In valence bond theory, a covalent bond forms when half-filled orbitals on two atoms overlap and their electrons pair.
Correct Answer
True
True. Each atom contributes one electron in a half-filled orbital, and the overlapping orbitals hold the shared pair between the nuclei. More overlap generally gives a stronger bond.
Category: Valence Bond Theory
Question 2
The bond length of H2 corresponds to the separation at which the energy of the two atoms is at its maximum.
Correct Answer
False
False. The bond length is the separation at which the energy is at its minimum. At that distance attraction and repulsion are balanced, and the H-H distance is 74 pm.
Category: Valence Bond Theory
Question 3
Every single covalent bond is a sigma bond.
Correct Answer
True
True. A single bond results from end-to-end overlap that concentrates electron density along the internuclear axis, which defines a sigma bond. Pi bonds appear only as the second or third bond in multiple bonds.
Category: Valence Bond Theory
Question 4
A pi bond has its electron density concentrated directly along the internuclear axis.
Correct Answer
False
False. A pi bond places electron density above and below the internuclear axis, with a nodal plane along the axis itself. Density concentrated along the axis describes a sigma bond.
Category: Valence Bond Theory
Question 5
A sigma bond can form from the end-to-end overlap of an s orbital on one atom and a p orbital on another, as in HCl.
Correct Answer
True
True. Sigma bonds can come from s-s, s-p or p-p end-to-end overlap. In HCl, the hydrogen 1s orbital overlaps a chlorine 3p orbital along the bond axis.
Category: Valence Bond Theory
Question 6
A pi bond between two carbon atoms is generally weaker than the sigma bond between them.
Correct Answer
True
True. Side-by-side overlap of p orbitals is less extensive than end-to-end overlap, so a pi bond contributes less energy. A C=C double bond is stronger than a C-C single bond but less than twice as strong, which shows the pi bond adds less energy than the sigma bond.
Category: Multiple Bonds
Question 7
A double bond consists of two sigma bonds.
Correct Answer
False
False. A double bond consists of one sigma bond and one pi bond. Only one end-to-end overlap can occur along the axis between two atoms, so any additional bonds are pi bonds.
Category: Multiple Bonds
Question 8
In an sp hybridized carbon atom, one p orbital remains unhybridized.
Correct Answer
False
False. sp hybridization uses the 2s orbital and one 2p orbital, leaving two unhybridized p orbitals. These two p orbitals can form the two pi bonds of a triple bond.
Category: Hybrid Orbitals
Question 9
The hybrid orbitals within one set are equivalent to each other in shape and energy.
Correct Answer
True
True. Mixing atomic orbitals produces a set of identical hybrid orbitals that differ only in direction. For example, the four sp3 orbitals of carbon in CH4 are equivalent and point to the corners of a tetrahedron.
Category: Hybrid Orbitals
Question 10
Hybrid orbitals exist in isolated atoms before any bonds form.
Correct Answer
False
False. Hybrid orbitals exist only in atoms that are bonded. Hybridization is a model used to explain the observed geometry of molecules, not a property of free atoms.
Category: Hybrid Orbitals
Question 11
In H2O, two of the oxygen atom's sp3 hybrid orbitals hold lone pairs.
Correct Answer
True
True. Oxygen in water has four regions of electron density, two bonds and two lone pairs, so it is described as sp3 hybridized. Two of its hybrid orbitals bond to hydrogen and the other two hold lone pairs.
Category: Hybrid Orbitals
Question 12
Second-period atoms such as carbon and nitrogen can form sp3d hybrid orbitals.
Correct Answer
False
False. sp3d and sp3d2 hybridization require d orbitals in the valence shell, which second-period atoms lack. Carbon and nitrogen can use at most four orbitals, one 2s and three 2p, so they form no more than four hybrid orbitals.
Category: Hybrid Orbitals
Question 13
Drawing a different resonance structure of SO2 does not change the hybridization assigned to the sulfur atom.
Correct Answer
True
True. Resonance structures differ only in the placement of pi bonds and electrons, while hybridization depends on sigma bonds and lone pairs. Sulfur in SO2 is sp2 in every resonance form.
Category: Multiple Bonds
Question 14
When atomic orbitals combine, the number of molecular orbitals formed equals the number of atomic orbitals combined.
Correct Answer
True
True. Orbitals are conserved in the combination, so two atomic orbitals give one bonding and one antibonding molecular orbital. This rule holds for any set of combining orbitals.
Category: Molecular Orbitals
Question 15
Electrons in antibonding molecular orbitals strengthen the bond between two atoms.
Correct Answer
False
False. Electrons in antibonding orbitals weaken the bond because these orbitals have reduced electron density between the nuclei. They are subtracted when calculating bond order.
Category: Molecular Orbitals
Question 16
A bonding molecular orbital is lower in energy than the atomic orbitals that combine to form it.
Correct Answer
True
True. The in-phase combination builds electron density between the nuclei, stabilizing the electrons. Its antibonding partner lies higher in energy than the original atomic orbitals.
Category: Molecular Orbitals
Question 17
A pi bonding molecular orbital has a nodal plane that contains the internuclear axis.
Correct Answer
True
True. A pi orbital forms from side-by-side p orbitals, which have zero electron density along the line through both nuclei. The electron density lies on both sides of that plane.
Category: Molecular Orbitals
Question 18
A higher bond order indicates a weaker bond between two atoms.
Correct Answer
False
False. A higher bond order indicates a stronger and generally shorter bond. A bond order of zero means no stable bond forms.
Category: Bond Order and Magnetism
Question 19
The Lewis structure of O2 correctly predicts that the molecule is paramagnetic.
Correct Answer
False
False. The Lewis structure of O2 shows every electron paired, which would make it diamagnetic, yet experiments show it is attracted to a magnetic field. Molecular orbital theory explains this by placing two unpaired electrons in degenerate pi antibonding orbitals.
Category: Bond Order and Magnetism
Question 20
Molecular orbital theory predicts that Li2 has a bond order of 0.
Correct Answer
False
False. Li2 has two valence electrons in the sigma 2s bonding orbital and none in its antibonding partner, giving a bond order of 1. Li2 molecules are found in lithium vapor.
Category: Bond Order and Magnetism