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The formation of atoms from fundamental particles, interesting as this might be to the physicist, is far from being the ultimate stage in the organization of matter. As we mentioned earlier, when atoms are close enough to one another that the outer electrons of one atom can interact with the other atoms, then attractions can be set up between atoms, strong enough to hold them together in what is termed a chemical bond. In the simplest cases the bond arises from the sharing of two electrons between a pair of atoms, with one electron provided by each of the bonded atoms. Bonds based on electron sharing are known as covalent bonds, and two or more atoms held together as a unit by covalent bonds are known as a molecule. One of the principal triumphs of the theory of quantum mechanics in chemistry (see Chapter 8) has been its ability to predict the kinds of atoms that will bond together, and the three-dimensional structures and reactivities of the molecules that result. In molecular diagrams, a covalent, electron-sharing bond is represented by a straight line connecting the bonded atoms. In the water molecule, one atom of oxygen (O) is bonded to two hydrogen (H) atoms. The diagram for the molecule can be drawn two ways:
The second version acknowledges the fact that a water molecule is not linear; the two H—O bonds make an angle of 104.5° with one another. Molecules of hydrogen gas, hydrogen sulfide, ammonia, methane, and methyl alcohol (methanol) have the following bond structures:
Figure 1.2: Shapes and relative sizes of some sample molecules. Two bonded atoms appear to interpenetrate because their electron clouds overlap. By convention, a tapered bond in a drawing represents a bond pointing out toward the observer, with the wide end or the taper closest, and a dashed line is used for a bond that points back behind the plane of the page.
These diagrams show only the connections between atoms in the molecules. They do not show the three-dimensional geometries (or shapes) of the
Each of the molecular diagrams shown can be condensed to a molecular formula, which tells how many atoms of each element are in the molecule,
The sum of the atomic weights of all the atoms in a molecule is its molecular weight. Using the atomic weights from the periodic table, we can calculate molecular weights. The molecular weight of hydrogen, H2, is
2 x 1.0080 u = 2.0160 u
A water molecule, H2O, has two atoms of hydrogen and one atom of oxygen, so:
Calculate the molecular weight of methyl alcohol.
1 carbon: 1 x 12.011 amu = 12.011 amu
(If you wonder why the last figure has been dropped, see the discussion of significant figures in Appendix 4.)
In Example 7 (example above, but not numbered) notice that the natural atomic weight of carbon is not 12.000 amu but 12.011 amu, since carbon occurs as a mixture of 98.89% carbon-12 and 1.11% carbon-13, with trace amounts of carbon-14.
What is the molecular weight of pure octane?
(8 x 12.011) + (18 x 1.008) = 114.23 amu
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