Download Chapter #2 The Chemical Context Of Life

Chapter #2
The Chemical Context Of Life
Introduction:
A Chemical Connection to Biology
I. Matter Consists of Chemical Elements In Pure Form and in Combination
Called compounds
1. Organisms are made of matter.
2. Matter is defined as anything that takes up space and has mass.
3. Matter exists in many diverse forms. Example: rocks, metal, oils, human.
A. Elements and Compounds
1. Matter is made up of elements.
2. An element is a substance that cannot be broken down to other substances
by chemical reactions.
3. Today, chemists recognize 92 elements occurring in nature. Ex. Gold.
4. Each element has a symbol, usually the first letter or two of its name.
5. A compound is a substance consisting of two or more different elements
combined in a fixed ratio. Ex. NaCl.
B. The Elements Of Life
1. Of the 92 natural elements, about 20-25% are essential elements.
2. Essential elements are elements that an organism needs to live a healthy
life and reproduce.
a) Humans need 25 elements.
3. The elements – C (carbon)
H (hydrogen) Make up 96% of
O (oxygen)
living matter
N (nitrogen)
Ca (calcium)
P (phosphorus)
K (potassium) Make up the remaining 4%
S (sulfur)
Na (sodium)
Cl (chlorine)
Mg (magnesium)
4. Trace elements are required by an organism in only minute quantities.
a) Iron (Fe) is a trace element needed by all forms of life.
b) In vertebrate (animals with a backbone), the element iodine (I) is an essential
ingredient of a hormone produced by the thyroid gland.
II. An Elements Properties Depend On the Structure Of Its Atoms
1. Each element consists of certain types of atoms.
2. An atom is the smallest unit of matter that still retains the properties of an element.
A. Subatomic Particles
1. The three kinds of subatomic particles that are relevant are
a) neutrons
b) protons
c) electrons
2. Protons & electrons are electrically charged.
a) Each proton has 1 unit of positive charge
b) Each electron has 1 unit of negative charge
c) A neutron is electrically neutral
3. Protons and neutrons are packed together in the atomic nucleus.
4. The neutrons and protons are almost identical in mass. Ex. 1.7 x 1024 Grams
5. Mass of an electron is about 1/2,000 that of a neutron or proton.
B. Atomic Number and Atomic Mass
1. All atoms of a particular element have the same number of protons in their nuclei.
2. The number of protons, which is unique to the element, is called the atomic number.
3. Unless otherwise indicated, an atom is electrically neutral.
a) Its protons must equal to the number of electrons.
4. The mass number is the sum of protons plus neutrons in the nucleus.
5. The number of neutrons can be deduced by subtracting the mass number – the atomic
number.
C. Isotopes
1. All atoms of a given element have the same number of protons, but a varying number
of neutrons. Therefore the different forms have a greater mass number. These
different forms of the same element are called – isotopes of the element.
2. Radioactive isotope is one in which the nucleus decays spontaneously giving off
particles and energy.
3. Radioactive isotopes have many useful applications in biology. For example: radioactive dating of fossils, tracers to follow atoms through metabolism, etc.
D. The Energy Levels of Electrons
1. Of the three kinds of subatomic particles, only electrons are directly involved in the
chemical reactions between atoms.
2. An atom’s electrons vary in the amount of energy they possess.
3. Energy is defined as the capacity to cause change. Example: Doing work
4. Potential energy is the energy that matter possess because of its location or structure.
5. Matter has a natural tendency to move to the lowest possible state of potential energy.
6. An electron’s potential energy is determined by its energy level.
7. Electrons are found in different electron shells, each with a characteristic average
distance and energy level.
a) The electrons on the shell closest to the nucleus, have the lowest potential energy.
E. Electron Distribution and Chemical Properties
1. The chemical behavior of an atom is determined by the distribution of electrons in the
atom’s electron shells.
2. Shell
1
2
3
Name of Shell
K
L
M
Maximum # of Electrons Shell Holds
2
8
8 (It holds 18 electrons, we will max @ 8)
3. The chemical behavior of an atom depends mostly on the number of electrons in its
outermost shell.
4. The outermost electron shell is called the valence electron shell.
5. The outer electrons are called valence electrons.
6. An atom with a completed valence shell is uncreative.
(These elements are inert – chemically uncreative)
7. Atoms with incomplete valence shells are chemically reactive.
F. Electron Orbitals
1. The three dimensional space where the electron is found is called an orbital.
2. No more than 2 electrons can occupy a single orbital.
3. The reactivity of atoms arises from the presence of unpaired electrons in one or more
orbitals of their valence shells.
4. Atoms interact in a way that completes their valence shells.
III. The Formation And Function Of Molecules Depend On Chemical Bonding Between
Atoms
1. Atoms with incomplete valence shells can interact with certain other atoms in
such a way that each partner completes its valence shell.
2. These interactions are held together by attractions called chemical bonds.
3. The strongest kinds of chemical bonds are; covalent bonds and ionic bonds.
A. Covalent Bonds
1. A covalent bond is the sharing of a pair of valence electrons by two atoms.
2. When two are more (similar) atoms are held together by covalent bonds – a molecule
is formed.
Examples: Molecular Formula:
Lewis Dot Structure:
Structural Formula:
3. The attractioin of a particular atom for the electrons of a covalent bond is called its
electronegativity.
4. Nonpolar covalent bond – equal sharing of electrons between two atoms of the same
element.
5. Polar covalent bond- When an atom is bonded to a more electronegative atom, the
electrons of the bond are not shared equally. This type of bond is called a polar
covalent bond. (Unequal sharing of electrons between atoms).
B. Ionic bonds
1. In some cases, two atoms are so unequal in their attraction for valence electrons that
the more electronegative atom strips an electron completely away from its partner.
2. A charged atom (or molecule) is called an ion.
3. When the charge is positive the ion is called a cation.
4. When the charge is negative the ion is called an anion.
5. Cation and anions attract each other, this attraction is called an ionic bond.
6. Compounds formed by ionic bonds are called ionic compounds or salts.
C. Weak Chemical Bonds
1. Weak bonds reinforce the shapes of large molecules and help molecules adhere to
each other.
2. A hydrogen bond (a weak bond) is an attraction between a hydrogen atom carrying
a partial positive charge and an electronegative atom.
3. Van deer Waals interactions (weak bond) occurs between transiently positive and
negative regions or molecules.
D. Molecular Shape And Function
1. A molecule has a characteristic size and shape.
2. A molecule’s shape is determined by the positions of its atomic valence orbitals.
3. Covalent bonds result in hybrid orbitals, which are responsible for the shapes of
water, methane, and many more complex biological molecules.
4. Shape is usually the basis for the recognition of one biological molecule by another.
IV. Chemical Reactions Make And Break Chemical Bonds