CHAPTER 2 The Chemical Basis of Life
31
Na
+
and individual Cl
−
in which each charged ion is surrounded byseveral ions of the opposite charge (see figure 2.5
b
).The properties of elements can change when they are combinedto form compounds. For example, the element hydrogen is extremelyflammable, and oxygen, although not flammable on its own, pro-motes fire. However, when combined, hydrogen and oxygen formthe very nonflammable compound water. Likewise, elements that aredangerous or toxic to humans can become useful as compounds. Forexample, sodium is very explosive when placed in water, and chlo-rine is a strong disinfectant in solutions, such as bleach and swim-ming pool water. Chlorine is so toxic that it was used as a poison gasin World War I, yet, when combined, sodium and chloride form therelatively safe and nonexplosive compound table salt.The kinds and numbers of atoms (or ions) in a molecule orcompound are typically represented by a formula consisting ofthe symbols of the atoms (or ions) plus subscripts denoting thequantity of each type of atom (or ion). The formula for glucose(a sugar) is C
6
H
12
O
6
, indicating that glucose has 6 carbon, 12hydrogen, and 6 oxygen atoms (table 2.3). The formulas for somecommon substances, such as O
2
for oxygen and CO
2
for carbondioxide, will be used throughout the text.The
molecular mass
of a molecule or compound can bedetermined by adding up the atomic masses of its atoms (or ions).The term
molecular mass
is used for convenience for ionic com-pounds, even though they are not molecules. For example, theatomic mass of sodium is 22.99 and that of chloride is 35.45. Themolecular mass of NaCl is therefore 58.44 (22.99 + 35.45).
in intermolecular forces. This differs from other chemical bonds.Intermolecular forces are much weaker than the forces producingchemical bonding. Intermolecular forces include hydrogen bondsand the properties of solubility and dissociation.
Hydrogen Bonds
Molecules with polar covalent bonds have positive and negative“ends.” Intermolecular force results from the attraction of thepositive end of one polar molecule to the negative end of anotherpolar molecule. When hydrogen forms a covalent bond with oxy-gen, nitrogen, or fluorine, the resulting molecule becomes verypolarized. If the positively charged hydrogen of one molecule isattracted to the negatively charged oxygen, nitrogen, or fluorineof another molecule, a
hydrogen bond
forms. For example, thepositively charged hydrogen atoms of a water molecule formhydrogen bonds with the negatively charged oxygen atoms ofother water molecules (figure 2.8). These hydrogen bonds areessential for the unique properties of water (see section 2.3).An important role of hydrogen bonds is to help build the shapeof complex molecules. The bonds can occur between different polarparts of a molecule to stabilize the final three-dimensional shape(see “Proteins” and “Nucleic Acids: DNA and RNA” in section 2.4).Table 2.4 summarizes the important characteristics of chemi-cal bonds (ionic and covalent) and intermolecular forces (hydro-gen bonds).
Solubility and Dissociation
Solubility
is the ability of one substance to dissolve in another—for example, sugar dissolving in water. Charged substances, suchas sodium chloride, and polar substances, such as glucose, readilydissolve in water, whereas nonpolar substances, such as oils, donot. We all have seen how oil floats on water. Substances dissolve
Intermolecular Forces
Intermolecular forces
are the weak electrostatic attractionsthat exist between oppositely charged parts of molecules, orbetween ions and molecules. There is no exchange of electrons