Download chemical bonds. - Dr. Gerry Cronin

Chapter 2
The Chemical Level of
Organization
Lecture slides prepared by Curtis DeFriez, Weber State University
Introduction
• Since chemicals compose your body (and all
body activities are chemical in nature), it is
important to become familiar with the language
and fundamental
concepts of
chemistry.
How Matter is Organized
• All forms of matter are composed of chemical
elements which are substances that cannot be
split into simpler substances by ordinary
chemical means.
– Elements are given letter abbreviations called
chemical symbols.
– Trace elements are
elements in our
bodies, present in
tiny amounts .
Structure of Atoms
• Units of matter of all chemical elements are
called atoms. An element is a quantity of matter
composed of atoms of the same type. Atoms
contain:
 Nucleus: protons (p+) & neutrons (no)
 Electrons (e-) surround the nucleus as a cloud
Structure of Atoms
• Electrons are very small and light (mass about
1/2000th that of proton or neutron), often
represented as a “planet” orbiting the “sun”
(atomic nucleus). In reality, they are found in a
“cloud” of probability. This
concept, however, made even
Einstein’s head hurt – we can
use the planets-orbiting-thesun model for this course!
Structure of Atoms
• Protons and neutrons form the nucleus of an atom;
electrons surround the nucleus.
• Protons are large, positively-charged particles
– The number of protons in the nucleus
(called the atomic number)
determines the element
(gold vs. silver vs. lead).
Structure of Atoms
• Neutrons are the second large particle that
make-up the nucleus of atoms.
– Unlike protons, neutrons have no charge.
– They do add mass, however, and determine the
variety, or “isotope” of a certain
element, i.e. carbon-12 vs.
carbon-14, which has 2
extra neutrons in nucleus
Structure of Atoms
• Mass is measured as a dalton (atomic mass
unit).
– Masses of subatomic particles
• Neutron - mass of 1.008 daltons
• Proton - mass of
1.007 daltons
• Electron has mass of
0.0005 dalton
Structure of Atoms
• Certain numbers are used to describe different
properties of atoms (of elements).
– Atomic number is the number of protons in an atom.
– Mass number is the sum of protons and neutrons in
an atom and indicates how much the atoms
“weighs”—this is always a whole number.
– Atomic mass (atomic weight) is the average mass of
all naturally occurring isotopes—since this is an
average, it is not exactly a whole number.
Structure of Atoms
• The “numbers” of some of the more important
elements for the human body are depicted in this
graphic:
Ions, Molecules, & Compounds
• Ions are atoms that have given up or gained an
electron in their outer electron shell (also called
the valence shell).
– Written with its chemical symbol and (+) or (–)
Ions, Molecules, & Compounds
• Molecules are formed when atoms share
electrons.
– Written as a molecular formula showing the number
of atoms of each element (H2O)
– The oxygen gas in the atmosphere we breath is really
not oxygen the atom, but a
pair of oxygen atoms linked
together into an oxygen
molecule (O2) .
Ions, Molecules, & Compounds
• A free radical is an electrically charged atom or group of
atoms with an unpaired electron in its outermost shell.
• They are unstable and highly reactive.
– They can become stable by giving up an electron or taking an
electron from another molecule .
• Antioxidants are
substances that
inactivate oxygenderived free radicals.
Chemical Bonds
• The atoms of a molecule are held together by
forces of attraction called chemical bonds.
• The likelihood that an atom will form a chemical
bond with another atom depends on the
number of electrons in its outermost or valence
shell.
Atomic Structure and Bonds
Interactions Animation
• Atomic Structure and the Basis of Bonds
You must be connected to the internet to run this animation.
Chemical Bonds
• Ionic bonds form when an atom loses or gains a
valence electron. Ions are formed.
– Positively and negatively charged ions are attracted to
one another.
– Cations are positively charged ions that have given up
one or more electrons (they are electron donors.)
– Anions are negatively charged ions that have picked up
one or more electrons that another atom has lost (they
are electron acceptors).
Chemical Bonds
Ionic Bond Formation
Chemical Bonds
• Covalent bonds are formed by the atoms of
molecules sharing one, two, or three pairs of
their valence electrons.
– Covalent bonds are the strongest chemical bonds.
– Single, double, or triple covalent bonds are formed
by sharing one, two, or three pairs
of electrons, respectively.
Chemical Bonds
• Covalent bonds may be nonpolar or polar.
– In a nonpolar covalent bond, atoms share the
electrons equally; nonpolar covalent bonds are the
most common types of covalent bonds, as shown in
this graphic depicting hydrogen gas, oxygen gas,
nitrogen gas, and methane gas:
Chemical Bonds
• Polar covalent bonds are formed by the
unequal sharing of electrons between atoms.
• Polar covalent bonds are extremely important
because the all-important water molecule
makes use of this bond.
