UNIT 6: COVALENT BONDING - VSEPR


Video: H.W.: NAMING COMPOUNDS - PART I: (BINARY COMPOUNDS)


ELEMENTS THAT EXIST AS DIATOMIC MOLECULES

 NAME:

FORMULA:

NAME:

FORMULA:

 hydrogen

H2

chlorine

Cl2 

oxygen 

O2

bromine

Br2

 nitrogen

N2

iodine

I2

 fluorine

F2

The list of formulas (above) are examples of elements that exist in nature as ___________________ molecules.


RULES FOR WRITING FORMULAS:

The first two rules for writing formulas are:

Rule 1: Represent each kind of element in a compound with the correct symbol for that element.

Rule 2: Use subscripts to indicate the number of atoms of each element in the compound. If there is only one atom of a particular element, no subscript is used.

Applying these rules for a molecule of one atom of oxygen, O, and two atoms of hydrogen, H, the formula could be written OH2. To avoid confusion, the symbols are written in a particular order.

Rule 3: Write the symbol for the MORE metallic element first.

Neither hydrogen nor oxygen is a metal. However, the location of hydrogen with the metallic elements in the periodic table suggests that is should appear before oxygen in the formula.

Similarly, for a compound containing oxygen and sulfur, the location of sulfur is below oxygen in the periodic table suggests that it is more metallic than oxygen and should be written first in the formula.

For example, SO2 is a compound formed when sulfur burns.

 

 The formula represents one molecule of sulfur dioxide
 
Sulfur is the more metallic element, and is written first. No subscript is used, since there is only one atom in the molecule.

 SO2
Oxygen is less metallic than sulfur; its symbol is written last.
 

 The subscript indicates that there are two oxygen atoms in each molecule
 

Using these three rules, you can write the correct formula for almost any compound if you know the elements it contains and the number of atoms of each element in one molecule (or formula unit) of that compound. However, the only way chemists can get that information is by experimental analysis.

Formulas are a convenient way to represent compounds, but compounds have names as well. There are Millions of compounds, so it is IMPOSSIBLE to memorize all their names (or Function).

Learning the rules for naming compounds will help you figure out their names.

Unfortunately many common compounds were named before it became obvious that a systematic method would be needed. H2O is called water be everyone (including chemists), even though its systematic name is dihydrogen monoxide.


NAMING BINARY MOLECULAR COMPOUNDS: ( NON-METAL with NON-METAL )

The first system is used to name binary compounds that exist as distinct molecules rather than as ionic compounds.

Binary compounds are compounds made up of two elements.

Binary molecular compounds contain two nonmetals, whereas binary compounds containing a metal and a nonmetal are usually ionic.

If you are not sure whether a compound is ionic or molecular, look to see if it contains a metal. If it does not, use the following system to name it.

GREEK PREFIXES:

 Prefix

Number

Prefix

Number

 mono-

1

penta-

5

di-

2

hexa-

6

tri-

3

hepta-

7

tetra-

4

octa-

8

The name tells what elements are in the molecule and includes Greek prefixes to indicate the number of atoms of each element present. The system involves these steps:

1. Name the elements in the same order that they appear in the formula.

2. Drop the last syllable (two syllables in some cases) in the name of the final element and add -ide.

3. Add ___________________ to the name of each element to indicate the number of atoms of that element in the molecule.

* * * In practice, the mono- prefix is omitted for the first element in the name.


Below describes the steps that are used to name CO2. Notice how each step in the naming process reduces ambiguity about what compound is being named.

The first step clarifies what elements are involved, but the resulting name does not make clear that the elements are in a compound.

Changing the ending of the second element, in step two, signals that you are talking about a compound rather than isolated elements.

Step three adds prefixes to indicate the number of atoms of each element present.

 CO2

Step 1:

Names of elements are written in the order they appear in the formula.

 Carbon Oxygen

 Carbon oxide

Step 2:

Drop the last syllable (or last two) in the second element and add -ide.

the "-ygen" in oxygen is dropped and "-ide" is substituted.

Step 3:

Add the correct prefix to each element.

Mono- is added to carbon and di- is added to oxide to indicate the number of carbon and oxygen atoms in the molecule

Mono-carbon Di-oxide

 Carbon Dioxide

Step 4:

Mono- is dropped from carbon since we DO NOT place the prefix mono- on the first name. All other prefixes WILL BE placed on the first element in the compound name.


Example: What is the name of the compound N2O4?

