Steps to Drawing Orbital Notation
1. First you have to write the electron configuration
2. You have to draw boxes or lines for each sublevel… 1 for s, 3 for p, 5 for d and 7 for
f.
3. Then fill the orbitals with arrows representing the electrons designated by the
superscript.
4. Be sure to follow Aufbau's Principle, Pauli's Exclusion Principle and Hund's Rule.
Example:
H
3
Li
C
F
Ne
P
Mn
E. Electron Configuration using the Noble Gas shortcut
If you notice, the electron configuration can be really tedious with the larger number of electrons.
For that reason, a Noble Gas shortcut can be used. THIS SHORTCUT CAN ONLY BE USED FOR
ELEMENTS AFTER ARGON.
a. Steps to writing an electron configuration using the Noble Gas Shortcut
1. Locate the atom on the periodic table and determine the number of electrons for the atom.
2. Find the noble gas before the atom, the one on the row above the atom. Place this noble gas
symbol in [ ]. This represents all the inner level electrons.
3. Subtract the electrons for the noble gas from the electrons for the atom.
4. Then start at the beginning of the row the element is on and write the configuration after the
noble gas until the superscripts and the electrons for the noble gas add up to the atom's
electrons. AFTER WRITING THE NOBLE GAS IN BRACKETS DO NOT START BACK
AT PERIOD 1.
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