• Molecules form when atoms bond to each other. Chemical bonds are based on electron sharing.
The degree of electron sharing varies from non-polar covalent bonds, to polar covalent bonds,
to ionic bonds.
• Of all small molecules, water is the most important for life.
• Chemical reactions tend to be spontaneous if they lead to lower potential energy and higher
• Most of the important compounds in organisms contain carbon.
The Building Blocks of Chemical Evolution
• Each element has a unique atomic number and contains a characteristic number of protons,
called the atomic number.
• The mass number (indicated in superscript) is the number of protons + neutrons of the most
• Elements commonly found in organisms have at least one unpaired valence electron. The
number of unpaired electrons in an atom is its valence.
Unfilled electron orbitals allow formation of chemical bonds, and atoms are most stable when each
electron orbital is filled.
Covalent bond: Each atom’s unpaired electrons are shared by both nuclei to fill their orbitals
Non-polar covalent bond: Electrons are evenly shared between two atoms and the bond is
Polar covalent bond: Electrons are asymmetrically shared
Ionic bond: Electrons are transferred from one atom to another Electronegativity ofAtoms
An atom in a molecule with a high electronegativity will hold the electrons more tightly and have a
partial negative charge (δ–), whereas the other atom will have a partial positive charge (δ+).
Cation:An atom that loses an electron and becomes positively charged.
Anion:An atom that gains an electron and becomes negatively charged.
How Many Bonds Can anAtom Have?
• The number of unpaired electrons determines the number of bonds an atom can make.
• Atoms with more than one unpaired electron can form multiple single bonds or double or triple
• The shape of a simple molecule is governed by the geometry of its bonds.
• The molecular formula indicates the numbers and types of atoms in a molecule (e.g., H2O,
• Structural formulas indicate which atoms are bonded together and whether the bonds are
single, double, or triple bonds.
• Other models show 3D geometry.
• The molecular weight of a molecule is the sum of the mass numbers of all the atoms in the
• One mole, or 6.022 ´ 1023 molecules, has a mass equal to the molecular weight expressed in
• The concentration of a substance in a solution is typically expressed as molarity (M), which is
the number of moles per liter.
The Properties of Water • Life is based on water because water is a great solvent.
• The covalent bonds in water are polar because oxygen and hydrogen differ in their
electronegativity: oxygen has a partial negative charge and hydrogen has a partial positive
• Ions and polar molecules stay in solution because of their interactions with water’s partial
Interaction between Hydrogen Bonds and Water
• Hydrogen bonds form between H2O and other polar molecules or ions. Ions and polar
molecules stay in solution because of their interactions with water’s partial charges.
• Hydrogen bonding makes it possible for almost any charged or polar molecule to dissolve in
water and hydrogen bonds are extremely important in biology.
The Structure of Water
• The structure of water is unique—its small size, bent shape, highly polar covalent bonds, and
• Water also has several striking physical properties: