Matter Is Made of Elements and Atoms 22
• Chemistry deals with the interactions of matter and energy.
Matter has mass and occupies space, while energy is the
ability to do work. Matter is made of up to 117 chemical
elements; only 26 of them are found naturally in the body.
Elements such as oxygen (O), carbon (C), hydrogen (H), and
nitrogen (N) make up 96% of the body's elements. Energy
comes in many forms—such as potential, kinetic, mechani-
cal, chemical, and heat—which can be converted from one
form to another.
• Chemical elements are made
VisuaMzmg atoms •
of atoms. An atom consists
Figure 2.3
of a nucleus, which contains
positively charged protons
and neutral neutrons, and
is surrounded by a cloud of
negatively charged elec-
trons that are located in
various electron levels, or
electron shells.
• Each element has a unique
atomic number, which is
the number of protons it
contains. Because atoms are
electrically neutral, the num-
ber of protons in an atom equals the number of electrons.
Most of an atom's mass is in its nucleus; the atomic mass
number is the total number of protons plus neutrons.
Atoms interact by exchanging electrons in their outer shells,
the valence shells. Atoms can give up valence electrons,
take them from other atoms, or share them.
The Building Blocks of Matter Fit Together
to Make Ions and Compounds 26 •
• When an atom gives up or
accepts one or more valence
electrons, it becomes an
ion. Ions can be positively
charged (cation) or nega-
tively charged (anion). Ions
remain electrically attracted
to ionic bonds and form
ionic compounds, such
as sodium chloride. Ionic
compounds have different
properties than the constitu-
ent elements.
Ionic bonds • Figure 2.4
Cl -
• When one or more atoms share one or more valence elec-
trons, they become covalent molecules. Each shared electron
pair forms a covalent bond. Atoms of the same element can
form covalent bonds to become molecules; when atoms of dif-
ferent elements form covalent bonds, they are called covalent
compounds. Two atoms may form up to four covalent bonds;
however, single, double, or triple bonds are most common.
• In some covalent compounds, the electron pairs are not
shared equally. One end of the molecule becomes partially
positive, while the other becomes partially negative (like
a bar magnet). These polar molecules attract each other
electrically. One type of weak chemical bond that forms
between the hydrogen of one polar molecule and a strongly
attractive atom (O, N) of another polar molecule is called a
hydrogen bond. Hydrogen bonds are easily made and bro-
ken; they stabilize the structures of many molecules (such as
DNA) and provide water with unique chemical properties.
Life Uses Important Chemicals 31
• Life's chemicals include non-carbon-containing inorganic
compounds and carbon-containing organic compounds.
The most important inorganic compound is water, which is
an ideal solvent for dissolving substances and has unique
properties that are suitable for life. In water, inorganic acids
dissociate into hydrogen ions (H+), bases dissociate into
hydroxyl ions (OH-), and salts dissociate into ions that are
neither H+ nor OH-. The concentration of H+ in a substance is
measured by the pH scale, which ranges from 0 to 14. Acids
have high H+ concentrations and pH less than 7, bases have
low H+ concentrations and pH greater than 7, and neutral
substances have equal concentrations greater than H+ and
OH-, with pH around 7.
Acids, bases, and salts • Figure 2.8
b. Base
• Organic compounds include carbohydrates, lipids, proteins,
nucleic acids, and ATP. They all have C, H, and O. In addition,
proteins have N and S, while nucleic acids have N and P.
• Carbohydrates are primary energy sources that consist of
simple sugars (monosaccharides, disaccharides) and com-
plex starches (polysaccharides). Starches are composed of
simple sugars linked together.
Introductory Chemistry
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