Chapter 2
The Chemical Context of Life
Objectives
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Know the elements that make up 96.3% of the
human body mass and their symbols
Distinguish between the following terms: proton,
neutron, electron, atomic number and mass
number
Know the difference between the following
terms:ionic bonds, hydrogen bonds, polar
bonds, nonpolar covalent bonds and van der
Waals interactions
Know the difference between isotopes and
Organization of Matter
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Organisms are composed of matter
Matter is defined as anything that takes up
space and has mass
Matter exists in many diverse forms
-rocks, metals,oils, gases and humans are just
a few examples of what seems an endless
assortment of matter
Elements and Compounds
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Matter is made up of elements
An element is a substance that cannot be
broken down to other substances by chemical
reactions
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There are 92 elements occurring in nature
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-Ex. Gold, copper, carbon and oxygen
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A compound is a substance consisting of two
or more different elements combined in a fixed
ratio
-Ex. NaCl sodium chloride
Essential Elements
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Of the 92 natural elements 20-25% are
essential elements
Just four elements make up 96% of living
matter
-(C)-Carbon, (H)-Hydrogen, (O)-Oxygen, and
(N)-Nitrogen
A few other elements make up the other 4%
Atoms
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Each element consists of a certain type of atom
that is different from the atoms of any other
element
An atom is the smallest unit of matter that still
retains the properties of an element
Atoms are composed of even smaller parts
called subatomic particles
Subatomic Particles
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Protons -positive charge
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-number of protons is the atomic number
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-part of the nucleus
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Neutrons- electrically neutral
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-part of the nucleus
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-protons + neutrons= mass number
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Electrons-negative charge
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-orbit the nucleus
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-number of electrons=number of protons
Isotopes
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All atoms of a given element have the same
number of protons.
Some atoms have more neutrons than other
atoms of the same element having a greater
mass these different atomic forms are called
Isotopes
Radioactive Isotopes- the nucleus decays
spontaneously
Radioactive Isotopes have many useful
applications in biology
The Energy Levels of Electrons
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An atom's electrons vary in the amount of
energy they possess
Energy is defined as the capacity to cause
change
Potential Energy is the energy that matter
possesses because of its location or structure
Electrons exist only at fixed levels of potential
energy called electron shells
Electron Distribution and Chemical Properties
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The chemical behavior of an atom is determined by the
distribution of electrons in an atom's electron shells
The periodic table of elements shows the electron distribution of
all the elements
The chemical behavior of an atom depends mostly on the
number of electrons in its outermost shell (valence shell)
-electrons in the valence shell are called valence electrons
-the number of valence electrons determines the chemical
properties of an atom
Electron Orbitals
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The three-dimensional space where an electron
is found 90% of the time is called an orbital
No more than two electrons can occupy a single
orbital
The 1st electron shell can hold up to two
electrons
The 2nd electron shell can hold up to eight
electrons
- Two in each orbital
Chemical Bonding between atoms
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Chemical bonds in atoms either gains, lose or
share electrons
There are two types of bonds formed between
atoms
-covalent bonds-sharing of a pair of valence
electrons by two atoms
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Polar- electrons shared unequally
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Nonpolar- electrons shared equally
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-ionic bonds-one atom gains electrons and
one loses an electron
Weak Chemical Bonds
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Many large biological molecules are held in
their functional form by weak bonds
-Hydrogen Bonding- noncovalent attraction
between a hydrogen and an electronegative
atom
van der Waals interactions- occur when
transiently positive and negative regions of
molecules attract each other
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-individually weak
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-occur only when atoms and molecules are very
Chemical Reactions
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Chemical reactions break or form chemical
bonds to change reactants into products
-Reactants-starting materials in a chemical
reaction
-Products- ending materials resulting from a
chemical reaction
All chemical reactions are theoretically
reversible
Chemical equilibrium is reached when the
forward and reverse reaction rates are equal