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

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%

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

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

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