Physical Chemistry : Quantum Chemistry
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Example Questions
Example Question #21 : Nuclear, Quantum, And Molecular Chemistry
Which of the following is/are true regarding the principal quantum number?
I. Principal quantum number signifies the energy level
II. Principal quantum number can never equal zero
III. Principal quantum number can be positive or negative
I and II
I, II, and III
I and III
II only
I and II
Quantum numbers are fancy coordinate systems that describe the potential location of an electron within an atom. The first quantum number is called the principal quantum number and it signifies the shell the electron is located in. Recall that electron shells are discrete orbits in an atom that have discrete energy; therefore, the principal quantum number signifies the energy level of an electron.
The principal quantum number is always an integer and is always greater than zero. If 
Example Question #1 : Quantum Numbers
How many subshells are there with n = 4 in an atom?
The types of subshells, from smallest to largest, are as follows: s, p, d, and f. These four subshells correspond respectively to the following quantum numbers: 0, 1, 2, and 3. The total number of sublevels with n = 4 is n or 4: 4s, 4p, 4d and 4f.
Example Question #3 : Quantum Numbers
Which of the following set of quantum numbers is not valid?
n=4
l=2
m=-3
n=3
l=1
m=0
n=2
l=1
m=1
n=3
l=2
m=-1
n=2
l=1
m=0
n=4
l=2
m=-3
The principle quantum number (n) and the angular quantum number (l) are acceptable. However, the magnetic quantum number (m) is restricted to lie between -l and l. Therefore, for l=2, the only possible numbers for m are -2, -1, 0, 1, 2.
Example Question #1 : Spectroscopy
Alkynes exhibit a distinctive peak in the 
There was not enough sample for the detector to read
The instrument is malfunctioning; ethyne should definitely show a peak at
Ethyne is has a center of symmetry about its triple bond, making it IR inactive
Ethyne does not contain triple bonds
The compound synthesized was not ethyne, and therefore the IR spectrum is incorrect
Ethyne is has a center of symmetry about its triple bond, making it IR inactive
Infrared spectroscopes uses dipole changes as a means of recording data. Because ethyne has a center of symmetry about its triple bond, there is no net dipole change when the bond stretches, and therefore no signal is measured by the detector. Such molecules are called IR inactive and a Raman spectrum must be obtained in order to observe the desired peaks.
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