 Chapter 2: The Chemical Context of Life
 Overview: A Chemical Connection to Biology
• Biology is a multidisciplinary science
• Living organisms are subject to basic laws of ________________
___________________
 Concept 2.1: Matter consists of chemical elements in pure form and in combinations
called compounds
• Organisms are composed of _________________
• Matter is anything that __________________________________
• Matter is made up of elements
• An ______________is a substance that _____________________ to other
substances by chemical reactions
• A _________________ is a substance consisting of ____________
__________________ in a fixed ratio
• A compound has characteristics __________________from those of its elements
 Essential Elements of Life
• About ______ of the ______naturally occurring elements are essential to life
• ___________________________ make up 96% of living matter
• Most of the remaining 4% consists of ______________________
_____________________________________
• ____________________ are those required by an organism in minute quantities (Ex:
iron, iodine)
•
 Concept 2.2: An element’s properties depend on the structure of its atoms
• Each element consists of unique ______________
• An atom is the ___________________________ that still retains the properties of an
element
• Atoms are composed of _________________________________ (physicists have
split the atom into more than 100 types----we are only concerned with three )
• Relevant subatomic particles include:
– __________________ (no electrical charge)
– __________________ (positive charge)
– __________________ (negative charge)
• Neutrons and protons form the __________________________
• Electrons form a _______________ around the nucleus
• Neutron mass and proton mass are almost identical and are measured in
______________(same as the ___________________, or _________)
 Atomic Number and Atomic Mass
• Atoms of the various elements differ in _________________
______________________________
• An element’s ________________ is the number of __________ in its nucleus.
Written as a subscript to the left of the symbol: 2He (In an
_____________________________, the number of protons is the same as the
number of electrons)
• An element’s ____________________ is the _______________
___________________ in the nucleus. Written as a superscript to the left of the
symbol:
• So to get the number of neutrons, subtract the atomic number from the mass
number.
• __________________, the atom’s ___________________, can be approximated
by the mass number (protons and neutrons each have a mass very close to 1
dalton)
 Isotopes
• All atoms of an element have the ________________________
____________________________________________
• _________________are two atoms of an element that differ in number of
neutrons
– Ex: Carbon has three isotopes:
• Carbon 12 makes up 99% of the carbon in nature. Most of the remaining 1% is
Carbon 13. Carbon 14 is very rare. (How many neutrons does each carbon
isotope have?) All behave identically in chemical reactions, however, Carbon 14 is
unstable or “radioactive” meaning, its nuclei have a tendency to lose particles.
• ___________________________ decay spontaneously, giving off particles and
energy. When decay leads to a change in the number of protons, it transforms
the atom to an atom of a different element.
– Ex: Carbon 14 decays to form nitrogen
• Some applications of radioactive isotopes in biological research are:
– _________________________
– ________________________________________
– ________________________________________
 The Energy Levels of Electrons
• _________________ is the capacity to cause change
• ___________________ is the energy that matter has because of its
______________ or _________________
• The electrons of an atom differ in their amounts of potential energy
• What makes up most of the ________________________?
– Answer: ____________________
– Electrons circle around the nucleus at fixed distances.
– They have ________________________ because of how they are arranged in
relation to the nucleus
– It takes work to move them further away from the nucleus. Thus, the further
away from the nucleus, the _____________
__________________________
• An electron’s state of potential energy is called its ____________, or
_________________
• An electron can change shells only by ______________________.
– Example: Sunlight can excite and electron to a higher energy level (this is the
first step in photosynthesis)
– When electrons lose energy, they “fall back” to a shell closer to the nucleus.
The lost energy is usually released as heat (this is what happens when sunlight
strikes a surface, excites electrons to a higher energy level. As they fall back,
the surface heats up and radiant energy is changed to thermal energy).
 Electron Distribution and Chemical Properties
• The ______________________ of an atom is determined by the
______________________________ in electron shells
• The _______________________________ shows the electron distribution for each
element
• ________________________ are those in the outermost shell, or
________________________
• The chemical behavior of an atom is mostly determined by the
___________________________
st
– 1 shell can hold ____________________
nd
– 2 shell can hold ___________________
rd
– 3 shell can hold ____________________
• Elements in the same column of the periodic table have the same number of valence
electrons and therefore react in similar manners.
• Elements with a _____________________ shell are chemically
______________________. They are in the last row of the periodic table.
 Electron Orbitals
• An __________ is the three-dimensional space where an electron is found ______ of
the time
– Think of an orbital as a component of an electron shell.
