Transcript Ch. 2 Chemistry

• Phones must be put away now, no
earbuds, no texting
• Do you have anything to turn in to
green basket?
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Chapter 2:
Chemical Context of Life
Atoms and Molecules
Elements and Compounds
Organisms are composed of matter, which
is anything that takes up space and has
mass
Matter is made up of elements, substances
that cannot be broken down to other
substances by chemical reactions
92 unique elements exist in nature
• More have been created in the lab
Compounds
•
A compound
•
Is a substance consisting of two or more
elements combined in a fixed ratio
•
Has characteristics different from those of its
elements
+
Sodium
Chloride
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Sodium Chloride
Essential Elements of Life
Essential elements
• Include carbon, hydrogen, oxygen,
nitrogen, phosphorus and sulfur.
• Make up 96% of living matter
Other Elements
A few other elements make up the
remaining 4% of living matter
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Elements
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
Subatomic Particles
• Atoms of each element are
composed of even smaller parts
called subatomic particles
• Relevant subatomic particles
include
• Neutrons, which have no electrical
charge
• Protons, which are positively charged
• Electrons, which are negatively
charged
Subatomic Particles
Protons and neutrons
Are found in the atomic nucleus
Electrons
Surround the nucleus in a “cloud”
Model of the Atom
•
Simplified models of an atom
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
(a)
This model represents the
electrons as a cloud of
negative charge, as if we had
taken many snapshots of the 2
electrons over time, with each
dot representing an electron‘s
position at one point in time.
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(b) In this even more simplified
model, the electrons are
shown as two small blue
spheres on a circle around the
nucleus.
Atomic Number and Atomic Mass
Atoms of the various elements
• Differ in their number of subatomic
particles
The atomic number of an element
• Is the number of protons
• Is unique to each element
The mass number of an element
• Is the sum of protons plus neutrons in
the nucleus of an atom
• Is an approximation of the atomic
mass of an atom
Isotopes
• Atoms of a given element may
occur in different forms called
Isotopes
• 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
Radioactive Isotopes
Can be used in biology and medicine
Cancerous
throat
tissue
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Energy Levels of Electrons
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
Energy Levels
The electrons of an atom
•
Differ in the amounts of potential
energy they possess
(a)
A ball bouncing down a flight
of stairs provides an analogy
for energy levels of electrons,
because the ball can only rest
on each step, not between
steps.
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Electron Shells
Energy levels
•
Are represented by electron shells
Third energy level (shell)
Second energy level (shell)
Energy
absorbed
First energy level (shell)
Energy
lost
Atomic
nucleus
(b) An electron can move from one level to another only if the energy
it gains or loses is exactly equal to the difference in energy between
the two levels. Arrows indicate some of the step-wise changes in
potential energy that are possible.
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Electron Configuration and
Chemical Properties
Valence electrons
• Are those in the outermost, or valence
shell
• Determines the chemical behavior of an
atom
The valence number of an atom is the
number of unpaired electrons in its
valence shell
This valence number determines the number of
bonds the atom can form.
Periodic Table
(Campbell & Reece, p.32)
The periodic table of the elements
•
Shows the electron distribution for all the
elements
Helium
Hydrogen
1H
Atomic mass
First
shell
2
He
4.00
Atomic number
2He
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
3B
Carbon
6C
Nitrogen
7N
Silicon
14Si
Phosphorus
15P
Oxygen
8O
Fluorine
9F
Neon
10Ne
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Sodium Magnesium Aluminum
13Al
11Na
12Mg
Third
shell
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Which drawing depicts an atom with a valence of 2?
a. A
b. B
c. C
d. D
e. E
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Orbitals
An orbital
• Is the three-dimensional space where
an electron is found 90% of the time
Orbitals
Each electron shell
•
Consists of a specific number of orbitals
Electron orbitals.
Each orbital holds
up to two electrons.
x
Y
Z
1s orbital
2s orbital
Three 2p orbitals
1s, 2s, and 2p orbitals
Electron-shell diagrams.
Each shell is shown with
its maximum number of
electrons, grouped in pairs.
(a) First shell
(maximum
2 electrons)
(b) Second shell
(maximum
8 electrons)
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(c) Neon, with two filled shells
(10 electrons)
Orbitals
Filling pattern of electron orbitals:
Electrons fill lower shells first before outer
shells
• First: two electrons in 1s orbital
• Then: two electrons 2s orbital
• Then: one electron in each of 2p orbitals
before adding more to any 2p if necessary
Carbon breaks these rules: sp3 hybridization
• Allows 4 unpaired electrons = 4 bonds
Structure of an Atom
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CH904
Bonds
• The formation and function
of molecules depend on
chemical bonding between
atoms.
Bonds
• The formation and function of
molecules depend on chemical
bonding between atoms.
• A covalent bond
• Is the sharing of a pair of valence
electrons
Covalent Bonds
Formation of a covalent bond
Hydrogen atoms (2 H)
1
2
3
In each hydrogen
atom, the single electron
is held in its orbital by
its attraction to the
proton in the nucleus.
When two hydrogen
atoms approach each
other, the electron of
each atom is also
attracted to the proton
in the other nucleus.
