2.1 Chemical Elements
Chemistry of Life
A. Matter
1. Matter takes up space and has mass.
2. All living and nonliving matter is composed of 92 naturally-occurring basic
elements (Periodic Table)
3. Elements cannot be broken down to substances with different chemical or physical
properties.
4. Six elements (C, H, N, O, P, S) make up 98% of living things.
B. Atomic Structure
1. Chemical and physical properties of atoms (e.g., mass) depend on the subatomic
particles.
a. Different atoms contain specific numbers of protons, neutrons, and electrons.
b. Protons and neutrons are in nucleus of atoms; electrons move around nucleus.
c. Protons are positively charged particles; neutrons have no charge; both have
about 1 atomic mass unit of weight.
d. Electrons are negatively charged particles.
2. The atomic mass of an
atom is about equal to the sum of its protons and neutrons.
3. All atoms of an element have the same number of protons, the atom's
atomic number.
C. Isotopes
1. Isotopes are atoms with the same number of protons but differ in number of
neutrons; e.g., a carbon atom has six protons but may have more or less than
usual six neutrons.
2. A carbon with eight rather than six neutrons is unstable; it releases rays
and subatomic particles and is a radioactive isotope.
3. Low levels of radiation such as radioactive iodine or glucose allow researchers
to trace the location and activity of the atom in living tissues; therefore
these isotopes are called tracers (used in CAT scans)
4. High levels of radiation can cause cancerous tissues and destroy cells; careful
use of radiation in turn can sterilize products and kill cancer cells.
E. Electrons and Energy
1. Electrons occupy an orbital at some level near or distant from the nucleus
of the atom.
2. An orbital is a volume off space where an electron is most likely to be found;
an orbital contains no more than two electrons.
3. The more distant the orbital, the more energy it takes to stay in the orbital.
4. When atoms absorb energy during photosynthesis, electrons are boosted to
higher energy levels.
5. The innermost shell of an atom is complete with two electrons; all other
shells are complete with eight electrons.
2.2 Elements and Compounds
A. Compounds
1. When two or more different elements react or bond together, they form a compound
(e.g., H2O).
2. A molecule is the smallest part of a compound that has the properties of
the compound.
3. Electrons possess energy and bonds that exist between atoms in molecules
contain energy.
B. Ionic Bonding
1. Ionic bonds form when electrons are transferred from one atom to another.
2. Losing or gaining electrons, atoms participating in ionic reactions fill
outer shells, and are more stable.
3. Example: sodium with one less electron has positive charge; chlorine has
extra electron that has negative
charge. Such charged particles are called ions.
4. Attraction of oppositely charged ions holds the two atoms together in an
ionic bond.
C. Covalent Bonding
1. Covalent bonds result when two atoms share electrons so each atom has octet
of electrons in the outer shell.
2. Hydrogen can give up an electron to become a hydrogen ion (H+) or share an
electron with another atom to complete its outer shell of two electrons.
3. Structural formulas represent shared atom as a line between two atoms; e.g.,
single covalent bond (H-H), double covalent bond (O=O)
4. Three dimensional shape of molecules is not represented by structural formulas
but shape is critical in understanding the biological action of molecules: action
of insulin, HIV receptors, etc.
D. Nonpolar and Polar Covalent
Bonds
1. In nonpolar covalent bonds, sharing of electrons is equal.
2. With polar covalent bonds, the sharing of electrons is unequal.
a. In water molecule (H2O), sharing of electrons by oxygen and hydrogen is not
equal; the oxygen atom with more protons dominates the H2O association.
b. Attraction of an atom for electrons in a covalent bond is called electronegativity;
oxygen atom is more electronegative than hydrogen atom.
c. Oxygen in water molecule, more attracted to electron pair, assumes small
negative charge
E. Hydrogen Bonding
1. A hydrogen bond is weak attractive force between slightly positive hydrogen
atom of one molecule and slightly negative atom in another or the same molecule.
2. Many hydrogen bonds taken together are relatively strong.
3. Hydrogen bonds between complex molecules of cells help maintain structure
and function.
2.3. Chemistry of Water
A. First Cells Evolved in Water
1. All living things are 70.90% water.
2. Because water is a polar molecule, water molecules are hydrogen bonded to
each other.
3. With hydrogen bonding, water is liquid between 0o C and 100 C which is critical
for life.
B. Properties of Water
1. The temperature of liquid water rises and falls more slowly than that of
most other liquids..
a. Calorie is amount of heat energy required to raise temperature of one gram
of water 1o C.
b. Because water holds more heat, its temperature falls more slowly than other
liquids; this protects
organisms from rapid temperature changes and helps them maintain normal temperatures.
2. Water has a high heat
of vaporization.
a. Hydrogen bonds between water molecules require a large amount of heat to
break.
b. This property moderates earthfs surface temperature; permits living
systems to exist here.
c. When animals sweat, evaporation of the sweat takes away body heat, thus cooling
the animal.
3. Water is universal solvent,
facilitates chemical reactions both outside of and within living systems..
a. Water is a universal solvent because it dissolves a great number of solutes.
b. Ionized or polar molecules attracted to water are hydrophilic.
c. Nonionized and nonpolar molecules that cannot attract water are hydrophobic.
4. Water molecules are cohesive and adhesive..
a. Cohesion allows water to flow freely without molecules separating, due to
hydrogen bonding.
b. Adhesion is ability to adhere to polar surfaces; water molecules have positive,
negative poles.
- Surface of water has structure, think of skipping a rock, or insects that walk on water
- Water sticks to itself, this allows water to be moved from the roots to leaves in plants
- Water becomes less dense
when it freezes, so it floats
C. Acids and Bases
1. Covalently bonded water molecules ionize; the atoms dissociate into ions.
2. When water ionizes or dissociates, it releases a small (107 moles/liter) but equal number of H+ and OH
ions; thus, its pH is neutral.
3. Water dissociates into hydrogen and hydroxide ions:
4. Acid molecules dissociate in water, releasing hydrogen ions (H+) ions: HCl
¨ H+ + Cl-.
5. Bases are molecules that take up hydrogen ions or release hydroxide ions.
NaOH ¨ Na+ + OH-.
6. The pH scale indicates acidity and basicity (alkalinity) of a solution.
1) One mole of water has 107 moles/liter of hydrogen ions; therefore, has neutral pH of 7.
2) Acid is a substance with pH less than 7; base is a substance with pH greater than 7.
3) As logarithmic scale, each lower unit has 10 times the amount of hydrogen ions as next higher pH unit;
* Buffers keep pH steady and within normal limits in living organisms.
