Transcript Polar covalent bond

Essential Chemistry for Biology
Chapter 2
Why Chemistry??
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Chemicals make
up everything!
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Body
Environment
Food
Basic Chemistry
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Matter
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Anything that takes up
space and has mass
Can exist as a liquid,
solid, or gas
Made up of atoms
Elements
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Basic building block of
matter
92 naturally occurring
elements
Basic Chemistry
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Only 6 elements that make up most
of the body weight of organisms
C
Carbon
H
Hydrogen
N
Nitrogen
O
Oxygen
P
Phosphorus
S
Sulfur
Trace Elements
Trace elements make up 0.01% of human
body weight, but still very important
Each Element Consists of ONE
Kind of Atom
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Smallest unit that retains the properties
of a given element
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And to enter a chemical reaction
Composed of subatomic particles:
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Protons (p+)
Neutrons
Electrons (e-)
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Move @ atomic nucleus
Usually (e-) = (p+)
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Electrically neutral
Atomic Symbol
Atomic Mass =
Number of Protons +
Number of Neutrons
Atomic Number = The Number of Protons
in the Nucleus
Why are electrons so important??
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determine the chemical behavior of
atoms
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Part of atom that comes close enough for
interactions
associated with energy
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the field of energy around an atom is
arranged as levels called electron shells
Electron Shells
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have specific numbers of
orbitals that may be filled
with electrons
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atoms that have incomplete
electron orbitals tend to be
more reactive
atoms will lose, gain, or share
electrons in order to fill
completely their outermost
electron shell
these actions are the basis of
chemical bonding
Valence Shell
Molecules and Compounds
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Molecules
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Chemical bonding that joins atoms
Some contain atoms of only one element
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Compounds
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Atoms of two or more different elements
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N2
H 2O
Mixture
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Two or more molecules mingling together
Chemical Bonds
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Atoms are held together by bonds
3 main types:
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Ionic bond
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Covalent bond
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Accept or donate electrons
Share one or more electrons
Hydrogen bond
Ionic Bond
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Atoms gain and lose
electrons
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Ion
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Balance between protons
and electrons shifts
Become “ionized”
Atom that has a charge
Losing an electron results
in a net positive
Gaining an electron results
in a net negative charge
Ionic bond
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Association of two ions
that have opposing
charges
Which will form an ionic bond?
Argon (Ar)
Chlorine (Cl)
Potassium (K)
Helium (He)
Can Mg and Cl form an ionic bond?
Chlorine (Cl)
Magnesium (Mg)
Covalent Bond
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Covalent bond
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Stable and stronger
than ionic bonds
Atoms share
electrons
 Each atom will
have a completed
outer shell
Bond Notation
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Single covalent
bonds are written
as H-H
Double covalent
bonds are written
as O=O
A covalent bond can be nonpolar
or polar
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Nonpolar
covalent bond
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Sharing of electrons
between atoms is
fairly equal
Polar covalent
bond
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Unequal sharing of
electrons
Hydrogen Bond
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Hydrogen bond
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Weak attraction
 Form and break
easily
Occurs between
polar molecules
 Polar bonds have
charges on either
end and often
attract to each
other
Water molecules contain two covalent bonds
Chemical Reactions
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Cells constantly rearrange molecules
by breaking existing chemical bonds
and forming new ones
Reactants
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Products
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Starting materials
End materials
Chemical reactions cannot create or
destroy matter,
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They only rearrange it
Hydrogen Bonds Give Water
Unique Properties
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Heat Storage
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water temperature changes slowly and holds
temperature well
Ice Formation
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few hydrogen bonds break at low temperatures
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water becomes less dense as it freezes because
hydrogen bonds stabilize and hold water molecules
farther apart
High Heat of Vaporization
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at high temperatures, hydrogen bonds can be broken
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water requires tremendous energy to vaporize because
of all the hydrogen bonds that must be broken
Hydrogen Bonds Give Water
Unique Properties
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Water molecules are sticky
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cohesion – when one water
molecule is attracted to
another water molecule
adhesion – when polar
molecules other than water
stick to a water molecule
Hydrogen Bonds Give Water
Unique Properties
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Water dissolves other polar
substances
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Solvent
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Solute
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A dissolved substance
Hydrophilic
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Ions and polar molecules
easily dissolve in it
water-loving molecules
form hydrogen bonds with
water
Hydrophobic
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water-fearing molecules
do not form hydrogen
bonds with water
Acids and Bases
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When water ionizes, it releases an equal
number of hydrogen ions (H+) and
hydroxide ions (OH-)
Some substances release more or fewer or
each
 Acids - Excess hydrogen ions
 Bases - Excess hydroxide ions
Acidic and Basic Solutions
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Acidic Solutions
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Sharp, sour taste
Molecules that
dissociate in water
 Release
hydrogen ions
(H+)
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Basic Solutions
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Bitter taste
Molecules that
either take up
hydrogen ions (H+)
or release
hydroxide ions
(OH-)
pH Scale
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The pH Scale
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Ranges from 0 - 14
pH below 7 is acidic
pH above 7 is alkaline
pH of 7 is neutral
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[H+] > [OH-]
[OH-] > [H+]
[H+] = [OH-]
Buffer
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Chemical or combination of chemicals that
keeps pH within normal limits
Water Ionizes
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The amount of ionized hydrogen
from water in a solution can be
measured as pH
pH = -log[H+]
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The pH scale is logarithmic, which
means that a pH scale difference of
1 unit actually represents a 10-fold
change in hydrogen ion
concentration
The pH scale