Download Electron Configurations

NASA‐Threads
Work & Mechanics
11‐Atoms and Molecules
Electron Configurations
So you learned about the Bohr model of an atom as well the
electronic configuration of that atom. If you have taken or are
taking any sort of an advanced chemistry class, then you
probably didn’t have much trouble with these concepts. Otherwise,
you may want some extra information on the subject.
Most of this below is “borrowed” from Sparknotes.com.
The first and most important rule to remember when attempting to determine
how electrons will be arranged in the atom is Hund’s rule, which states that the
most stable arrangement of electrons is that which allows the maximum number
of unpaired electrons. This arrangement minimizes electron-electron repulsions.
Here’s an analogy. In large families with several children, it is a luxury for each
child to have his or her own room. There is far less fussing and fighting if siblings
are not forced to share living quarters: the entire household experiences a lowerintensity, less-frazzled energy state. Likewise, electrons will go into available
orbitals singly before beginning to pair up. All the single–occupant electrons of
orbitals have parallel spins, are designated with an upward-pointing arrow, and
have a magnetic spin quantum number of +1/2.
As we mentioned earlier, each principal energy level, n, has n sublevels. This
means the first has one sublevel, the second has two, the third has three, etc. The
sublevels are named s, p, d, and f.
Number of Names of
Energy level
sublevels
sublevels
principal quantum
number, n
1
1
s
2
2
s, p
3
3
s, p, d
4
4
s, p, d, f
At each additional sublevel, the number of available orbitals is increased by
two: s = 1, p = 3, d = 5, f = 7, and as we stated above, each orbital can hold only
two electrons, which must be of opposite spin. So s holds 2, p holds 6 (2
electrons times the number of orbitals, which for the p sublevel is equal to
3), d holds 10, and f holds 14.
NASA‐Threads
Work & Mechanics
Sublevel
s p d
11‐Atoms and Molecules
f
Number of orbitals
1 3 5 7
Maximum number of electrons
2 6 10 14
Quantum number, l
0 1 2 3
Orbital Notation
Orbital notation is basically just another way of expressing the electron
configuration of an atom. It is very useful in determining quantum numbers as
well as electron pairing. The orbital notation for sulfur would be represented as
follows:
Notice that electrons 5, 6, and 7 went into their own orbitals before electrons 8,
9, and 10 entered, forcing pairings in the 2p sublevel; the same thing happens in
the 3plevel.
Now we can determine the set of quantum numbers. First, n = 3, since the
valence electron (the outermost electron) is a 3p electron. Next, we know
that p sublevels have an l value of 1. We know that ml can have a value
between l and -l, and to get theml quantum number, we go back to the orbital
notation for the valence electron and focus on the 3p sublevel alone. It looks like
this:
Simply number the blanks with a zero assigned to the center blank, with negative
numbers to the left and positive to the right of the zero. The last electron was
number 16 and “landed” in the first blank as a down arrow, which means its ml =
-1 and ms = -1/2, since the electron is the second to be placed in the orbital and
therefore must have a negative spin.
So, when determining ml, just make a number line underneath the sublevel, with
zero in the middle, negative numbers to the left, and positive numbers to the
right. Make as many blanks as there are orbitals for a given sublevel. For
assigning ms, the first electron placed in an orbital (the up arrow) gets the +1/2
and the second one (the down arrow) gets the -1/2.
Example
Which element has this set of quantum numbers: n = 5, l = 1, ml = -1, and ms = 1/2?
NASA‐Threads
Work & Mechanics
11‐Atoms and Molecules
Explanation
First, think about the electron configuration: n = 5 and l = 1, so it must be a
5pelectron. The ms quantum number corresponds to this orbital notation
picture:
Be sure to number the blanks and realize that the -1/2 means it is a pairing
electron! The element has a configuration of 5p4; so it must be tellurium.
Example
Complete the following table:
Element
Valence electron
configuration
Valence orbital notation
Set of quantum
numbers
[Ar] 3d 6
5, 1, 0, +1/2
4p 5
Answer:
element
Valence
electron
configuration
K
[Ar] 4s 1
4, 0, 0, +1/2
Fe
[Ar] 4s23d 6
3, 2, -2, 1/2
N
1s 2 2s 2 2p 3
2, 1, 1, +1/2
Sn
[Kr]
5s 2 4d 1 0 5p 2
5, 1, 0, +1/2
Br
[Ar]
4s 2 3d 1 0 4p 5
4, 1, 0, -1/2
Valence orbital notation
Set of
quantum
numbers
(n,l, ml, ms)
NASA‐Threads
Work & Mechanics
11‐Atoms and Molecules