Question

The Group \(5 \mathrm{A}\) (15) elements can form molecules or ions that involve three, five, or six covalent bonds; \(\mathrm{NH}_{3}, \mathrm{AsCl}_{5},\) and \(\mathrm{PF}_{6}^{-}\) are examples. Draw the Lewis structure for each of these substances, and predict the molecular structure and hybridization for each. Why doesn't \(\mathrm{NF}_{5}\) or \(\mathrm{NCl}_{6}^{-}\) form?

Short answer

The Lewis structures for NH3, AsCl5, and PF6- are as follows: NH3: Trigonal pyramidal molecular geometry with sp3 hybridization. H | H-N-H : AsCl5: Trigonal bipyramidal molecular geometry with sp3d hybridization. Cl | Cl-As-Cl | Cl PF6-: Octahedral molecular geometry with sp3d2 hybridization. F \ F-P-F | \ F F NF5 and NCl6- do not form stable compounds because nitrogen, being in the 2nd period of the periodic table, has a smaller size and can only accommodate a maximum of 4 electron pairs (including bond pairs and lone pairs) in its valence shell, which corresponds to an sp3 hybridization. This is due to the unavailability of d orbitals for 2nd period elements.

Step-by-step solution

Lewis Structure

First, we need to draw the Lewis structures for NH3, AsCl5, and PF6-. To do this, we count the total number of valence electrons for each compound, and distribute them to form bonds and satisfy the octet rule for each atom. For NH3 (ammonia): 1. Nitrogen has 5 valence electrons, and each hydrogen atom has 1 valence electron. The total valence electrons for ammonia are 5 (from N) + 3*1 (from H) = 8 electrons. 2. Nitrogen forms three single covalent bonds with three hydrogen atoms, using 6 valence electrons. 3. The remaining 2 valence electrons form a lone pair on nitrogen. The Lewis structure for NH3 is: H | H-N-H : (the lone pair is represented by two dots) For AsCl5 (arsenic pentachloride): 1. Arsenic has 5 valence electrons, and each chlorine atom has 7 valence electrons. The total valence electrons for AsCl5 are 5 (from As) + 5*7 (from Cl) = 40 electrons. 2. Arsenic forms single covalent bonds with five chlorine atoms, using 10 valence electrons. 3. Each chlorine atom has 3 lone pairs, using the remaining 30 valence electrons. The Lewis structure for AsCl5 is: Cl | Cl-As-Cl | Cl For PF6- (hexafluorophosphate ion): 1. Phosphorus has 5 valence electrons, each fluorine atom has 7 valence electrons, and the overall ion has an extra electron (due to the -1 charge). The total valence electrons for PF6- are 5 (from P) + 6*7 (from F) + 1(extra electron) = 48 electrons. 2. Phosphorus forms single covalent bonds with six fluorine atoms, using 12 valence electrons. 3. Each fluorine atom has 3 lone pairs, using the remaining 36 valence electrons. The Lewis structure for PF6- is: F \ F-P-F | \ F F

Molecular Geometry and Hybridization

We will use the VSEPR theory to predict the molecular geometry for each compound, and then determine the corresponding hybridization. For NH3: 1. The nitrogen atom in NH3 has 3 bonding electron pairs and 1 lone pair. Therefore, the VSEPR electron pair arrangement is tetrahedral. The molecular geometry is trigonal pyramidal, due to the presence of 3 bonded atoms and 1 lone pair. 2. Hybridization for a tetrahedral electron pair arrangement is sp3. For AsCl5: 1. The arsenic atom in AsCl5 has 5 bonding electron pairs and no lone pair. The VSEPR electron pair arrangement and the molecular geometry are both trigonal bipyramidal. 2. Hybridization for a trigonal bipyramidal electron pair arrangement is sp3d. For PF6-: 1. The phosphorus atom in PF6- has 6 bonding electron pairs and no lone pair. The VSEPR electron pair arrangement and the molecular geometry are both octahedral. 2. Hybridization for an octahedral electron pair arrangement is sp3d2.

Stability of NF5 and NCl6-

NF5 and NCl6- are not stable compounds because nitrogen, being in the 2nd period of the periodic table, has a smaller size and can only accommodate a maximum of 4 electron pairs (including bond pairs and lone pairs) in its valence shell, which corresponds to an sp3 hybridization. This is due to the unavailability of d orbitals in the 2nd period elements. In the hypothetical NF5 molecule, nitrogen would have to form 5 bond pairs, which is not possible due to its limited number of available orbitals. Similarly, in the hypothetical NCl6- ion, nitrogen would have to form 6 bond pairs, which is also not possible due to the same reasoning. Hence, NF5 and NCl6- do not form stable compounds.

Key concepts

VSEPR theory

VSEPR theory is an essential tool to predict the shape of molecules based on the repulsion between electron pairs around a central atom. In VSEPR, which stands for Valence Shell Electron Pair Repulsion, electron pairs (bonding or lone pairs) arrange themselves as far apart as possible around the central atom to minimize repulsion.
This arrangement determines the molecule's overall geometry.

• For example, NH3 (ammonia) has a tetrahedral electron arrangement but takes a trigonal pyramidal shape because of one lone pair.
• AsCl5 (arsenic pentachloride) exhibits a trigonal bipyramidal structure, as it has five bonding pairs and no lone pairs around arsenic.
• PF6- (hexafluorophosphate ion) has six bonding pairs and adopts an octahedral geometry.

Understanding these geometries is crucial for predicting molecular properties such as polarity and reactivity.

Hybridization

Hybridization is a concept that explains the mixing of atomic orbitals to form new hybrid orbitals, which influence molecular geometry and bonding. Different hybridizations correspond to specific molecular geometries.

• In NH3, the nitrogen atom undergoes sp3 hybridization, forming four equivalent orbitals that shape the molecule into a trigonal pyramidal geometry.
• As for AsCl5, arsenic undergoes sp3d hybridization, enabling it to accommodate five electron pairs in a trigonal bipyramidal arrangement.
• In PF6-, the phosphorus atom utilizes sp3d2 hybridization, allowing six surrounding fluorines, resulting in an octahedral geometry.

Understanding hybridization helps to relate the theoretical models to the actual geometry of molecules, providing insight into how molecules interact and react under different conditions.

Covalent bonds

Covalent bonds are the glue that holds molecules together by the sharing of electron pairs between atoms. This type of bonding is crucial for the three molecules described in the exercise. Each bond involves a pair of shared electrons between two atoms.
In NH3, the nitrogen shares its electrons with hydrogen, forming three covalent bonds. Phosphorus in PF6- forms covalent bonds with fluorine atoms. Arsenic in AsCl5 manages to bond with five chlorine atoms via its available valence electrons.
These shared electron pairs are responsible for the stable configurations of molecules. Covalent bonds are directional, giving rise to specific geometric arrangements as predicted by VSEPR theory and explained through hybridization.

Molecular geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It is determined by the number of bonding and lone electron pairs around the central atom.

• For NH3, the presence of one lone pair on nitrogen alters the ideal tetrahedral arrangement to a trigonal pyramidal shape, commonly found in ammonia.
• AsCl5 presents a trigonal bipyramidal geometry, where the central arsenic atom forms bonds in two geometric planes, due to five available bonding pairs.
• PF6- is characterized by an octahedral shape, with six fluorine atoms symmetrically surrounding the phosphorus atom.

Understanding molecular geometry is fundamental in chemistry as it affects molecular properties like polarity, color, magnetism, and reactivity, which are key to comprehending the behavior of substances.