 

 

1) Practice Problems: Name the following compounds.

a) SiC

b) CS2

c) SF6

d) SiO2

e) B2O3

f) N2O5

g) S4O6

h) P5Cl9

2) Write the formula of the following compounds.

a) chlorine di-oxide

b) di-chlorine mono-oxide

c) tri-phosphorus tetra-oxide

d) tetra-sulfur di-nitride

e) iodine hepta-fluoride

f) tetra-arsenic octa-oxide

g) penta-silicon hepta-oxide

h) tri-carbon hexa-chloride

i) phosphorous mono-chloride

HOMEWORK - NAMING COMPOUNDS - PART I: (BINARY COMPOUNDS)

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Video: H.W.: WORKSHEET: PREDICTING BOND TYPES

BONDING CONCEPTS:

Chemical Bond: is a strong attractive force between atoms or ions in a compound.

Factors that affect bonding is: 1) The electrical nature of bonding. and 2) The __________________________ to other atoms.

There are millions of stable compounds formed from fewer than 100 elements.

In compounds formed from representative elements, the atoms have acquired an electron configuration that is isoelectronic with that of a noble gas element.

For Representative Elements: the electrons in the "s" and "p" orbitals.

- Full outer shell = 8 electrons

- Empty outer shell = 0 electrons


The Law of Definite Composition: That the proportion of elements in a given compound is fixed. - To know the proportion of a compound can only be determined from the lab.

Molecular arrangement: The structure shows how elements are arranged - Also, the structure helps predict prooperties.


SHAPE OF A MOLECULE AND WHY IT IS IMPORTANT:

Shape is another aspect of structure that influences chemical properties.

Shape is crucial in determining whether a reaction will (or will not) occur - Example: Biological Proteins or Enzymes.

Strength of a bond is due to its BOND ENERGY. Bond energy is the energy involved in the process of a bond forming and breaking.


IONIC BONDING:

CATION: Atom loses one (or more) electrons. Cations are positive in charge. (Metals = Cations)

ANION: Atom gains one (or more) electrons. Anions are __________________________ in charge. (Nonmetals = Anions)

The electrostatic attraction is the mechanism for ionic bonds. Electrostatic attraction is due to opposite charges which are attracted to each other.

Ionic Bond: A chemical bond by the electrostatic attraction between a cation and anion.


PROPERTIES OF IONIC COMPOUNDS:

- Liquid state of an ion __________________________ electricity.

- Mobile charged particles are necessary in order for a substance to conduct an electrical current.

- At room temperature, crystals of ionic compounds exist as regular, three-dimensional arrangements of cations and anions.

CRYSTAL LATTICES: The three dimensional arrangement of atoms.

Forming Ionic Compounds: 1) React a metal with a non-metal. 2) The metal must transfer one (or more) electrons to the non-metal.

Na: [Ne] 3s1 + Cl: [Ne] 3s23p5 ® Na: [Ne] + Cl: [Ne] 3s23p6


COVALENT BONDING:

Covalent bonding occurs when two or more nonmetals share electrons - attempting to obtain a stable octet of electrons at least part of the time.

H2, Cl2, O2, N2, F2 ... etc, Exist as a diatomic molecule, but when each of these gases change their physical state to the liquid state - each DOES NOT conduct electricity. Therefore, each DOES NOT contain ions.

Even though the Hydrogen molecule (for example), H2, does not contain ions, each atom of hydrogen is still made up of charged particles (protons and electrons).

When two Hydrogen atoms get close , the attraction between electrons and protons occur.

The electrons are then shared.


BOND ENERGY:

The energy needed to break a bond is the Bond Energy.

The distance between the __________________________ is referred to as the bond length.

COVALENT BONDS and LEWIS DOT SYMBOLS:

Electron dot symbols are also used for representing covalent bonds. The formation of a molecule of hydrogen can be illustrated as:

Just as in the case of hydrogen, when two chlorine atoms approach, the unpaired electrons are shared and a covalent bond is formed. The formation of molecular chlorine can be illustrated as:


UNEQUAL SHARING OF ELECTRONS:

Could a covalent bond form between a hydrogen atom and a chlorine atom? If you draw the Lewis Dot Symbol for each atom, you will find that they both have an unpaired electron. IF THE UNPAIRED ELECTRONS are shared by both the hydrogen and chlorine nucleus, a Covalent Bond IS FORMED.

- If both nuclei are identical ( same number of protons ), the electrons will be shared EQUALLY.

- If one nucleus has a stronger attraction for the electrons than the other nucleus, the likelihood is that the shared electrons will be __________________________ to the stronger nucleus.


POLAR COVALENT BONDS:

Even though electrons are shared, the fact that they are more strongly attracted to the chlorine atom results in a partial negative charge at the chlorine atom and a partial positive charge at the hydrogen atom.

Such a bond is called a DIPOLE. A dipole has two separated, equal but opposite charges.

A covalent bond that has a dipole is called a __________________________ Covalent Bond.

When two different elements (therefore, an unequal sharing of electrons) form a covalent bond, the bond is usually a Polar Covalent Bond, as in HCl.

