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The Chemical Context
of Life
chapter 2
2
Energy & Matter
Universe is composed of 2 things ……
Energy

Ability to do work
o
Force on an object that causes it to move
Matter
Anything that has mass and occupies space
 Atoms/elements

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Elements and Compounds
Element -- cannot be broken down to other
substances by ordinary physical or chemical
means
Compound -- consists of two or more elements
in a fixed ratio
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Essential Elements of Life
Around 25 elements are essential to life
C, H, O, and N -- 96% of living matter
Ca, P, K, and S -- most of the rest
Trace elements -- required by an organism in
minute quantities
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An atom is the smallest unit of
matter that still retains the
properties of an element
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Subatomic Particles
Atoms -- subatomic particles
Neutrons (no electrical charge)
 Protons (positive charge)
 Electrons (negative charge)

 Nucleus -- Neutrons and protons
 Electrons -- cloud around nucleus
 Mostly empty space
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LE 2-4
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
Site: colonial.net
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Physicist Brian Greene puts it in these terms …..
http://io9.com/a-friendly-reminder-that-you-are-mostly-nothing-471778130
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LE 2-8
Hydrogen
1H
2
Atomic number
He
Atomic mass
First
shell
4.00
Helium
2He
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Sodium
11Na
Magnesium
12Mg
Aluminum
12Al
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Third
shell
Atomic Number & Atomic Mass
Atoms -- differ in number of subatomic particles
atomic number -- number of protons
 mass number -- protons + neutrons
 atomic mass -- approximated by the mass number
(mass number + electrons)

o
Mass electron … 0.000544
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Atoms and Isotopes
Atoms -- differ in number of subatomic particles

NOTE: atoms: protons == electrons
Isotopes -- two atoms of an element that differ in
number of neutrons

Stability/ radioactivity
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Electron Configuration and
Chemical Properties
 Distribution of electrons in electron shells determines
behavior
 The periodic table shows electron distributions
 Elements in common rows and/or columns similar
properties
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Electron Orbitals
three-dimensional space where electrons are
(90%)
each shell -- specific number and kind of
orbitals
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LE 2-9
Electron
orbitals
y
x
z
1s orbital
2s orbital
Three 2p orbitals
1s, 2s, and 2p orbitals
Electron-shell
diagrams
First shell
(maximum
2 electrons)
Second shell
(maximum
8 electrons)
Neon, with two
filled shells
(10 electrons)
Valence Electrons
Valence electrons – outermost level or “shell”
Chemical behavior -- valence electrons
Interactions – chemical bonds
Complete Valence Shell – 8 electrons
Atoms with incomplete valence shells –
sharing/transferring valence electrons
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Oxygen (O2)
Water (H2O)
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Molecule -- two or more atoms held
together by chemical bonds
Single bond = sharing of one pair of valence
electrons
Double bond = sharing of two pairs of valence
electrons
Triple bond =
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Characteristic Bonding Patterns
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Types of Bonds
Strong Chemical Bonds
Covalent Bonds
 Ionic Bonds

Weak Bonds
Hydrogen Bonds
 Van der Waals Forces

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Covalent Bonds
Hydrogen atoms (2 H)
Sharing valence electrons
Part of each atom’s valence
shell
Hydrogen
molecule (H2)
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Covalent bonds can form between
atoms of the same element or atoms
of different elements
compound vs molecule
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Covalent Bonds
Electronegativity -- measure an atom’s attraction
for electrons in a covalent bond

more electronegative -- stronger the pull
 Polar covalent bond – atoms share the electron
unequally -- one atom is more electronegative
 Nonpolar covalent bond -- atoms share the
electron equally
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LE 2-12
–
O
H
+
H
H2O
+
Ionic Bonds
Ion – unequal numbers of electrons and
protons
Anion -- negatively charged ion
 Cation -- positively charged ion

Ionic bond -- attraction between an anion
and a cation
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Ionic Bonds
Highly reactive atoms sometimes strip
electrons from their bonding partners
NaCl -- transfers electrons
Na
Cl
Na+
Cl–
Sodium atom
(an uncharged
atom)
Chlorine atom
(an uncharged
atom)
Sodium ion
(a cation)
Chlorine ion
(an anion)
Sodium chloride (NaCl)
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Compounds formed
by ionic bonds are
called ionic
compounds, or salts
Salts, such as sodium
chloride (table salt),
are often found in
nature as crystals
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Free Radicals
 Molecule whose atoms have one or more
unpaired electrons in their outer shells
Highly unstable and reactive
Steal electrons
 Cause cell death

Potentially involved in causing heart disease,
Alzheimer’s, cancer, and aging
Antioxidants (vitamins C and E) render free
radicals harmless
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LE 2-12
Na
Cl
Na+
Cl–
Sodium atom
(an uncharged
atom)
Chlorine atom
(an uncharged
atom)
Sodium ion
(a cation)
Chlorine ion
(an anion)
Sodium chloride (NaCl)
–
O
+
H
H
H2O
+
Weak Chemical Bonds
covalent bonds are the strongest
weak chemical bonds also important
reinforce shapes
 help molecules adhere to each other

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Hydrogen Bonds
Happens between molecules
a hydrogen atom covalently bonded to one
electronegative atom is also attracted to
another electronegative atom in a different
molecule
In living cells, the electronegative partners
are usually oxygen or nitrogen atoms
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LE 2-15
–
+
Water
(H2O)
+
Hydrogen bond
–
Ammonia
(NH3)
+
+
+
Van der Waals Interactions
Molecules or atoms that are very close
together can be attracted by fleeting charge
differences
These weak attractions are called van der
Waals interactions
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Van der Waals Interactions
Collectively,
such interactions
can be strong, as
between
molecules of a
gecko’s toe hairs
and a wall
surface
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Common Bonds
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Chemical reactions make and
break chemical bonds
Chemical reactions lead to new arrangements of
atoms
The starting molecules of a chemical reaction
are called reactants
The final molecules of a chemical reaction are
called products
Reactions must balance
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Photosynthesis is an important chemical
reaction
Sunlight powers the conversion of CO2 and
H2O to glucose (C6H12O6) and O2
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LE 2-UN44
2 H2
O2
Reactants
2 H2O
Reaction
Products
Reactions Differ
Some rxns “go to completion” -- All
reactants converted to products
Many rxns reversible: Products of the
forward reaction become reactants for the
reverse reaction

CaCO3 ↔ CaO + CO2
Chemical Equilibrium is reached when the
forward and reverse reaction rates are equal
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http://www.bbc.co.uk/news/uk-england-leeds11959381
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