6 . D
When atomic orbitals combine, they form molecular orbitals. When two atomic orbitals with the
same sign are added head-to-head or tail-to-tail, they form bonding molecular orbitals. When
two atomic orbitals with opposite signs are added head-to-head or tail-to-tail, they form
antibonding molecular orbitals. Atomic orbitals can also hybridize, forming sp^3 , sp^2 or sp orbitals.7 . B
Like atomic orbitals, molecular orbitals each can contain a maximum of two electrons with
opposite spins. The 2n^2 rule in choice (D) refers to the total number of electrons that can exist in
a given energy shell, not in a molecular orbital.8 . B
π bonds are formed by the parallel overlap of unhybridized p-orbitals. The electron density is
concentrated above and below the bonding axis. A σ bond, on the other hand, can be formed by
the head-to-head overlap of two s-orbitals or hybridized orbitals. In a σ bond, the density of the
electrons is concentrated between the two nuclei of the bonding atoms.9 . A
Each single bond has one σ bond, and each double bond has one σ and one π bond. In this
question, there are five single bonds (five σ bonds) and one double bond (one σ bond and one π
bond), which gives a total of six σ bonds and one π bond. Thus, the correct answer is choice (A).10 . C
The four bonds point to the vertices of a tetrahedron, which means that the angle between two
bonds is 109.5°, a characteristic of sp^3 orbitals. Hence, the carbon atom of CH 4 is sp^3 -hybridized.
11 . B
Bond strength is determined by the degree of orbital overlap; the greater the overlap, the greater
the bond strength. A π bond is weaker than a single bond because there is significantly less
overlap between the unhybridized p-orbitals of a π bond (due to their parallel orientation) than
between the s-orbitals or hybrid orbitals of a σ bond. sp^3 -hybridized orbitals can be quite stable,
as evidenced by the number of carbon atoms with this hybridization forming stable compounds.
12 . B
The carbon bond in hydrogen cyanide (H–C≡N:) is triple-bonded, and because triple bonds
require two unhybridized p-orbitals, the carbon must be sp-hybridized; sp-hybridized orbitals