All Physical Chemistry Resources
Example Questions
Example Question #5 : Quantum Numbers
Which of the following is true regarding ethene? (The electrons in answer choices refer to carbon electrons)
The electrons in bond have the same set of quantum numbers
The electrons in the bond have the same orbital angular momentum number
The electrons in the bonds and the bond have different principal quantum number
The electrons in the bonds have the same set of quantum numbers
The electrons in the bond have the same orbital angular momentum number
Ethene, or , has a carbon-carbon single () and double () bond. Recall that a bond can be found in hybridized orbitals whereas a bond cannot. This means that the carbon atoms in ethene hybridize the single ‘s’ orbital and two of the ‘p’ orbitals, forming a hybridization. The bonds () are found in these three hybridized orbitals. The remaining unhybridized ‘p’ orbital will house the two electrons in the bond.
The orbital angular momentum number is the second quantum number and it signifies the type of orbital. An electron found in a ‘p’ orbital will always have an . Since both electrons in the bond are found in the ‘p’ orbital, the ‘l’ value for both electrons is the same.
The principal quantum number is the first quantum number and it signifies the shell or energy level of an electron. All electrons involved in bonds are found in carbon’s outermost shell (2nd shell); therefore, they will all have an . Remember that no two electrons can have the same set of quantum numbers (regardless of whether the electrons are found as lone pairs, in bonds, or in bonds).
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 the electron is found within the first shell, if then the electron is found within the second shell, and so and and so forth. Also, since it is always greater than zero, the principal quantum number can never be negative.
Example Question #21 : Quantum Chemistry
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 #22 : Quantum Chemistry
Alkynes exhibit a distinctive peak in the region of an IR spectrum, caused by the stretching of the triple bond. After performing a synthesis of ethyne in the laboratory, she takes an infrared spectrum of the synthesized compound and no such peak appears. Which of the following statements best explains the reason for this phenomenon?
The compound synthesized was not ethyne, and therefore the IR spectrum is incorrect
Ethyne does not contain triple bonds
Ethyne is has a center of symmetry about its triple bond, making it IR inactive
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
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.
Example Question #1 : Covalent Bonding
Which statement best characterizes a covalent bond?
Electrical conduction
Crystal structure
Sharing electrons between atoms
Cations and anions
Electron sea
Sharing electrons between atoms
To achieve an octet of valence electrons, atoms can share electrons so that all atoms participating in the bond will have full valence shells. Covalent bonds, by definition, result from the sharing of one or more pairs of valence electrons.
Example Question #2 : Covalent Bonding
How many single covalent bonds would the element sulfur be expected to form in order to obey the octet rule?
The key to this problem is that electrons in covalent bonds are shared and therefore "belong" to both of the bonded atoms. Sulfur is a nonmetal in group 6A , and therefore has 6 valence electrons. In order to obey the octet rule, it needs to gain 2 electrons . It can do this by forming 2 single covalent bonds.
Example Question #3 : Covalent Bonding
How many single covalent bonds would the element selenium be expected to form in order to obey the octet rule?
0
The key to this problem is that electrons in covalent bonds are shared and therefore "belong" to both of the bonded atoms. Selenium is a nonmetal in group 6A , and therefore has 6 valence electrons. In order to obey the octet rule, it needs to gain 2 electrons. It can do this by forming 2 single covalent bonds.
Example Question #4 : Covalent Bonding
Which of the following constitutes a covalent bond?
All of these
Constructive interference between atomic orbitals
Electron pair shared between two neighboring atoms
Spin pairing of electrons
Atoms of similar electronegativities
All of these
A chemical bond is considered covalent if there is sharing of one or more pairs of electrons between atoms. As opposed to a covalent bond, an ionic bond can involve the transfer of electrons from one atom to another resulting in a high charge differential between two atoms in order for them to acquire a full octet.
As described by the Pauli Exclusion Principle, every pair of electrons must consist of spin-up paired with spin-down. It states that no more than two electrons may occupy an orbital, and in full electron orbitals, the spin of one must cancel the spin of the other so their spins will have a zero net spin/angular momentum.
Chemical bonds are made up of orbitals, which are simply waves that have wave functions. Wave functions tell us the likelihood that an electron can be found within an orbital. Constructive interference of two wave functions/orbitals occur when the two waves are in phase and result in a new wave function, or in other words a chemical bond.
Example Question #1 : Molecules
Which of the following molecules does not contain polar bonds?
Polar bonds form between all atoms of different electronegativity. Bromine is a diatomic molecule and thus both atoms of bromine have the same electronegativity. This bond between bromine is perfectly nonpolar, meaning that both atoms share the electron density equally.
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