• Each electron shell consists of a specific number of orbitals
st
– 1 shell has one spherical (s) orbital = _____
nd
– 2 shell has four orbitals: one spherical _____and three dumbbell shaped (p)
orbitals = ______
– No orbital can hold more than _______________
 Concept 2.3: The formation and function of molecules depend on chemical bonding
between atoms
• Atoms with incomplete valence shells can ___________________ valence electrons
with certain other atoms
• These interactions usually result in atoms staying close together, held by attractions
called ____________________
 Covalent Bonds
• A _____________________ is the ________________________
______________________ by two atoms
• In a covalent bond, the shared electrons count as
part of ______________________________________
• A _______________ consists of two or more atoms
held together by ___________________________
• A single covalent bond, or _________________, is the
sharing of ___________________________________
• A double covalent bond, or __________________, is
the sharing of
_________________________________
• The notation used to represent atoms and bonding is
called a __________________________
– For example,
• This can be abbreviated further with a
______________________
– For example,
•
•
•
•
•
•
•
Covalent bonds can form between atoms of the ______________ or atoms of
_______________________
A ____________________ is a combination of two or more
________________elements
__________________________ is called the atom’s ___________
– Usually equals the number of unpaired electrons required to complete the
outer shell
– i.e. The number of covalent bonds it can form
___________________ is an atom’s ________________________
__________________________
The more electronegative an atom, the more ________________ shared electrons
toward itself
In a _________________________, the atoms share the electron ______________
In a _____________________________, one atom is more electronegative, and the
atoms ___________________________
Unequal sharing of electrons causes a _____________________ or
____________________________ for each atom or molecule
 Ionic Bonds
• Atoms sometimes ____________________ from their bonding partners
• An example is the _______________________________________
• After the transfer of an electron, both atoms have ____________
• A charged atom (or molecule) is called an __________
• A _____________ is a _______________________
• An __________ is a _________________________
• An ___________________ is an attraction between an
_____________________________
• Compounds formed by ionic bonds are called ________________
_______________________
• Salts, such as sodium chloride (table salt), are often found in nature as
__________________
 Weak Chemical Bonds
• Most of the _________________________in organisms are
_________________________ that form a cell’s molecules
• Weak chemical bonds, such as ___________________ and
______________________, are also important
• Weak chemical bonds _________________________________ and
____________________________________
1. Hydrogen Bonds
– A ________________________ forms when a
hydrogen atom covalently bonded to one
electronegative atom is also attracted to another
electronegative atom
– In living cells, the electronegative partners are
usually
_______________________________atoms
2. Van der Waals Interactions
• If electrons are distributed asymmetrically in molecules or atoms, they can
result in “__________” of positive or negative charge
• ____________________________are attractions between molecules that
are close together as a result of these charges.
– These interactions are weak and occur only when atoms are very close.
– Collectively, such interactions can be strong, as between molecules of a
gecko’s toe hairs and a wall surface
 Molecular Shape and Function
• A molecule’s ______________ is usually very important to its
________________
• A molecule’s shape is determined by the _______________
_________________________________
• In a ______________________, the s and p orbitals may
___________________, creating specific molecular shapes
• Biological molecules ________________ and ___________
________________________ with a specificity based on
__________________________
• Molecules with similar shapes can have similar biological effects
– Ex: Opiates and endorphins have similar shapes. This allows them to bind
to the same receptors in the brain and produce similar effects
 Concept 2.4: Chemical reactions make and break chemical bonds
• _____________________ are the making and breaking of chemical bonds
• The starting molecules of a chemical reaction are called
___________________
• The final molecules of a chemical reaction are called ____________________
• ____________ are used to indicate the ________________ of the reactants
to the products.
• _____________________ is an important chemical reaction
• Sunlight powers the conversion of carbon dioxide and water to glucose and
oxygen
• Some chemical reactions go to completion: all reactants are converted to products
• All chemical reactions are __________________: products of the forward reaction
become reactants for the reverse reaction
• ______________________ is reached when the forward and reverse reaction rates are
equal
You should now be able to:
1. Identify the four major elements
2. Distinguish between the following pairs of terms: neutron and proton, atomic number
and mass number, atomic weight and mass number
3. Distinguish between and discuss the biological importance of the following: nonpolar
covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals
interactions