chapter2_chemical_context_of_life.ppt

Atom- Molecule-Element- Compound Relationship

Chemical Building Blocks Of Life
• A few other elements
– Make up the remaining 4% of living matter
Table 2.1

Dissecting An Atom

Energy Levels Of Electrons
• Energy levels
– Are represented by electron shells
Third energy level (shell)
Second energy level (shell)
First energy level (shell)
Energy
absorbed
Energy
lost
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.
(b)
Atomic
nucleus
Figure 2.7B

The Periodic Table
• The periodic table of the elements
– Shows the electron distribution for all the
elements
Second
shell
Helium
2He
First
shell
Third
shell
Hydrogen
1H
2
He
4.00
Atomic mass
Atomic number
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
3B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Sodium
11Na
Magnesium
12Mg
Aluminum
13Al
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Figure 2.8

Electron Shells and Orbital
• Each electron shell
– Consists of a specific number of orbitals
Electron orbitals.
Each orbital holds
up to two electrons.
1s orbital 2s orbital Three 2p orbitals 1s, 2s, and 2p orbitals
(a) First shell
(maximum
2 electrons)
(b) Second shell
(maximum
8 electrons)
(c) Neon, with two filled shells
(10 electrons)
Electron-shell diagrams.
Each shell is shown with
its maximum number of
electrons, grouped in pairs.
x
Z
Y
Figure 2.9

Covalent Bond
Figure 2.10
• Formation of a covalent bond
Hydrogen atoms (2 H)
Hydrogen
molecule (H2)
+ +
+ +
+ +
In each hydrogen
atom, the single electron
is held in its orbital by
its attraction to the
proton in the nucleus.
1
When two hydrogen
atoms approach each
other, the electron of
each atom is also
attracted to the proton
in the other nucleus.
2
The two electrons
become shared in a
covalent bond,
forming an H2
molecule.
3

(a)
(b)
Name
(molecular
formula)
Electron-
shell
diagram
Structural
formula
Space-
filling
model
Hydrogen (H2).
Two hydrogen
atoms can form a
single bond.
Oxygen (O2).
Two oxygen atoms
share two pairs of
electrons to form
a double bond.
H H
O O
Figure 2.11 A, B
• Single and double covalent bonds

Name
(molecular
formula)
Electron-
shell
diagram
Structural
formula
Space-
filling
model
(c)
Methane (CH4).
Four hydrogen
atoms can satisfy
the valence of
one carbon
atom, forming
methane.
Water (H2O).
Two hydrogen
atoms and one
oxygen atom are
joined by covalent
bonds to produce a
molecule of water.
(d)
H
O
H
H H
H
H
C
Figure 2.11 C, D
• Covalent bonding in compounds

Polarity & Electro negativity
Figure 2.12
This results in a
partial negative
charge on the
oxygen and a
partial positive
charge on
the hydrogens.
H2O
d–
O
H H
d+ d+
Because oxygen (O) is more electronegative than hydrogen (H),
shared electrons are pulled more toward oxygen.
• In a polar covalent bond
– The atoms have differing electronegativities
– Share the electrons unequally

Ionic Bond
Cl–
Chloride ion
(an anion)
–
The lone valence electron of a sodium
atom is transferred to join the 7 valence
electrons of a chlorine atom.
1 Each resulting ion has a completed
valence shell. An ionic bond can form
between the oppositely charged ions.
2
Na Na
Cl Cl
+
Na
Sodium atom
(an uncharged
atom)
Cl
Chlorine atom
(an uncharged
atom)
Na+
Sodium on
(a cation)
Sodium chloride (NaCl)
Figure 2.13
• An ionic bond
– Is an attraction between anions and cations

Hydrogen Bonds
d – d +
d +
Water
(H2O)
Ammonia
(NH3)
O
H
H
d +
d –
N
H
H H
A hydrogen
bond results
from the
attraction
between the
partial positive
charge on the
hydrogen atom
of water and
the partial
negative charge
on the nitrogen
atom of
ammonia.
d+ d+
Figure 2.15
• A hydrogen bond
– Forms when a hydrogen atom covalently
bonded to one electronegative atom is also
attracted to another electronegative atom

Van der Waals Interactions
• Van der Waals interactions
– Occur when transiently positive and negative
regions of molecules attract each other

Structure Of A Molecule
s orbital
Z
Three p orbitals
X
Y
Four hybrid orbitals
(a) Hybridization of orbitals. The single s and three p orbitals
of a valence shell involved in covalent bonding combine to
form four teardrop-shaped hybrid orbitals. These orbitals
extend to the four corners of an imaginary tetrahedron
(outlined in pink).
Tetrahedron
Figure 2.16 (a)
• In a covalent bond
– The s and p orbitals may hybridize, creating
specific molecular shapes

Space-filling
model
Hybrid-orbital model
(with ball-and-stick
model superimposed)
Unbonded
Electron pair
104.5°
O
H
Water (H2O)
Methane (CH4)
H
H H
H
C
O
H
H
H
C
Ball-and-stick
model
H H
H
H
(b) Molecular shape models. Three models representing molecular shape are shown for
two examples; water and methane. The positions of the hybrid orbital determine the
shapes of the molecules
Figure 2.16 (b)

Structural Similarities
Morphine
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Natural
endorphin
(a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds to
receptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match.
(b) Binding to endorphin receptors. Endorphin receptors on the surface of a brain cell
recognize and can bind to both endorphin and morphine.
Natural
endorphin
Endorphin
receptors
Morphine
Brain cell
Figure 2.17

Chemical Reactions
Reactants Reaction Product
2 H2 O2 2 H2O
+
+
• Chemical reactions
– Convert reactants to products

chapter2_chemical_context_of_life.ppt

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