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Chapter 4 Section 3
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Chapter 4 Sec 3: Rules Governing Electron Configurations
Pauli Exclusion Principle states that no two electrons in the same atom
can have the same four quantum numbers. Every electron is different than
all the other electrons in an atom.
No 2 electrons can be in the same
space
Electron Configurations: the arrangement of electrons in an atom.
An arrangement – the “certain way”
the electrons are together
Configuration Notations: The model used to exhibit electron
configurations.
Electron arrangement written down
Aufbau Filling Principle An electron occupies the lowest orbital
(energy) that can receive it.
Energies are filled from the bottom
up
Hund’s Rule states that orbitals of equal energy are each occupied by one
electron before any are occupied by a second electron and all electrons in
singly occupied orbitals must have the same spin.
all orbitals in a group get one electron
before any get 2; all first electrons
have the same spin direction
Examples of the p-orbital filling
This figure shows how two (a), three (b), and four (c) electrons fill the p
sublevel of any given main energy level according to Hund’s rule. Since
up and down are arbitrarily assigned, convention has given that we start
with up and then down after the “up spin” has filled a sublevel.
sublevel = orbital
orbital notation examples:
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Orbital Notation
In Orbital Notation, an unoccupied orbital is represented by a line,
with the orbital’s name written underneath the line – the next step after the Bohr
model
_______
1s
An orbital containing one electron is represented as
two electrons is represented as
opposite spins.
. An orbital containing
, showing the electrons paired and with
Putting it all together:
The lines are labeled with the principal quantum number and sublevel letter beneath
the line.
For example, the orbital notations for Hydrogen, Helium, and Boron are written as
follows.
Hund’s rule is followed when using this notation. For Example Oxygen’s notation
is:
Electron-configuration notation eliminates the lines and
arrows of orbital notation. Instead, the number of electrons in a sublevel is shown
by adding a superscript to the sublevel designation.
Hydrogen’s Electron-configuration notation is 1s1.
The superscript indicates that one electron is
present in hydrogen’s 1s orbital.
Helium’s configuration is 1s2. The superscript
indicates that there are two electrons in Helium’s
1s orbital.
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The electron configuration for Boron is 1s22s22p1.
The superscript indicates the number of electrons
in each orbital.
The electron configuration for Oxygen is
1s22s22p4. The superscript indicates the number
of electrons in each orbital.
Electron-configuration notation general Rule
First write the Principle Quantum number (the
energy level: same as the period on our periodic
table). Then write the angular momentum
quantum number (the orbital shape: s, p, d, or f)
Finally, superscript the number of electrons in the
orbital. (maxes: s 2, p6, d10, and f14)
n l # of electrons in the orbital
Principle Quantum Number: n (1 – 7)
Angular Momentum Quantum Number: l (s, p,
d, or f)
# of electrons in the orbital (just the e –’s in the “l”
orbital
«Num»
Coefficient is the
principle quantum #; the
letter is the angular
momentum quantum #;
the superscript is the
number of electrons, this
indicates both the
magnetic quantum # and
spin
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Nobel Gas Configuration
Neon’s electron configuration is 1s22s22p6
Phosphorus’s electron configuration is 1s22s22p63s23p3
Notice how Neon’s configuration is the first part of Phosphorus’s configuration.
Neon
1s22s22p6
Phosphorus 1s22s22p63s23p3
Just like you can substitute in a math equation, chemist substitute Neon’s
configuration with Neon’s symbol Square bracketed to distinguish that it is Neon’s
configuration and not Neon.
Similarly, all the noble gas configurations can be
used to substitute into configurations as
shorthand.
P’s Nobel - Gas Notation: [Ne] 3s23p3
This shorthand configuration is called Nobel - Gas
Notation. See page 111, 112, and 114 for more
examples.
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Inner-Shell Electrons: electrons that are not in
the highest occupied energy level.
Highest Occupied Level: the electron-containing
(has electrons in it) main energy level with the
highest principal quantum number.
Draw Orbital Notation, Electron Configuration
Notation, and Nobel - Gas Notation for fluorine,
chlorine, bromine and iodine. (The halogens)
F: 1s2 2s2 2p5 F: [He] 2s2 2p5
Cl: 1s2 2s22p6 3s23p5 Cl: [Ne] 3s23p5
Br: 1s2 2s22p6 3s23p63d10 4s24p5
Br: [Ar] 3d10 4s2 4p5
I: 1s2 2s22p6 3s23p63d10 4s24p64d10 5s25p5
I: [Kr]4d105s25p5
Also a note about the Halogens and a few other elements of the Periodic Table: There are some elements
that do not exist as single atoms in their elemental form. They exist as “pairs” and are called Diatomic
elements
The diatomic elements are hydrogen, nitrogen, oxygen, and the halogens: fluorine, chlorine, bromine,
iodine, and astatine. Astatine is so rare in nature (its most stable isotope has a half-life of only 8.1 hours)
that it is usually not considered. Many metals are also diatomic when in their gaseous states.
HW: