Chemistry Comes Alive
Hydrogen bonds are weak bonds formed between one hydrogen
atom, already covalently linked to an electronegative atom, and
another electronegative atom (such as nitrogen or oxygen). Tey
bind together diﬀerent molecules (e.g., water molecules) or
diﬀerent parts of the same molecule (as in protein molecules).
Chemical reactions involve the formation, breaking, or
rearrangement of chemical bonds.
Patterns of Chemical Reactions
Chemical reactions are either anabolic (constructive) or
catabolic (destructive). Tey include synthesis, decomposition,
and exchange reactions. Oxidation-reduction reactions may
be considered a special type of exchange (or decomposition)
Energy Flow in Chemical Reactions
Bonds are energy relationships and there is a net loss or gain of
energy in every chemical reaction.
In exergonic reactions, energy is liberated. In endergonic
reactions, energy is absorbed.
Reversibility of Chemical Reactions
If reaction conditions remain unchanged, all chemical reactions
eventually reach a state of chemical equilibrium in which the
reaction proceeds in both directions at the same rate.
All chemical reactions are theoretically reversible, but many
biological reactions go in only one direction because of energy
requirements or the removal of reaction products.
Factors Inﬂuencing the Rate of Chemical Reactions
Chemical reactions occur only when particles collide and valence
shell electrons interact.
Te smaller the reacting particles, the greater their kinetic energy
and the faster the reaction rate. Higher temperature or reactant
concentration, as well as the presence of catalysts, increases
chemical reaction rates.
Most inorganic compounds do not contain carbon. Tose found
in the body include water, salts, and inorganic acids and bases.
Water is the single most abundant compound in the body. It
absorbs and releases heat slowly, acts as a universal solvent,
participates in chemical reactions, and cushions body organs.
Salts are ionic compounds that dissolve in water and act as
electrolytes. Calcium and phosphorus salts contribute to the
hardness of bones and teeth. Ions of salts are involved in many
Acids and Bases
Acids are proton donors; in water, they ionize and dissociate,
releasing hydrogen ions (which account for their properties)
Te building blocks of elements are atoms.
Atoms are composed of positively charged protons, negatively
charged electrons, and uncharged neutrons. Protons and
neutrons are located in the atomic nucleus, constituting
essentially the atom’s total mass. Electrons are outside the nucleus
in the electron shells. In any atom, the number of electrons equals
the number of protons.
Atoms may be identiﬁed by their atomic number (p
) and mass
). Te notation
He means that helium (He) has
an atomic number of 2 and a mass number of 4.
Isotopes of an element diﬀer in the number of neutrons they
contain. Te atomic weight of any element is approximately equal
to the mass number of its most abundant isotope.
Many heavy isotopes are unstable (radioactive). Tese so-
called radioisotopes decompose to more stable forms by
emitting alpha or beta particles or gamma rays. Radioisotopes
are useful in medical diagnosis and treatment and in
How Matter Is Combined: Molecules and Mixtures
Molecules and Compounds
A molecule is the smallest unit resulting from the chemical
bonding of two or more atoms. If the atoms are diﬀerent, they
form a molecule of a compound.
Mixtures are physical combinations of solutes in a solvent.
Mixture components retain their individual properties.
Te types of mixtures, in order of increasing solute size, are
solutions, colloids, and suspensions.
Solution concentrations are typically designated in terms of
percent or molarity.
Distinguishing Mixtures from Compounds
Compounds are homogeneous; their elements are chemically
bonded. Mixtures may be homogeneous or heterogeneous; their
components are physically combined and separable.
The Role of Electrons in Chemical Bonding
Electrons of an atom occupy areas of space called electron shells
or energy levels. Electrons in the shell farthest from the nucleus
(valence shell) are most energetic.
Chemical bonds are energy relationships between valence shell
electrons of the reacting atoms. Atoms with a full valence shell
or eight valence shell electrons are chemically unreactive (inert).
Tose with an incomplete valence shell interact with other atoms
to achieve stability.
Types of Chemical Bonds
Ionic bonds are formed when valence shell electrons are
completely transferred from one atom to another.
Covalent bonds are formed when atoms share electron pairs. If
the electron pairs are shared equally, the molecule is nonpolar. If
they are shared unequally, it is polar (a dipole).