Transcript unpaired electrons?

Chapter 6 Chemical bonding
and molecular shapes
How atoms connect to one another?
6.1 An atomic model is needed to
understand how atoms bond
Electrons in the outermost occupied shell of any atom
are responsible for the atom’s chemical properties.
The electrons that participate in chemical bonding are
called valence electrons ( 价 电 子 ),the shell they
occupy is called the valence shell (价层) of an atom.
Valence electrons can be conveniently represented as a
series of dots surrounding an atomic symbol.
(Electron-dot structure or Lewis dot symbol).
Valance electrons can be either paired or unpaired.
Paired electrons usually do not form chemical bonds
with other elements.
•
Cl
••
Chlorine
Unpaired
electrons
•
C
•
••
•
••
Paired
electrons
Unpaired
electron
•
Carbon
Question: Why Carbon atom has four (not two)
unpaired electrons?
6.2 Atoms can lose or gain electrons to
become ions (离子)
When the number of protons in the
nucleus of an atom equals to the
number of electrons in the atom,
the charges balance and the atom
is electrically neutral. If one or
more electrons are lost or gained,
the atom takes on a net electric
charge. Any atom having a net
electric charge is called an ion. If
the electrons are lost, the ion’s net
charge is positive. If the electrons
are gained, the ion’s net charge is
negative.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
Shell model can be used to deduce the type of
ion an atom tends to form: Atoms tend to
lose or gain electrons so that they end up
with an outermost occupied shell that is
filled to capacity.
1+
2+
3+
4-
3-
2-
1-
0
6.3 Ionic bonds result from a transfer of
electrons
When an atom that tends to lose electrons is
placed in contact with an atom that tends to
gain them, the result is that an electron
transfer and the formation of two oppositely
charged ions. The two oppositely charged
ions are thus attracted to each other by the
electric force, which holds them together.
This electric force of attraction between two
oppositely charged ions is called an ionic
bonds (离子键). All compounds containing
ions are referred to as ionic compounds (离
子化合物).
How to judge the composition of an
ionic compound? Balance of electric
charge
Ruby
Sapphire
6.4 Covalent bonds result from a
sharing of electrons
The electrical attraction in
which atoms are hold
together by their mutual
attraction
for
shared
electrons is called an
covalent bond (共价键). A
compound composed of
atoms held together by
covalent bonds is a
covalent compound (共价
化合物).
F
7e-
+
F
F F
7e-
8e- 8e-
More than one electron can be shared in an atom: double
bond (双键) and triple bond (叁键).
single covalent bond
F
F
O
C
O
double bonds
N
N
triple bond
Question: What’s in common for ionic
bonds and covalent bonds and what are
the differences?
6.5 Valence electrons determine
molecular shape
How to determine the three
dimensional structure of
a covalent compound?
Valence shell electron-pair
repulsion (VESPR): any
given pair of valenceshell electrons strives to
get as far away as
possible from all other
electron pairs in the
shell.
6.6 Polar covalent bonds result from an
uneven sharing of electrons
What is the distribution of a
shared pair of electrons in
a covalent bond?
In HF the shared electrons
are drawn more close to F
atom, so the fluorine side
of bond is electrically
negative while the
hydrogen side of bond is
electrically positive. This
kind of charge separation
is called a dipole (偶极).
In H2, evenly shared
in HF, unevenly shared
The ability to draw bonding
electrons is decided by the atom’s
electronegativity (电负性).
Polar bond and nonpolar bond?
When the two atoms have the same electronegativity, no
dipole is formed and the bond is called a nonpolar
bond (非极性键).
When the two atoms have the different electronegativity,
a dipole is formed and the bond is called a polar
bond (极性键).
electron poor
region
C:C
electron rich
region
H
F
d+
d-
6.7 Molecular polarity results from an
uneven distribution of electrons
If all bonds in a molecule are nonpolar, the
molecules as a whole is also nonpolar. (H2, O2)
If a molecule consists of polar bonds, the polarity
of the molecule is decided by the polarity of the
bonds and also the shape of the molecule.
Molecular shape of H2O and NH3
• H2O
H
H
O
..
O ..
H
nonpolar
polar
H
O: sp3 hybrid orbitals, two paired
electrons, two unpaired electrons to
form covalent bonds with two H
atoms
The molecular polarity decides a lot of
the physical properties of substances
Substance
Boiling Point(℃)
Polar
Water,H2O
100
Ammonia,NH3
-33
Nonpolar
Oxygen,O2
-183
Hydrogen,H2
-253
Nitrogen,N2
-196
Carbon dioxide,CO2
-79
Boiling Points of some polar and nonpolar substance
Chapter 7 Molecular Mixing
Water that has been fully aerated at room
temperature, contains only 1 oxygen molecule
for every 200,000 water molecules. Why? The
attractive forces between water molecules and
oxygen molecules are very weak.
