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Sizes and Shapes of Molecules
and the approximate interatomic distance (D) between two atoms (A
and B) will be given by
D \_B = RA + RB
PROBLEM:
Calculate the C-Cl distance in the CCl 4 molecule.
SOLUTION:
The interatomic distance is the sum of the covalent radii so, with data from Table
9-1,
DC-U = R<. + R<\ = 0.77 + 0.99 = 1.76 A
We have already observed in the preceding problem that the C-Cl bond energy is
about 76 4 kcal/mole, and that it possesses about 6% ionic character.
SHAPES OF MOLECULES
To predict the shape of a molecule, we can apply a set of simple principles.
Before we outline them, however, you should realize that they have little predictive value when you try to answer the question, "What molecule is formed
when two or more particular elements react?" From the same elements, but
under differing conditions, it is possible to prepare many different molecules,
not just one. Instead, our principles will answer the question, "What is the
shape of the molecule with a particular formula?"
The concept of the c entral atom is convenient to use in discussing the shapes
of molecules. In a simple molecule, one of the atoms usually is "central" to the
whole molecule. For example, in CCl 4 the central atom is C, the one to which all
the other atoms are attached.
We shall use the term ligand (L) in its broadest sense to refer to those atoms
that are attached to the central atom. What we mean by the shape of a molecule
or ion is the geometrical arrangement of the ligands about the central atom.
General Predictive Principles
In broad outline, our shape-prediction approach will assume that the valence
electrons of the central atom (M) are all spin-paired (that is, their axes are
parallel but spinning in opposite directions), and that these pairs of electrons
will repel each other in such a way that they occupy positions of minimum
repulsion (that is, positions of minimum potential energy). The electron pairs
will try to get as far away from each other as they can and still stay in the
molecule. The central atom's valence electron pairs will fall into one of two
Sizes and Shapes of Molecules
and the approximate interatomic distance (D) between two atoms (A
and B) will be given by
D \_B = RA + RB
PROBLEM:
Calculate the C-Cl distance in the CCl 4 molecule.
SOLUTION:
The interatomic distance is the sum of the covalent radii so, with data from Table
9-1,
DC-U = R<. + R<\ = 0.77 + 0.99 = 1.76 A
We have already observed in the preceding problem that the C-Cl bond energy is
about 76 4 kcal/mole, and that it possesses about 6% ionic character.
SHAPES OF MOLECULES
To predict the shape of a molecule, we can apply a set of simple principles.
Before we outline them, however, you should realize that they have little predictive value when you try to answer the question, "What molecule is formed
when two or more particular elements react?" From the same elements, but
under differing conditions, it is possible to prepare many different molecules,
not just one. Instead, our principles will answer the question, "What is the
shape of the molecule with a particular formula?"
The concept of the c entral atom is convenient to use in discussing the shapes
of molecules. In a simple molecule, one of the atoms usually is "central" to the
whole molecule. For example, in CCl 4 the central atom is C, the one to which all
the other atoms are attached.
We shall use the term ligand (L) in its broadest sense to refer to those atoms
that are attached to the central atom. What we mean by the shape of a molecule
or ion is the geometrical arrangement of the ligands about the central atom.
General Predictive Principles
In broad outline, our shape-prediction approach will assume that the valence
electrons of the central atom (M) are all spin-paired (that is, their axes are
parallel but spinning in opposite directions), and that these pairs of electrons
will repel each other in such a way that they occupy positions of minimum
repulsion (that is, positions of minimum potential energy). The electron pairs
will try to get as far away from each other as they can and still stay in the
molecule. The central atom's valence electron pairs will fall into one of two