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 ar

e responsible for the atom’s chemical properties.The electrons that participate in chemical bonding are ca

lled 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• •

• •

•C•

•

•Unpaired electrons

Unpaired electronPaired

electrons

Chlorine 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 protons11 electrons

Na+ 11 protons10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 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 F+

7e- 7e-

F F

8e- 8e-

More than one electron can be shared in an atom: double bond ( 双键 ) and triple bond ( 叁键 ).

O C O

double bonds

N N

triple bond

F F

single covalent 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 valence-shell 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 s

hared 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 electroneg

ativity ( 电负性 ).

Polar bond and nonpolar bond? When the two atoms have the same electronegativity, n

o 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 ).

H F

electron richregion

electron poorregion

C : C

6.7 Molecular polarity results from an

uneven distribution of electrons If all bonds in a molecule are nonpolar, the molec

ules 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 O H nonpolar

H

H

O....

O: sp3 hybrid orbitals, two paired electrons, two unpaired electrons to form covalent bonds with two H atoms

polar

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 a

toms (submicroscopic particles)

 

Attraction Relative strength

Ion-dipole Strongest

Dipole-dipole

Dipole-induced dipole ( 诱导偶极 )

Induced dipole-induced dipole weakestQuestion: 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 dipole-dip

ole 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

O OH

3-ethyl-3-oxetanemethanol (EOM)

o

阳离子开环聚合 (SCROP)

超支化聚醚 HBPO

Ill-defined 两亲性超支化多臂共聚物HBPO-star-PEO

颜德岳教授等人所合成的特殊大分子举例：

宏观多壁螺旋管

Yan, D., Zhou, Y. & Hou, J. Science 303, 65-67(2004)

宏观分子自组装

大分子 HBPO-star-PEO在丙酮中的自组装行为：

Dipole-induced dipole interaction

oxygen dissolved in water, carbon dioxide in water

Isolated oxygen molecule

Induced dipole-induced dipole interaction (di

spersion 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 fluorine-containing 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 gaseousSolid: 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 molecule-to-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 ℃

180g NaNO3 in 100ml water

87g NaNO3 in 100ml water

93g of NaNO3

precipitates out of

solution

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 + greaseGrime 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 NaO

H

Fat molecule Three fatty acid soap molecules Glycerol molecule

• Question: Apart from coupling, any other technique to stick two materials with different properties together?