• In water, oxygen attracts the hydrogen
electrons more strongly, making
oxygen slightly electronegative as
indicated by the negative Greek
delta sign.
Chemical Bonds
• Hydrogen bonds are weak interactions
(approximately 5% as strong as covalent bonds)
between hydrogen and adjacent electronegative
atoms like oxygen or sulfur.
– Hydrogen bonds result from attraction of oppositely
charged parts of molecules—they
should not be confused with covalent
bonding to hydrogen which
Chemical Bonds
• Hydrogen bonds are useful in establishing links
between molecules or between distant parts of a
very large molecule. Large 3-D molecules (like
proteins) are often held together by a great many
hydrogen bonds.
In water, hydrogen bonding provides
considerable cohesion which
creates a very high surface tension
(as this bug demonstrates).
Chemical Bonds
Interactions Animation
• Chemical Bonding
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Chemical Reactions
• Chemical reactions occur when electrons in the
valence shell are shared or transferred. New
bonds form and/or old bonds are broken.
• Metabolism is the “sum of all the chemical
reactions in the body”.
• Law of conservation of energy:
– The total mass of reactants equals the total mass of
the products .
Chemical Reactions
• Energy (the capacity to do work) is transferred
in a chemical reaction.
• Kinetic energy is the energy of matter in
motion.
• Potential energy is energy stored by matter due to
an object’s position
in space, or
stored in
chemical bonds.
Chemical Reactions
• An exergonic reaction releases energy (usually in
the form of heat during catabolism of food) by
breaking a bond with more energy than the one
being formed.
• An endergonic reaction requires that energy be
added, usually from a
molecule called
ATP, to form
a bond.
Chemical Reactions
• Activation
Energy is the
energy required
to break
chemical bonds
in the reactant
molecules so a
reaction can
start.
Chemical Reactions
• Factors that cause a collision (and a chemical
reaction to take place) include the temperature
and the concentration of the reactants, and the
presence or absence of a catalyst.
• Catalysts are chemical compounds that speed up
chemical reactions by lowering the activation
energy needed for a reaction to occur.
– Catalysts are neither consumed nor produced in the
reaction. They are used over-and-over again, often
several million times per second!
Chemical Reactions
• In a chemical reaction, a catalyst helps to
properly orient the colliding particles of matter
so that a reaction can occur at a lower collision
speed. A catalyst does not alter the difference
in potential
energy between the reactants
and products (it only lowers
the amount of energy needed
to get the reaction started.)
Chemical Reactions
• Types of chemical reactions can be broadly
classified as:
– Synthesis reactions – Anabolism
• A + B ➙ AB
– Decomposition reactions – Catabolism
• AB ➙ A + B
– Exchange reactions
• AB + CD ➙ AD + CB
– Reversible reactions
• AC ↔ A + C
Types of Reactions
Interactions Animation
• Types of Reactions and Equilibrium
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Chemical Reactions
• Types of Reactions and Equilibrium
Interactions Animation
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Inorganic & Organic Compounds
Inorganic compounds are structurally simple molecules
that usually lack carbon - like the salt potassium
chloride (KCl) depicted here:
• Organic compounds always contain carbon and
are usually large, complex molecules.
– Usually contain hydrogen
– Always have covalent bonds
Inorganic Compounds
• Water is the most important and abundant
inorganic compound in all living systems.
• Water’s most important
property is polarity, the
uneven sharing of valence
electrons that enables
reactants to collide to
form products.
Inorganic Compounds
Sodium and Chloride ions dissolve in the polar water
molecules.
Water
Interactions Animation
• Polarity and Solubility of Molecules
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Water and Fluid Flow
Interactions Animation
• Water and Fluid Flow
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,
Inorganic Compounds
• Water as a solvent:
– In a solution, the solvent dissolves the solute.
– Substances which contain polar covalent bonds and
dissolve in water are hydrophilic, while substances
which contain non-polar covalent bonds are
hydrophobic.
– The polarity of water and its bent shape allow it to
interact with several neighboring ions or molecules.
– Water’s role as a solvent makes it essential for health
and survival.
Inorganic Compounds
• Water has a high heat capacity, meaning it can
absorb or release a relatively large amount of
heat with only a modest change in its own
temperature.
• This property is due to the large number of
hydrogen ions in water.
Heat of vaporization (amount of heat needed to
change from a liquid to a gas) is also high.
– Evaporation of water from the skin removes large
amounts of heat.
Three Common Mixtures
• A mixture is a combination of elements or
compounds that are physically blended together
but are not bound by chemical bonds.
• In a solution, a substance called the solvent
dissolves another substance called the solute.
Usually there is more solvent than solute in a
solution.
– In our bodies, the most common solvent is water:
We are 65-80% water (depending on age - babies are
very “wet”… as anyone who has had one can attest.)