Atoms and Molecules:  The Chemical Basis of Life

Chapter 2




     Elements are not changed in chemical reactions

     Atoms are the basic particles of elements

    Atoms contain protons, neutrons, and electrons

     Atoms form molecules and Compounds

     Atoms are joined by chemicl bonds

     Oxidation involves the loss of electrons; reduction involves the gain of electrons

     Water is essental to life

     Acids are proton donors; Bases are proton accpetors.



Prefixes Meanings
electr(i,o)- Amber; electricity
equi- equal
hydr(o)- water
neutro- neutral
non- not
   pro- Before, infront of, forward
radio- beam, ray
tetra- four

Suffixes Meanings
-hedron face
-philic loving, friendly, lover
-phobic fearing

Provide definitions for the following words by using the above prefixes and suffixes as a guide:



I.  Elements are not changed in chemical reactions

A. 92 naturally occurring elements range from hydrogen to uranium

B. Four elements (CHON) make up 96% of most living organisms

C. Trace elements are necessary, but present in small quantities

II. Atoms are the basic particles of elements

A. Atoms are the smallest component of an element that retains the

chemical properties of the element

III. Atoms contain protons, neutrons, and electrons

A. A proton is a subatomic particle with one unit of positive charge

B. A neutron has the same mass as a proton, but no charge

C. Protons and neutrons compose the nucleus of the atom

1. Atomic Mass Units are also known as daltons

2. One AMU equals the approximate mass of a proton or neutron

C. Electrons orbit the nucleus and have a very small mass

D. An atom is uniquely identified by its number of protons

1. The atomic number is equal to the number of protons

2. Atomic numbers are written in subscript to the left of the

chemical symbol

E. Protons plus neutrons determine atomic mass

1. The atomic mass is written in superscript to the left of the

chemical symbol

2. The atomic weight is the ratio of atomic mass compared to the atomic mass of carbon (nonisotopic) and is nondimensional

F. Isotopes differ in number of neutrons

1. Some isotopes are unstable (radioisotopes)

2. Many radioisotopes are important in scientific research

G. Electrons can be involved in chemical changes

H. Electrons occupy orbitals corresponding to energy levels

1. Electrons occupy a space called an orbital

2. Orbitals contain a maximum of two electrons

IV. Atoms form molecules and compounds

A. Two or more atoms combined chemically form a molecule

1. Molecules may be composed of different or similar atoms

B. Chemical formulas describe chemical compounds

1. Chemical formulas indicate ratios of atoms in a molecule

2. Structural formulas show the arrangement of atoms in a molecule

C. One mole of any substance contains the same number of units

1. The molecular mass of a compound is the sum of the atomic masses of the atoms composing the molecule

2. The molecular weight is dimensionless

3. The number of units in a mole is Avogadro's number

D. Chemical equations describe chemical reactions

1. Reactants are on the left side of the equation

2. Products are on the right side of the equation

V. Atoms are joined by chemical bonds

A. The electrons in the outer shell are the valence electrons

1. Atoms tend to be reactive if the valence shell is not full

2. Atoms in a compound are held together by chemical bonds

B. Electrons are shared in covalent bonds

1. Covalent bonds result in filled valence shells

2. The Lewis structure of the atoms represents the valence electrons

3. A single electron pair shared between two atoms is a single

covalent bond

4. Double and triple covalent bonds are formed by sharing two

or three electron pairs, respectively

5. Hydrogen forms a maximum of 1 covalent bond

6. Carbon forms a maximum of 4 covalent bonds

C. Orbitals may change shape when covalent bonds form

1. Molecules composed of two molecules are linear

2. Molecules composed of more than two atoms form complicated shapes

D. Covalent bonds can be nonpolar or polar

1. Electrons orbit the most electronegative of the atoms in a molecule

2. Covalent bonds in which atoms are of varying electronegativity are polar

3. Polar molecules have a portion of the molecule which is partially

positive and partially negative in charge

E. Ionic bonds form between cations and anions

1. An atom becomes an ion when it gains or loses one or more electrons

2. Cations are positively charged ions

3. Anions are ions with a negative charge

4. An ionic bond forms between cations and anions

5. Ionic compounds tend to dissociate in water

F. Hydrogen bonds are weak attractions involving partially charged

hydrogen atoms

1. In the water molecule, hydrogen is partially positively charged

2. Hydrogen bonds form between the hydrogen atom in a water

molecule and a partially negatively charged atom

3. Hydrogen bonds individually are weak, but collectively very strong

G. Molecules may interact through van der Waals forces and

hydrophobic forces

1. Like hydrogen bonds, van der Waals forces are individually weak, but may be strong in great numbers

2. Hydrophobic interactions occur between nonpolar molecules

VI. Oxidation involves the loss of electrons; reduction involves the

gain of electrons

A. Oxidation and reduction reactions occur simultaneously (redox reactions)

B. Reduction is a process in which an atom, ion, or molecule gains electrons

1. Reduction is so named, as the gain of electrons reduces

the positive charge

C. Redox reactions are important in both cellular respiration

and photosynthesis

VII. Water is essential to life

A. Water is important because most organisms are composed of water

or live in water

B. Water molecules are polar

1. Hydrogen atoms in the water molecule have a partial positive charge

2. Each water molecule can form up to 4 hydrogen bonds

C. Water is the principal solvent in living things

1. Water readily dissolves polar and ionic compounds

2. Water does not dissolve hydrophobic substances

D. Hydrogen bonding makes water cohesive and adhesive

1. Water molecules tend to hydrogen bond to each other,

making water cohesive

2. Water molecules tend to hydrogen bond to other molecules,

making water adhesive

3. Capillary action is a result of adhesion and cohesion

4. Cohesion results in water moving upwards in plants

5. Hydrogen bonding also results in surface tension

E. Water helps maintain a stable temperature

1. Water has a high specific heat; it takes much energy to raise

the temperature of water, due to hydrogen bonding

2. Temperature stability is important to aquatic organisms

3. The high heat of vaporization of water results in cooling during sweating or other evaporative processes

VIII. Acids are proton donors; bases are proton acceptors

A. Water tends to slightly dissociate into hydrogen and hydroxide ions

B. The concentration of hydrogen and hydroxide ions in pure water

is 10-7 moles per liter

C. Acids are substances that dissociate in a solution to yield hydrogen ions

1. Acids are proton donors

2. Acids have a higher hydrogen ion concentration higher than the hydroxide ion concentration

D. Bases are substances that dissociate in a solution to yield hydroxide ions

1. Bases are proton acceptors

2. Bases have a lower hydrogen ion concentration higher than the hydroxide ion concentration

E. pH is a convenient measure of acidity

1. The pH of a solution is the negative log of the hydrogen ion

concentration in moles per liter

2. A pH below 7 is acidic, above 7 is basic

3. The pH of most living cells is slightly above 7.0

F. Buffers minimize pH change

1. Weak acids and weak bases act as buggers

G. An acid and a base react to form a salt

1. When an acid and base are mixed, water is formed from the hydrogen ion of the acid, and the hydroxide ion of the base

2. The cation of the base and the anion of the acid form the salt

3. Electrolytes are salts, acids or bases dissolved in water which can conduct an electrical current

4. Nonelectrolytes are molecules which do not dissociate in water

and therefore do not conduct an electrical current