CHAPTER 3: Question 3
3D explanations
a) A nitrogen atom has five electrons in its outer shell. In the case of
this compound, one of these electrons is removed to leave the positive
charge on the nitrogen atom. The remaining four electrons are required
to form the four covalent bonds (the nitrogen atom uses one electron to
form each bond). Hence, the nitrogen atom is surrounded by four bond pairs
of electrons and the structure will be tetrahedral around the nitrogen
atom as shown below. Since the structure is tetrahedral and the four substituents
attached to the nitrogen atom are all different, the molecule will be chiral
and the nitrogen atom will be a stereocentre.
b) A sulfur atom has six electrons in its outer shell. In the case
of this compound, one of these electrons is removed to leave the positive
charge on the nitrogen atom. Three of the remaining electrons are required
to form the three covalent bonds (the sulfur atom uses one electron to
form each bond), and the remaining two electrons will form a lone pair
of electrons. Hence, the sulfur atom is surrounded by four pairs of electrons
(three bond pairs and a lone pair), the bond structure will be a trigonal
pyramid, and the electron structure will be tetrahedral around the sulfur
atom as shown below. Since the electron structure is tetrahedral and the
four substituents (including the lone pair of electrons as a substituent)
attached to the sulfur atom are all different, the molecule will be chiral
and the sulfur atom will be a stereocentre.
c) A phosphorus atom has five electrons in its outer shell. In the
case of this compound, one of these electrons is removed to leave the positive
charge on the phosphorus atom. The remaining four electrons are required
to form the four covalent bonds (the phosphorus atom uses one electron
to form each bond). Hence, the phosphorus atom is surrounded by four bond
pairs of electrons and the structure will be tetrahedral around the phosphorus
atom as shown below. Since the structure is tetrahedral and the four substituents
attached to the phosphorus atom are all different, the molecule will be
chiral and the phosphorus atom will be a stereocentre.
d) A phosphorus atom has five electrons in its outer shell. These electrons
are all required to form the four s-bonds and
one p-bond (the phosphorus atom uses one electron
to form each bond). In determining molecular geometries, p-bonds
are ignored so since the phosphorus atom is surrounded by four s-bond
pairs of electrons, the structure will be tetrahedral around the phosphorus
atom as shown below. Since the structure is tetrahedral and the four substituents
attached to the phosphorus atom are all different, the molecule will be
chiral and the phosphorus atom will be a stereocentre.
e) A xenon atom has eight electrons (not including the ten d electrons)
in its outer shell. Four of these electrons are required to form the four
covalent bonds (the xenon atom uses one electron to form each bond), leaving
four electrons to form two lone pairs associated with the xenon atom. Hence,
the xenon is surrounded by six electron pairs (four bond pairs and two
lone pairs), so the electron structure will be octahedral. The largest
repulsion will be between the two lone pairs of electrons, so these will
be orientated at 180o to one another as shown below. This results
in a square planar bond geometry, so the compound is not chiral.
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