7.1 Submicroscopic particles electrically
attract one another
Four types of interaction between ions, molecules and
atoms (submicroscopic particles)
Attraction
Ion-dipole
Dipole-dipole
Dipole-induced dipole (诱导偶极)
Induced dipole-induced dipole
Relative strength
Strongest
weakest
Question: What’s in common for these four interactions?
The molecular-molecular interaction
determines many of properties of substance
Ion- dipole interaction: table salt dissolved in water
Dipole-dipole interaction
An unusually strong dipoledipole attraction: hydrogen
bond (氢键) (occurs between
hydrogen atom covalently
bonded
to
a
highly
electronegative atom, usually
nitrogen,
oxygen
and
fluorine)
The hydrogen bond is
responsible for many of the
unusual properties of water
and many of biomolecules,
such as DNA.
Hydrogen Bond
Question: The role of hydrogen
bonds in DNA
Hydrogen bonding play important
role in self-assembly
• Self-assembly is the only scientific issues
in the 25 most important issues in 21st
century claimed by Science
颜德岳教授等人所合成的特殊大分子举例：
O
OH
阳离子开环聚合(SCROP)
3-ethyl-3oxetanemethanol
(EOM)
超支化聚醚HBPO
o
Ill-defined 两亲性
超支化多臂共聚物
HBPO-star-PEO
大分子HBPO-star-PEO在丙酮中的自组装行为：
宏
观
分
子
自
组
装
宏
观
多
壁
螺
旋
管
Yan, D., Zhou, Y. & Hou, J. Science 303, 65-67(2004)
Dipole-induced dipole interaction
oxygen dissolved in water, carbon dioxide in water
Isolated oxygen molecule
Induced dipole-induced dipole interaction
(dispersion force: 色散力)
Iodine,I2,a solid at
room temperature
Fluorine,F2,a gas at
room temperature
Fluorine-containing molecules
usually have very small induced
dipole, and thus have very weak
interaction with other molecules
Question: Why fluorinecontaining molecules usually
have very small induced dipole?
• Question: Other applications for
fluorine-containing molecules?
7.2 A solution is a single-phase
homogeneous mixture
Solution can be solid, liquid and gaseous
Solid: ruby (red chromium compounds in aluminum oxide)
Blue sapphire (green iron compounds and blue titanium compounds in aluminum oxide)
Metal alloys
Gaseous: air (breath in: 78% nitrogen, 21% oxygen, 1% others)
(exhale: 75% nitrogen, 15% oxygen, 5% carbon dioxide, 6% water vapor)
Solvent (溶剂), Solute (溶质), Dissolving (溶解)
How much a given solute can dissolve in a given solvent?
Unsaturated solution (不饱和溶液), Saturated solution (饱和溶液)
Concentration (浓度) = amount of solute/amount of solvent
Concentration can be weight and molar
7.3 Solubility (溶解性) is a measure of
how well a solute dissolves
Solubility depends on attractions between
solute/solvent particles.
When the molecule-to-molecule attractions among
solute molecules are comparable to the moleculeto-molecule attractions among solvent molecules,
the result can be no practical point of saturation, as
in the case of ethanol/water. Ethanol and water can
mix homogeneously in any proportion. (infinitely
soluble)
Solubility changes with temperature
100℃
20℃
93g of
NaNO3
precipitates
out of
solution
180g NaNO3 in
100ml water
87g NaNO3 in
100ml water
Gases are more soluble
at low temperature and
high pressure
Nonpolar gases readily
dissolve
in
perfluorocarbons:
Oxygen has much
higher solubility in
perfluorocarbons than
in air. This has many
applications.
• 问题：既然含氟材料与其它材料的相互
作用很小(不能粘结)，TEFLON是如何粘
到锅底上的？
7.4 Soap works by being both polar and
nonpolar
grime = dirt + grease
Grime is difficult to remove from
hands or clothing using just water,
because grime is nonpolar and
water is polar. Grime can be
dissolved and washed away by
nonpolar substances such as
trichloroethane.
Grime can also be washed away
by soap and water, such soap is a
substance having both polar and
nonpolar properties.
Soup molecules play as a
coupling agent.
Detergents
Calcium and magnesium ions in hard water can replace the sodium ions in
soap to form insoluble substances. Sodium carbonate is usually added
to detergent as a water-softening agent.
Treat with
NaOH
Fat molecule
Three fatty acid soap molecules
Glycerol molecule
• Question: Apart from coupling, any other
technique to stick two materials with
different properties together?