1. UNIT I: THE CHEMISTRY OF LIFE
CH. 2 THE CHEMICAL CONTEXT OF LIFE

2. I. Chemical Elements and Compounds
Matter consists of chemical elements in pure form and
in combinations called compounds
ELEMENTS: CANNOT BE DOWN TO OTHER SUBSTANCES
COMPOUND: CONTAINS TWO OR MORE ELEMENTS
Figure 0201
B. Life requires about 25 chemical elements
CARBON, OXYGEN, HYDROGEN, AND NITROGEN = 96%

3. II. Atoms and Molecules
Atomic structure determines the behavior of an element
ATOM: SMALLEST UNIT OF AN ELEMENT
NUCLEUS: CONTAINS PROTONS (+) & NEUTRONS (0)
OUTER SHELLS: CONTAINS ELECTRONS (-)
THE # OF ELECTRONS IN AN ELECTRICALLY NEUTRAL ATOM EQUALS THE NUMBER OF PROTONS
ISOTOPES: One of several atomic forms of an element, each containing a different number of neutrons and thus differing in atomic mass.
Figure 0205

4. B. Atoms combine by chemical bonding to form molecules
CHEMICAL BONDS FORM WHEN ATOMS INTERACT AND COMPLETE THEIR
VALENCE SHELLS
A COVALENT BOND IS SHARING OF A PAIR OF VALENCE ELECTRONS BY TWO
ATOMS
Figure 0210 COVALENT BONDING
MOLECULES CONSIT OF TWO OR MORE COVALENTLY BONDED ATOMS
ELECTRONS OF A POLAR COVALENT BOND ARE PULLED CLOSER TO THE
MORE ELECTRONEGATIVE ATOM
Figure 0211 POLAR COVALENT BONDING IN WATER
TWO ATOMS MAY DIFFER SO MUCH IN ELECTRONEGATIVITY THAT ONE OR MORE
ELECTRONS ARE ACTUALLY TRANSFERRED FROM ONE ATOM TO ANOTHER
THE RESULT IS: ANION (-) & CATION (+)
IONIC BOND: ATTRACTION B/W TOW IONS OF OPPOSITE CHARGE
Figure 0212 IONIC BONDING

5. C. Weak chemical bonds play important roles in the chemistry
of life
HYDROGEN BONDS ARE RELATIVELY WEAK INTERACTIONS B/W A PARTIAL (+) HYDROGEN ATOM & PARTIAL (-) ATOM OF ANOTHER POLAR MOLECULE
Figure 0214
WEAK BONDS REINFORCE THE SHAPES OF LARGE BIOLOGICAL MOLECULES AND ALSO HELP MOLECULES ADHERE TO EACH OTHER