Biology 101 Chapter 2

The Chemical Basis of Life


Chemistry = the study of matter and the changes matter undergoes.


Nature of Matter:


Element defined: the basic kinds or types of matter, a substance that cannot be broken down further by chemical means.


-        substances whose atoms have the same # of protons

-        92 elements occur naturally, 25 found in living things

-        118 known elements

-        the 4 most important elements (>97% mass)

1)  Carbon [C]

2)  Oxygen [O]

3)  Hydrogen [H]

4)  Nitrogen [N]

-        trace elements


Atom defined: the smallest fundamental particles of matter that still retain the identity of an element.


Atomic Structure

Same for all atoms

Atoms have two basic regions

1)  Nucleus

2)  Orbitals (shells)


Composed of 3 smaller particles (subatomic particles)

1.   Protons

2.   Electrons

3.   Neutrons






# in atom










Lost or Gained





Can alter


Atoms in an uncombined state have the same number of protons and electrons; therefore they are electrically neutral.


Most of the mass of an atom is in the nucleus.

Most of the volume is empty space occupied by orbitals.

Periodic table always refers to atoms in an uncombined state.


The Periodic Table:

Atomic Symbols:

The Atomic Number:

The Atomic Mass:

Isotopes and mass number


Molecule defined:

Diatomic molecules: H2, O2, N2, F2, Cl2, Br2, I2


Compound defined:

Atomic Structure Part II


Electron Arrangements

Electron shells:

-        certain energy levels

-        2e- in 1st

-        8e- in outermost

Valence electrons = bonding electrons and the octet rule


Chemical Bonds

3 types:

Ionic Covalent Hydrogen


Ionic bonds and ions:

-        atoms can gain or lose electrons and become charged

-        ions can form independent of ionic bonds


Ion = an atom or molecule with an electrical charge resulting from the gain or loss of 1 or more electrons.


Ionic bond: attraction of opposite charges, fairly strong, most common, found in metals and salts. Ex. NaCl

Covalent bond: (molecular bond) in which two atoms share 1 or more pairs of electrons, strongest bond, organic compounds.

Structure of a covalent bond

Single, double and triple bonds


Hydrogen bond: most rare, weakest, water and some organics.

       Always formed between an H atom already covalently bonded and an atom in another molecule

       Unique because of H atom structure


The Properties of Water (due to H-bonds)

1.   Polarity

2.   Cohesion + surface tension

3.   Temperature Stability

4.   Universal Solvent

Solution, solvent and solute


Acids and Bases

pH = measures the level of acidity of a solution, level of H+ (ion) in solution.


Hydrogen ion (H+) plus Hydroxide ion (OH-) = H2O


Acid: any compound that releases H+ to solution

Base: or alkali, any compound that removes H+ from solution (most have OH-)

pH Scale:

-        ranges from 0-14

-        0-6.9 acidic, 7 neutral, 7.1-14 basic

-        Water is neutral