AP/IB Biology Chemistry Review 
THE BASICS
Everything that exists is either matter or energy.
Matter (including LIVING matter) is made up of atoms.
Atoms have three major subatomic particles:
Protons: positive charge, 1 AMU ( Dalton), determine the element.
Neutrons: neutral charge, 1 AMU, determine the isotope
P = atomic number (order on the periodic table)
P + N = atomic mass, mass number, atomic weight
C – 12 has 6 protons (carbon) and 6 neutrons.
C – 14 has 6 protons and 8 neutrons.
Electrons: negative charge, no significant mass, determine the bonding.
Atoms bond (chemically react) with other atoms to stabilize the valence electron orbital. The Octet Rule states that most atoms try to get 8 valence electrons. Atoms can gain, lose, or share electrons to accomplish this.
Valence = outermost
Therefore, the bonding characteristics of an atom are determined by the number of electrons that the atom needs to gain, lose, or share to reach that stable state. Atoms with one or two valence electrons will probably lose those electrons. The orbital below the valence orbital will be stable. (Hydrogen cannot do this b/c it only has 1 orbital). Atoms with 6 or 7 valence electrons will probably gain electrons to reach 8 valence electrons.
IONS:
Because atoms are electrically neutral before bonding (same number of protons and electrons), any atom that gains or loses electrons will have a charge. Gaining electrons means that the overall charge of the atom will be negative. Losing electrons will give an atom an overall positive charge. The charged particles are called ions (cations = positive ions, anions = negative ions). Ions with opposite charges will be attracted to each other and will form ionic bonds. The overall charge of the bonded atoms will be zero.
For example: Na has 1 valence electron, Cl has 7. Na loses the electron (Na +) and Cl gains it (Cl -). You write the bonded compound: NaCl.
For example: Ca has 2 valence electrons, Cl has 7. Ca loses both electrons (Ca +2). Since Cl can only take on 1 electron, two chloride ions must bond with one calcium ion for the entire unit to be electrically neutral.
CaCl 2
Compounds formed by ionic bonding tend to disassociate in water because the negative ions are attracted to the hydrogen (positive) side of the water molecule, and the positive ions are attracted to the oxygen (negative) side.
COVALENT BONDS
Atoms that neither gain nor lose electrons tend to share electrons with other atoms to achieve stability. Shared electrons count towards the valence in BOTH atoms. Carbon has 4 valence electrons. It will share 4 electrons with other atoms to achieve stability (octet rule). Hydrogen has 1 valence electron but it only has 1 energy level and is stable with 2 valence electrons.
Carbon atoms can share 4 electrons with another carbon atom (quadruple bond) or it can share 4 electrons with 4 different hydrogen atoms (four single bonds). CH 4
Water is a covalent molecule with oxygen sharing two electrons with hydrogen atoms.
If the electrons are shared equally, the covalent bond is nonpolar. If the electrons are drawn more towards one atom in the molecule, the covalent bond is polar. The side with the highest affinity (attraction) towards the electrons will be electronegative. The opposite side is positive.
The important elements to remember in this class are those that predominate in living tissues. These include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur. A few others will pop up from time to time. When structural diagrams are displayed for these (and other) substances, the number of lines drawn from an atom reflect the valence of the atom.