Chapter #2 – The Chemical Context Of Life - Notes
I. A Chemical Connection to Biology
1. A single flowering tree species near the Amazon’s headwaters in Peru
dominate tracts of forest.
2. The organism responsible for this pruning is the ant species. This organisms
use formic acid as a herbicide.
3. This chapter focuses on the chemical components that make up all matter.
I. Matter consists of Chemical Elements in Pure Form and in Combination
Called Compounds
1. Organisms are composed of matter.
2. Matter is anything that takes up space and has mass.
3. Matter whether living or nonliving is composed of
a) elements
b) compounds
c) mixtures
4. An element is a substance that cannot be broken down to other substance by
chemical reactions.
5. Today, chemists recognize 92 elements occurring in nature.
A. Essential Elements of Life
1. About 25 of the 92 natural elements aree known to be essential to life.
2. Carbon, hydrogen, oxygen and nitrogen make up approximately 96% of
living matter.
3. Calcium, phosphorus, potassium, sulfur, sodium, chlorine and magnesium
make up about 4% of human body weight.
C. Trace elements
1. Trace elements are those required by an organism in only minute quantities.
2. Some trace elements such as iron (Fe) is required by all forms of life.
3. In humans & other vertebrates, the element iodine (I) is an essential ingredient of the hormone produced by the thyroid gland.
4. Thyroid deficiency in the diet causes a condition called a goiter.
5. A compound is a substance consisting of two or more different elements
combined in a fixed ratio. Example – table salt (Na Cl).
6. A mixture is two or more substances that are put together but retain their individual properties & are not chemically combined. (Sand + salt)
II. An Elements’s Properties Depend on the Structure of its Atoms
1. Each element consists of a certain kind of atom that is different from the
atoms of any other element.
2. An atom is the smallest unit of matter that still retains the properties of an
element.
A. Subatomic Particles
1. Atoms are composed of smaller particles called sub-atomic particles.
2. The three most stable subatomic particles are:
a) neutrons –No charge, electrically neutral
b) protons -- + charge
(Packed in the nucleus with neutrons)
c) neutrons -- - charge (Orbit around nucleus. Held by electrostatic attraction to positively charged nucleus)
3. The Dalton is the unit used to measure the atomic mass.
B. Atomic Number & Atomic Mass
1. An electrically neutral atom has equal numbers of electrons & protons.
2. The number of protons determines the atomic number. The number of protons
is constant.
3. The atomic mass is measured in Daltons & is roughly equal to the sum of
protons plus neutrons. At. Mass = P + N.
4. The number of neutrons can be deduced by subtracting the atomic number from
the atomic mass. N = P -- Atomic Mass.
C. Isotopes
1. Isotopes are atoms of an element that have the same atomic number but varying
number of neutrons & hence different mass numbers.
2. Some isotopes are stable & others are radioactive.
3. A radioactive isotope is one in which the nucleus decays spontaneously,
Giving off particles and energy.
4. Radioactive isotopes have many useful application in science.
D. The Energy Levels Of Electrons
1. In an atom, electrons occupy specific energy shells; the electrons in a shell
have a characteristic energy level.
E. Electron Distribution & Chemical Properties
1. Electron distribution in shells determines the chemical behavior of an atom.
2. The K – shell can hold a maximum of 2 electrons, the L – shell can hold a
maximum of 8 electrons, M – shell can hold a maximum of 8 electrons (in
a biology class this is true). We adhere to the Octet rule.
.
3. An atom that has an incomplete valence shell is reactive, one that has a
complete valence shell is nonreactive.
F. Electron Orbitals
1. Electrons exist in orbitals, three-dimensional spaces with specific shapes that
are components of electron shells.
2. No more than 2 electrons can occupy a single orbital.
III. The Formation and Function of Molecules Depend on Chemical Bonding
Between Atoms
1. Chemical bonds are attractions that hold molecules together.
2. The strongest kinds of chemical bonds are covalent bonds & ionic bonds.
A. Covalent Bonds
1. A covalent bond is the sharing of a pair of valence electrons by two atoms.
An example – two hydrogen atoms interact & share a pair of valence electrons.
(some atoms share two pairs of electrons, others share three pairs of electrons)
2. The notation which represents both atoms and bonding is called a structural
formula- H—H
3. The above can be abbreviated even further by writing H2- this is the molecular
formula indicating the molecule consists of two atoms of hydrogen.
4. The bonding capacity of an atom is called its valence & usually equals the
number of unpaired electrons required to complete the atom’s outermost
(valence) shell.
5. The attraction of a particular ind of atom for the electrons of a covalent bond
is called its electronegativity.
6. A nonpolar covalent bond is one in which the electrons are shared equally
between atoms.
7. Polar covalent bonds are formed by unequal sharing of electrons between
atoms.
B. Ionic Bonds
1. If two atoms are so unequal in their attraction for valence electrons that the
more electronegativity atom strips an electron completely away from its
partner.
2. The electron transfer between the two atoms moves one unit of negative charge
from one to the other atom. This will cause the atom to have a net electrical
charge.
3. A charged atom (or molecule) is called an ion.
4. When the charge is positive, the ion is called a cation.
5. Conversely, the othe atom has gained an extra electron, giving it a net electrical
charge of -- 1, --2, etc.
6. This atom has become an ion specially called an anion, or negatively charged
ion.
7. Two ions of opposite charge can form an ionic bond.
8. Compounds formed by ionic bonds are called ionic compounds, or salts.
9. Ionic compounds dissolve in water.
C. Weak Chemical Bonds
D. Weal Hydrogen Bonds
1. A hydrogen bond is formed when a hydrogen atom covalently bonded to one
electronegative atom is also attracted to another electronegative atom.
2. Van der Waals interactions occur between transiently positive & negative
regions of molecules.
3. Weak bonds reinforce the shapes of large molecules and help molecules
adhere to each other.
E. Molecular Shape & Function
1. A molecule’s shape is determined by the position of its atoms’ valence orbitals.
2. Shape is usually the basis for the recognition of one biological molecule by
another.
F. Chemical Reactions Make & Break Chemical Bonds
1. The making & breaking of chemical bonds, leading to changes in the
composition of matter, are called chemical reactions.
2. Chemical reactions change reactants into products while conserving matter.
Example: The example will be placed on the board. See p. 42.