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8/9/2019 Chapter2 Molecules and Water
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The Nature of Molecules and theProperties of Water
Chapter 2
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2
The Nature of Atoms
All matters are composed of atoms.
Understanding the structure of atoms iscritical to understanding the nature of
biological molecules.
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Atomic Structure
Atoms are composedof
protons !"
neutrons neutral"
electrons #"
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Atomic Number
%ifferent atoms are defined b& the number of
protons' a (uantit& called the atomic number.
Atomic Number ) Number of Protons
*or e+ample' ,C has , protons and the atomic number is ,
Atoms -ith the same atomic number hae the
same chemical properties and belong to the same
element.
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Atomic Mass 0 Weight
1ach proton and neutron has a mass-eight" of appro+imatel& dalton.
1lectrons -eigh onl& 0$4 of a dalton'contributing almost nothing to the mass of anatom. Therefore
Atomic mass ) the sum of protons andneutrons
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1lectrons
Negatiel& charged
5ocated in orbitals that lie at ar&
distances around the nucleus P!N".
A neutral atom contains the same number
of protons and electrons
,
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6ons
Atoms in -hich the number of electrons
does not e(ual the number of protons are
ions. Cations 7 more protons than electrons' thus
positiel& charged Na!"
Anion 7 fe-er protons than electrons thus
negatiel& charges Cl#
"
8
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6sotopes
Isotopes are atoms of the same element thathae different atomic mass numbers due todifferent numbers of neutrons.
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1lectrons determine the chemical
behaior of atoms
1lectrons are located in orbitals surrounding the nucleus.
1ach orbital can contain onl& 2 electrons.
1lectrons that contain the same amount of energ& are
located in the same electron shell. 1ach shell can hold certain amount of electrons 2 electrons in the
st shell' in the 2nd : 3rd shell
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Atoms contain discrete energ& leel
The lo-est energ& leel the one nearest the nucleus. When an atom absorbs energ&' an electron moes to a
higher energ& leel
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Chemical reactions are processes in -hich
electrons are transferred from one atom to
another. 5oss of an electron is o+idation.
;ain of an electron is reduction.
<edo+ <eactions
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1lements *ound in 5iing S&stem
There are 94 naturall& occurring elements.
=nl& 2 elements shaded in green in the
periodic table" are found in liing
organisms in substantial amounts>4.4?".
*our elements ma@e up 9,.3? of human
bod& -eight
# carbon' h&drogen' o+&gen' nitrogen
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Valence electrons are the electrons in the outermost
energ& leel of an atom.
Atoms -ith completed alence shell are nonreactieB
atoms -ith unfilled alance shell are reactie.
Chemical properties depend on interactions bet-een
alence electrons of different atoms.
alence 1lectrons and Chemical Properties
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Chemical Donds
Molecules are groups of atoms held
together in a stable association.
Compounds are molecules containing
more than one t&pe of element.
Atoms are held together in molecules or
compounds b& chemical bonds. Chemical bonds form bet-een reactie atoms.
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Chemical Donds
Ionic bonds are formed b& the attraction ofoppositel& charged ions.
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Chemical Donds
Covalent bonds form -hen atoms share alence
electrons. The number of coalent bond depends on the number of
electron pairs shared b& the atoms.
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Chemical Donds
*or atoms that differ in electronegatiit&' atoms arenot shared e(uall&' forming polar coalent bonds. Une(ual sharing results partial positie and partial
negatie charges in the region
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Chemical reactions
Chemical reactions inole the formation orbrea@ing of chemical bonds.
Drea@ing bonds needs the input of energ& and
forming bonds releases energ&.
Whether a chemical reaction occurs is
influenced b& Temperature
concentration of reactants and products
aailabilit& of a catal&st
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Chemical <eactions
Chemical reactions are -ritten -ith thereactants first' follo-ed b& the products.
,E2= ! ,C=2 C,E2=, ! ,=2
reactants products
Chemical reactions are often reersible.
C,E2=, ! ,=2 ,E2= ! ,C=2
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Water Chemistr&
All liing organisms are dependent on
-ater.
The polar structure of -ater is the basis
for its uni(ue properties.
The most important propert& of -ater is
the abilit& to form hydrogen bonds.
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Water Chemistr&
Within a -ater molecule' the bonds bet-een
o+&gen and h&drogen are polar polar coalent
bonds" and partial electrical charges deelop.
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Water Chemistr&
E&drogen bonds are -ea@ attractions bet-een
the partiall& negatie o+&gen of one -atermolecule and the partiall& positie h&drogen of adifferent -ater molecule.
E&drogen bonds can form bet-een -ater
molecules or bet-een -ater and anothercharged molecule.
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Water Chemistr&
The polarit& of -ater causes it to becohesie and adhesie. cohesion -ater molecules stic@ to other
-ater molecules b& h&drogen bonding
adhesion -ater molecules stic@ to other
polar molecules b& h&drogen bonding
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Water Chemistr&
Eigh Surface Tension is due
to h&drogen bonding bet-een
-ater molecules
Adhesion of-ater to the
glass surface
dra-s -ater
up-ard
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2,
Properties of Water
Water has a high specific heat. A large amount of energ& is re(uired to change the
temperature of -ater. Water is cooler than land in
summer.
Water has a high heat of aporiFation. The eaporation of -ater from a surface causes
cooling of that surface. Water transpiration and
human s-eeting can cool the bod&.
Solid -ater is less dense than li(uid -ater. 6ce formation protects liing organism in deep -ater.
Water is a good solent. Water dissoles polar molecules and ions.
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Properties of Water
1+ample of -ater as a good solent
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Properties of Water
Water causes h&drophobic molecules toaggregate or assume specific shapes. h&drophilic G-ater#loingH
h&drophobic G-ater#fearingH Water can form ions.
E2= =E# ! E!
h&dro+ide ion h&drogen ion 6n pure -ater' I=E#J) IE!J ) 4#8 M
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Acids and Dases
Acid a chemical that releases E! ions.
1+ample ECl
Base a chemical that accepts E
!
ions.1+ample Na=E
;reater E! concentration lo-er pE
5o-er E!
concentration
higher pE
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The pE scale measures h&drogen ion
concentration
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Duffers help stabiliFe pE
Duffer is a chemical that accepts0releases E!
asnecessar& to @eep pE constant
• Most biological buffers consist of a pair of
molecules' one an acid and one a base.
