Download Chemistry Week 04 - nchsdduncanchem1

Quantum Numbers:
four numbers used to describe the electrons in an atom.
The Bohr model was a one-dimensional model that used one quantum number to describe the
electrons in the atom. Only the size of the orbit was important, which was described by the n
quantum number. Schr–dinger described an atomic model with electrons in three dimensions.
This model required three coordinates, or three quantum numbers, to describe where electrons
could be found.
The three coordinates from Schr–dinger's wave equations are the principal
(n), angular (l), and magnetic (m) quantum numbers. These quantum numbers
describe the size, shape, and orientation in space of the orbitals on an atom.
1. Principal (shell) quantum number -
n
Describes the energy level within the atom.

Energy levels are 1 to 7

Maximum number of electrons in n is 2 n
2. Momentum (subshell) quantum number -
2
l
Describes the sublevel in n

Sublevels in the atoms of the known elements are s - p - d - f

Each energy level has n sublevels.

Sublevels of different energy levels may have overlapping energies.
The momentum quantum number also describes the shape of the orbital.

Orbitals have shapes that are best described as spherical (l = 0), polar (l = 1), or
cloverleaf (l = 2).

Orbitals even take on more complex shapes as the value of the angular quantum
number becomes larger.
3. Magnetic quantum number -
m
Describes the orbital within a sublevel

s has 1 orbital

p has 3 orbitals

d has 5 orbitals

f has 7 orbitals
Orbitals contain 1 or 2 electrons, never more.
m also describes the direction, or orientation in space for the orbital.
This diagram shows the three possible orientations of a p orbital - px, py, pz. Atomic
orbitals are studied in great detail in a college "theoretical inorganic chemistry" class.
If you are curious, study these websites:
4. Spin quantum number -
s
This fourth quantum number describes the spin of the electron.

Electrons in the same orbital must have opposite spins.

Possible spins are clockwise or counterclockwise.
Rules governing the combinations of quantum numbers:

Three quantum numbers n, l, and m are integers.

The principal quantum number (n) cannot be zero.


The angular quantum number (l) can be any integer between 0 and n - 1.


For n = 3, l can be either 0, 1, or 2.
The magnetic quantum number (m) can be any integer between -l and +l.


n must be 1, 2, 3, etc.
For l = 2, m can be either -2, -1, 0, +1, or +2.
The spin quantum number (s) is arbitrarily assigned the numbers +1/2 and -1/2.
Pauli Exclusion Principle:
No two electrons in an atom have the same set of four quantum numbers.
Hund's Rule:
Electrons will enter empty orbitals of equal energy, when they are available.
Quantum Chemistry: Describes the way atoms combine to form molecules and the way
molecules interact with one another, using the rules of quantum physics.
One of the key insights in quantum chemistry is that, because an electron is not a classical particle
located at a definite point in space, even a single electron can "surround" the nucleus of an atom,
filling a volume roughly as big as the whole atom. Instead of thinking of electron shells neatly
nested inside one another, it is better to visualize electrons in interpenetrating orbitals, like a lot of
ripples on a pond. Each individual electron cloud extends down to "touch" the nucleus, and all the
electrons in an atom come under the direct influence of the nucleus, although some are influenced
more strongly than others. Since there is not enough room for all the orbitals to fit next to the
nucleus, some of them are concentrated further out from the nucleus than others. Some
arrangements of electrons in orbitals are more stable than others, and atoms will interact to reach
these stable arrangements (the basis for chemical bonds).
Concept Understanding:
1. When was Niels Henrik David Bohr born?
2. When was Erwin Rudolf Josef Alexander Schr–dinger born?
3. What is the maximum number of electrons possible in the 6th electron energy level?
4. How many sublevels are found in the 3rd electron energy level?
5. How many orbitals are in the d sublevel of an energy level?
6. What is the maximum number of electrons in the outer electron energy level of any atom?
7. The atomic theory tells us that the fifth electron energy level has five sublevels. Why do we only
have letters for four sublevels - s, p, d, f ?
8. What are the possible numbers for m?
9. What are the possible numbers for the fourth quantum number?
Electron configuration:
The shape of today's periodic table shows energy levels, sublevels, and orbitals.
 Except for helium, you can read the electron configuration directly from the table.
 If you mentally move helium beside hydrogen, above beryllium, it will be in place to read its
electron configuration.
 Remember the energy overlaps in the d and f sublevels.
 There are a "few" electron configuration exceptions, chromium being the first, but this class
will not be concerned with the exceptions.
2
2
The electron configuration for chlorine is 1s 2s
 The large numbers represent the energy level.
 The letters represent the sublevel.
2p6 3s2 3p5

The superscripts indicate the number of electrons in the sublevel. Think of each square
on the table as an electron. Counting the number of squares will give you the number of
electrons in each energy level and sublevel.
There are two ways to check an electron configuration:

The last notation in the electron configuration should represent the location
of the element on the periodic table.
Example: the 3p5 in the electron configuration for chlorine above indicates its
location as the fifth square in the p sublevel on the third row of the periodic table.

The total of the superscripts in an electron configuration will equal the
atomic number of the element.
Example: the total of the superscripts in the electron configuration for chlorine
above is 17, its atomic number.