The two electrons
become shared in a
covalent bond,
forming an H2
molecule.
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
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Hydrogen
molecule (H2)
Covalent Bonds
A molecule
• Consists of two or more atoms held
together by covalent bonds
A single bond
• Is the sharing of one pair of valence
electrons
A double bond
• Is the sharing of two pairs of valence
electrons
Covalent Bonds
•
Single and double covalent bonds
Name
(molecular
formula)
Electronshell
diagram
(a) Hydrogen (H2).
Two hydrogen
atoms can form a
single bond.
(b) Oxygen (O2).
Two oxygen atoms
share two pairs of
electrons to form
a double bond.
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Structural
formula
H
H
O
O
Spacefilling
model
Covalent Bonds
•
Covalent bonding in molecules
Name
(molecular
formula)
Electronshell
diagram
(c) Water (H2O).
Two hydrogen
atoms and one
oxygen atom are
joined by covalent
bonds to produce a
molecule of water.
(d) Methane (CH4).
Four hydrogen
atoms can satisfy
the valence of
one carbon
atom, forming
methane.
Structural
formula
O
H
H
H
H
C
H
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H
Spacefilling
model
Electronegativity
Electronegativity
• Is the attraction of a particular kind of
atom for the electrons in a covalent
bond
The more electronegative an atom
• The more strongly it pulls shared
electrons toward itself
In a nonpolar covalent bond
• The atoms have similar
electronegativities
• Share the electron equally
Thinking Question
Electronegativity values increase
as the atom becomes smaller and
as you go from left to right on the
Periodic Table. Why do you think
this is so?
Polar Covalent Bonds
In a polar covalent bond
•
The atoms have differing
electronegativities
•
Share the electrons unequally
Because oxygen (O) is more electronegative than hydrogen (H),
shared electrons are pulled more toward oxygen.
d–
This results in a
partial negative
charge on the
oxygen and a
partial positive
charge on
the hydrogens.
O
d+
H
H
H2O
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d+
Water Models
•Make 10 or more paper cut-outs of water atoms
•Arrange them to demonstrate water crystal lattice
Because oxygen (O) is more electronegative than hydrogen (H),
shared electrons are pulled more toward oxygen.
d–
This results in a
partial negative
charge on the
oxygen and a
partial positive
charge on
the hydrogens.
O
d+
H
H
H2O
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d+
Covalent Bonds Reviewed
Ionic Bonds
In some cases, atoms strip electrons
away from their bonding partners
Electron transfer between two atoms
creates ions
Ions
• Are atoms with more or fewer
electrons than usual (charged atoms)
• Such as Na+, Cl-, K+, PO43-
Ionic Bonds
An anion
• Atom that gains electrons
• Is negatively charged
• Cl-, PO43-
A cation
• Atom that lost electrons
• Is positively charged
• Na+, K+
Ionic Bonds
An ionic bond
•
Is an attraction between anions and
cations
1 The lone valence electron of a sodium
2 Each resulting ion has a completed
atom is transferred to join the 7 valence
electrons of a chlorine atom.
valence shell. An ionic bond can form
between the oppositely charged ions.
–
+
Na
Na
Sodium atom
(an uncharged
atom)
Cl
Cl
Chlorine atom
(an uncharged
atom)
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Cl
Na
Na+
Sodium on
(a cation)
Cl–
Chloride ion
(an anion)
Sodium chloride (NaCl)
Ionic Bonds
Ionic compounds
•
Are often called salts, which may form
crystals
Na+
Cl–
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Hydrogen Bonds
A hydrogen bond
•
Forms when a hydrogen atom covalently bonded
to one electronegative atom is also attracted to
another electronegative atom
d–
Water
(H2O)
d+
O
H
d+
d–
Ammonia
(NH3)
N
H
d+
H
d+
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A hydrogen
H
bond results
from the
attraction
between the
partial positive
charge on the
hydrogen atom
of water and
the partial
negative charge
H
d+ on the nitrogen
atom of
ammonia.
Molecular Shape and
Function
The precise shape of a molecule
• Is usually very important to its
function in the living cell
• Is determined by the positions of
its atoms’ valence orbitals
Molecular Shape
Space-filling
model
Ball-and-stick
model
Hybrid-orbital model
(with ball-and-stick
model superimposed)
Unbonded
Electron pair
O
O
H
Water (H2O)
104.5°
H
H
H
H
C
C
H
H
Methane (CH4)
H
H
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H
H
H
Chemical Reactions
Chemical reactions
•
•
Are the making and breaking of
chemical bonds
Lead to changes in the
composition of matter
Chemical Reactions
•
Chemical reactions
•
Convert reactants to products
+
2 H2
+
O2
Reactants
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2H2O
Reaction
Product
Equilibrium
Chemical equilibrium
• Is reached when the forward and
reverse reaction rates are equal
Key Points of Chapter 2
• Matter consists of chemical
elements in pure form and in
combinations called compounds
• An element’s properties depend on
the structure of its atoms
• The formation and function of
molecules depends on chemical
bonding between atoms
• Chemical reactions make and break
chemical bonds