The Chemistry of Life Biology is a multidisciplinary science. Living organisms are subject to basic laws of physics and chemistry. One example is the bombardier beetle, which uses chemistry to defend itself.

Atoms of elements play important roles in biology What element is lacking in her diet?

Biochemistry Biochemistry studies the properties and interactions of biologically important molecules. The important molecules in cells are water, fats, proteins like enzymes, nucleic acids and the carbohydrates including sugars, starches and cellulose.

Organic Molecules These are all organic molecules which contain carbon, hydrogen and oxygen. Organic molecules can include atoms of nitrogen, phosphorus, sulfur, sodium and iron.

The Atom Atoms contain positive protons and neutral neutrons in the nucleus surrounded by a cloud of negative electrons. The electrons in the outmost orbital are called valence electrons and are involved in forming bonds between atoms. A chemical bond has energy.

Atoms combine to make molecules The bonds between atoms of the same molecule are intramolecular bonds. The three types of intramolecular bonds are: Covalent Bonds Polar Covalent Bonds Ionic Bonds

1. Covalent Bonds The covalent bond is formed by sharing a pair of valence electrons between two non-metal atoms. The bond can be represented in many ways. Hydrogen (H2) Name (molecular formula) Electron- shell diagram Structural formula Space- filling model

Bond Energy Organic molecules have mostly covalent bonds. The shared pair of electrons contain energy. Forming a bond requires adding energy. Breaking a bond releases energy. The strongest bonds contain the most energy.

2. Polar Covalent Bonds This bond occurs when a pair of valence electrons is unequally shared between two atoms. The atom with a larger nucleus attracts the electron more and has a slightly negative charge. The other atom is slightly positive. The grey area is the electron cloud.

Sodium chloride (NaCl) 3. Ionic Bond An ionic bond is formed between a metal and a non-metal atom. It is formed by the transfer of an electron from atom to atom. This results in the formation of positive cations, and negative anions. Ions are held together by electrostatic attraction. Na Sodium atom (an uncharged atom) Cl Chlorine atom Na+ Sodium ion (a cation) Cl– Chlorine ion (an anion) Sodium chloride (NaCl)

Cations are positive ions because they have less negative electrons than positive protons. Anions are negative ions because there are more negative electrons than positive protons.

The Role of Electronegativity Electronegativity is a measure of the strength with which an atom can attract electrons. Electronegativity values can be found on a periodic table. The electronegativity difference between two atoms is calculated by subtracting the smaller value from the larger value. See p 563 Carbon En = 2.5 Hydrogen En = 2.1 ∆En = 2.5 – 2.1 = 0.4

Bond Types and Electronegativity A difference of < 0. 5  covalent bond A difference of 0.5 – 1.6  polar covalent bond A difference of > 1.6  ionic bond

Review Properties of Covalent Bonds Pure Covalent Bonds Results from largely equal sharing of electrons. Happens when bond partners have similar electronegativities or an electronegativity difference of <0.5 Polar Covalent Bonds Results from unequal sharing of electrons. Happens when bond partners have electronegativity differences between 0.5 and 1.6

Bonds in Water Bond Type Polar covalent Oxygen electronegativity = 3.44 Hydrogen electronegativity = 2.2 Electronegativity Difference 3.44 – 2.2 = 1.24 Bond Type Polar covalent In a water molecule the oxygen is slightly negatively charged. H O H2O + –

Inter- Molecular Interactions The properties of molecules can influence their interactions with other molecules. These are weaker bonds than intramolecular bonds. hydrogen bonding van der waals forces including dipole-dipole and London forces