Biology Now, 2e

50 ■ CHAPTER 03 Chemistry of Life

CELLS

Figure 3.10

Versatile carbon

All organic compounds are built from carbon atoms interacting with atoms of other elements through

strong covalent bonds. A carbon atom can bond with up to four other atoms because it has four valence

electrons in its outer shell. Recall that atoms must fill their outer shells to be stable and that covalent

bonds require the sharing of valence electrons. Count the electrons circling each of the atoms in the

molecules shown; all the outer shells are full.

Q1: In methane gas, how many electrons is each hydrogen atom sharing? How many is the

carbon atom sharing?

Q2: In carbon dioxide, how many electrons is each oxygen atom sharing? How many is the

carbon atom sharing?

Q3: Draw a molecule of formaldehyde (CH 2 O). How many electrons is the oxygen atom sharing

with the carbon atom? How many is the carbon atom sharing with the oxygen atom and with

each hydrogen atom?

H

H

H H

H

H H

H

O O

C

H

H

C

H

H

C

CO 2 : Carbon dioxide

C

CH 4 : Methane

C 3 H 8 : Propane

H

H

H C H

H

H

C

C 2 H 5 OH: Ethanol

C

O