Essential Cell Biology 5th edition

46 CHAPTER 2 Chemical Components of Cells
QUESTION 2–3
Discuss whether the following
statement is correct: “An ionic
bond can, in principle, be thought
of as a very polar covalent bond.
Polar covalent bonds, then, fall
somewhere between ionic bonds
at one end of the spectrum and
nonpolar covalent bonds at the
other end.”
To get an idea of what bond strengths mean, it is helpful to compare
them with the average energies of the impacts that molecules continually
undergo owing to collisions with other molecules in their environment—
their thermal, or heat, energy. Typical covalent bonds are stronger than
these thermal energies by a factor of 100, so they are resistant to being
pulled apart by thermal motions. In living organisms, covalent bonds are
normally broken only during specific chemical reactions that are carefully
controlled by highly specialized protein catalysts called enzymes.
Ionic Bonds Form by the Gain and Loss of Electrons
In some substances, the participating atoms are so different in electronegativity
that their electrons are not shared at all—they are transferred
completely to the more electronegative partner. The resulting bonds,
called ionic bonds, are usually formed between atoms that can attain
a completely filled outer shell most easily by donating electrons to—or
accepting electrons from—another atom, rather than by sharing them.
For example, returning to Figure 2–5, we see that a sodium (Na) atom
can achieve a filled outer shell by giving up the single electron in its third
shell. By contrast, a chlorine (Cl) atom can complete its outer shell by
gaining just one electron. Consequently, if a Na atom encounters a Cl
atom, an electron can jump from the Na to the Cl, leaving both atoms
with filled outer shells. The offspring of this marriage between sodium, a
soft and intensely reactive metal, and chlorine, a toxic green gas, is table
salt (NaCl).
When an electron jumps from Na to Cl, both atoms become electrically
charged ions. The Na atom that lost an electron now has one less
electron than it has protons in its nucleus; it therefore has a net single
positive charge (Na + ). The Cl atom that gained an electron now has one
more electron than it has protons and has a net single negative charge
(Cl – ). Because of their opposite charges, the Na + and Cl – ions are attracted
to each other and are thereby held together by an ionic bond (Figure
2–12A). Ions held together solely by ionic bonds are generally called salts
rather than molecules. A NaCl crystal contains astronomical numbers of
Na + and Cl – ions packed together in a precise, three-dimensional array
with their opposite charges exactly balanced: a crystal only 1 mm across
contains about 2 × 10 19 ions of each type (Figure 2–12B and C).
Figure 2–12 Sodium chloride is held
together by ionic bonds. (A) An atom
of sodium (Na) reacts with an atom of
chlorine (Cl). Electrons of each atom are
shown in their different shells; electrons
in the chemically reactive (incompletely
filled) outermost shells are shown in red.
The reaction takes place with transfer of a
single electron from sodium to chlorine,
forming two electrically charged atoms, or
ions, each with complete sets of electrons
in their outermost shells. The two ions have
opposite charge and are held together by
electrostatic attraction. (B) The product of
the reaction between sodium and chlorine,
crystalline sodium chloride, contains sodium
and chloride ions packed closely together
in a regular array in which the charges are
exactly balanced. (C) Color photograph of
crystals of sodium chloride.
sodium atom (Na) chlorine atom (Cl) positive
sodium ion (Na + )
(A)
(B)
(C)
negative
chloride ion (Cl – )
sodium chloride (NaCl)
1 mm