Bio 178 Lecture 2The Nature of Molecules

Reading

• Chapter 2

Quiz Material

• Questions on P 34

• Chapter 2 Quiz on Text Website (www.mhhe.com/raven7)

Outline

• Atoms

• Chemical Bonds

• Water

• Acids, Bases, and Buffers

The Chemistry of Life - Definitions• Matter

Any substance that has mass and occupies space.

• MassAmount of a substance.

• Weight

How strongly mass is pulled by gravity.

Example of Mass Vs. Weight:

Astronauts in space and on earth.

The Structure of AtomsWhat are Atoms?The smallest particle into which a substance can be divided and still retain its chemical properties.

Atomic NumberThe number of PROTONS an atom possesses.

Eg. OxygenAtomic number = 8

Atomic Mass

Sum of the masses of protons and neutrons.

Masses:

Protons = 1.009 Da, Neutrons = 1.007 Da, Electrons = 1/1840 Da

Eg. O2

Atomic Mass = 16 Da

Elements and IsotopesElementAtoms with the same atomic number have the same chemical properties.

IsotopesAtoms of an element that have the same number of protons but a different number of neutrons.

126C

136C

146C

Radioactive IsotopesUnstable isotopes - the nucleus of the isotope has a tendency to break into elements with different atomic numbers.

Eg. 14C decays to 14N (1 neutron in 146C1 proton14

7N)

Half-Life

Time taken for 1/2 the atoms in a sample to decay.

Example: The half-life of 146C is 5,600 yrs. How old is a

fossil that contains 1/8 the proportion of 146C in the

atmosphere?

ElectronsDetermine the chemical behavior of atoms.

Neutral AtomsHave no net charge.

IonsAtoms in which:# electrons # protons Electrical charge

CationIon with > # protons than electrons.

AnionIon with > # electrons than protons.

Electron Orbitals

OrbitalThe area around an atom’s nucleus where an electron is most likely to be found.

Potential Energy of Electrons (Energy of Position)Negative charge of electrons Attraction to Nucleus Keeping electrons in orbit work

Which requires more energy of position? Orbitals close to or far away from the

nucleus?

Eg. Photosynthesis

Electron Energy Levels

Electron Orbitals

Electron Energy Levels

RedoxOxidation-Reduction occurs when an electron is transferred from one atom to another (OILRIG).

The Periodic Table

Chemical Bonds

Molecule

Compound

A molecule that contains atoms of more than one element.

Types of Chemical Bond

• Ionic

• Covalent

• Hydrogen

Two or more atoms held together by chemical bonds.

Ionic Bonds*Not important in most biological molecules.

When do they occur?When 2 atoms are so unequal in their attraction for electrons that an electron is transferred from one atom to the other and ions are formed.

Ionic BondsCRYSTALS are formed - each ion is attracted to ions of opposite charge (NOT between particular ions, so DISCRETE NaCl molecules do not form).

Properties

• Strong (not as strong as covalent)

• Non-directional

Covalent Bonds*Important in biological molecules.

When do they occur?When electrons are SHARED between 2 atoms so that their outer energy level is completed.

Properties

• Strong (important for bonding within molecules)

• Directional (bonds between 2 specific atoms, resulting in specific SHAPE)

Strength of Covalent Bonds

Dependent on the number of shared electrons.

Double Bonds

2 pairs of shared electrons.

Eg. O=O

Triple Bonds

Strongest covalent bonds.

Polar Covalent Bonds

• Occur when the nucleus of one atom (involved in a covalent bond) is more electronegative than the other atom(s).

• This results in partial charges () on the atoms.

Example

Water

Hydrogen bonds*Important in biological molecules

When do they occur?When a hydrogen atom is covalently bonded to an electronegative atom it will have a partial positive charge and will also be attracted to another electronegative atom.