The Chemical Basis of Life
Chapter 2
• Objectives
• Identify the four elements that make up 96% of living
matter.
• Distinguish between the following pairs of terms: neutron
and proton, atomic number and mass number, atomic
weight and mass number
• 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
2
Organization of Matter
• Matter-anything that occupies space and has
mass
–
–
–
–
–
Includes solids, liquids and gases
Made up of 92 naturally-occurring elements
Atom-smallest unique unit of element
Molecule-bonded unit of two or more atoms
Compound-substance in which relative
percentages of two or more elements never vary
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• Essential elements
– include carbon, hydrogen, oxygen, and nitrogen
– make up 96% of living matter
• a few other elements make up the remaining 4% of
living matter
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2
Structure of Atoms
• Each element consists of a certain kind of
atom that is different from those of other
elements
– An atom is the smallest unit of matter that still
retains the properties of an element
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• Atoms of each element are composed of
three even smaller parts called subatomic
particles
– Protons-part of nucleus
• positive charge
• number of protons=atomic number
– Neutrons-part of nucleus
• no charge
• protons+neutrons=atomic mass
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– Electrons-orbit nucleus
• negative charge
• number of electrons=number of protons
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• Atoms of a given element may occur in
different forms
– Isotopes of a given element differ in the number
of neutrons in the atomic nucleus
• have the same number of protons
– Radioactive isotopes spontaneously give off
particles and energy
• can be used in biology
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4
The Energy Levels of Electrons
• An atom’s electrons vary in the amount of
energy they possess
– electron’s have potential energy
• due to position in relation to nucleus (electrostatic)
• represented by energy levels or electron shells
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Electron Configuration and
Chemical Properties
• The chemical behavior of an atom is defined
by its electron configuration and distribution
– the periodic table of the elements shows the
electron distribution for all the elements
• Outermost shell of an atom called the
valence shell
– electrons in the valence shell called valence
electrons
• number of valence electrons determines the chemical
properties of atoms
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5
Electron Orbitals
• Each energy shell is divided into one or more
3d spaces called orbitals
– each orbital can contain only two electrons
– the maximum number of electrons an energy shell
can hold is determined by the number of orbitals
the energy shell is divided into
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6
Bonds Between Atoms
• Chemical bond-atoms gain, lose or share
electrons
– two types of bonds formed between atoms
• covalent bond
• ionic bond
– type of bond formed determined by two factors
• electron configuration of the outer or valence shell of
electrons
• electronegativity of the atom
– electronegativity is a measure of the attraction of an atom
for electrons
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Covalent Bonds
• Covalent bond
– atoms share one or more pairs of electrons
• nonpolar-share electrons equally
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• Polar covalent bonds
– share electrons unequally
• more electronegative atom in the covalent bond pulls the
electrons toward itself
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8
Ionic Bonds
• Ionic bond
– one atom gains electron, one atom loses electron
– + and - charges mutually attractive
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9
Weak Chemical Bonds
• Several types of weak chemical bonds are
important in living systems
– Hydrogen Bonding
• atom of molecule interacts with hydrogen already in
polar bond
• stabilizes nucleic acids and proteins
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10
• Van der Waals interactions occur when
transiently positive and negative regions of
molecules attract each other
• Weak chemical bonds
– Reinforce the shapes of large molecules
– Help molecules adhere to each other
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Atoms and Molecules
• A molecule’s biological function is related to
its shape
– molecules have characteristic sizes and shapes
symbolized by ball-and-stick and space-filling
models
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• The shape of the molecule is the basis for
one molecule recognizing another molecule
when they interact
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Chemical Reactions
• Chemical reactions break or form chemical
bonds to change reactants into products
– matter is conserved during reaction
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• Most chemical reactions are reversible
– chemical equilibrium is reached when the forward
and backward reaction rates are equal
• Living cells carry out thousands of chemical
reactions that rearrange matter in significant
ways
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