- A partial negative charge is represented with the lower case Greek letter delta with a negative sign

- A partial positive charge is represented with the lower case Greek letter delta with a positive sign

Covalent bonds in which electrons are Equally Shared by two nuclei, as in H2 or Cl2, are called Nonpolar Covalent Bonds.


SUMMARY OF THE BOND TYPES:

NONPOLAR COVALENT BOND: Equal sharing of electrons, because the nuclear attraction for the electron pair is EQUAL.

POLAR COVALENT BOND: Unequal sharing of electrons, because one atom has a greater nuclear attraction for the electron pair than the other atom.

IONIC BOND: The metal donates its electrons to obtain a positive charge and the nonmetal accepts the electrons from the metal to obtain a negative charge.


ELECTRONEGATIVITY:

ELECTRONEGATIVITY is the measure of the attraction an atom has for a shared pair of electrons in a bond.

The above example shows Hydrogen and Chlorine bonding to form a polar covalent bond.

Chlorine has a greater attraction for the electron pair than hydrogen. Therefore, chlorine is said to be more electronegative than hydrogen.

In general, the Bonding Electrons will be more strongly attracted to the atom that has a higher electronegativity.

Francium has the LOWEST electronegativity.

Fluorine has the HIGHEST electronegativity.


PREDICTING THE "TYPES" OF BONDS:

__________________________ in Electronegativities is used as a guide to determine the degree of electron sharing in a bond.

- As the electronegativity difference between the atoms increases, the degree of sharing decreases.

- If the difference in electronegativities is 1.7 or more, the bond is GENERALLY considered more IONIC than COVALENT.

- If the electronegativity difference is between 0.1 and 1.7, the bond is a POLAR COVALENT bond that is GENERALLY considered more covalent than ionic.

- If the electronegativity difference is ZERO, the bond is considered to be a NONPOLAR COVALENT bond.


EXAMPLE PROBLEM:

Classify the bond in each of the following as ionic, polar covalent, or nonpolar covalent for: KF, O2, and ICl. (show the partial charge for any polar covalent bonds.)

KF . . . . . . . Electronegativity for K = 0.8, and F = 4.0; ( 4.0 - 0.8 = 3.2 ) - [ ionic bond ]

 

O2 . . . . . . . Electronegativity for O = 3.5; ( 3.5 - 3.5 = 0 ) - [ nonpolar covalent bond ]

ICl . . . . . . Electronegativity for I = 2.5, and Cl = 3.0; ( 3.0 - 2.5 = 0.5 ) - [ polar covalent bond ];


3) PRACTICE PROBLEMS: Identify the type of bond in the following substances: ( show the partial charge for any polar covalent bonds. )

a) HBr

 

b) NaF

 

c) N2

 

GO TO "PREDICTING BOND TYPES" WORKSHEET


Video: H.W.: WORKSHEET: BONDING TYPES & LEWIS DOT SYMBOLS

ELECTRON DOT CONFIGURATION (LEWIS DOT SYMBOLS):

1916, Lewis: Developed a system of arranging dots (representing valence electrons) around the symbols of the elements.

- The Symbol represents the Nucleus and Core Electrons for that Element.

- Dots represent the Valence Electrons.

Example: Write the electron dot symbol for phosphorus.

Answer: P = Phosphorus, Group 5A, therefore, there are 5 valence electrons.


4) PRACTICE PROBLEMS: Draw the Lewis Dot Symbol for the Given.

a. Iodine

 

b. Krypton

 

c. Arsenic

 

d. Oxygen

 


Example Problem: Use Electron Dot Symbols to represent the formation of Magnesium Fluoride from atoms of Mg and F.

Mg is in Group 2A, ( Forms a Cation, Mg+2 )

F is in Group 7A, ( Forms an Anion, F-1 )

Mg + F2 ® MgF2

or

Hopefully, you noticed the brackets in the products. The brackets are used for IONS ONLY - representing the "new" charge on the atom because of the gain or loss of electrons.


5) PRACTICE PROBLEMS: Draw the Lewis Dot Symbol for the Given.

a. sodium oxide

 

 

b. magnesium chloride

 

 


THE OCTET RULE:

Predicting the bonding arrangement that occurs between atoms in a molecule is based on two important observations.

- The first fact, is that noble gases are unreactive and form very few compounds.

The reason noble gases do not generally react is because the outermost "s" and "p" orbitals are __________________________ , making them particularly stable.

- The second fact, is that ionic compounds of the representative elements are generally made up of anions and cations that have noble gas configurations.

From observations like these, chemists have formulated the OCTET RULE. The octet rule is based on the assumption that atoms form bonds to achieve a noble gas configuration. Each atom would then have 8 ( an octet of ) electrons in its valence orbitals.


LEWIS DOT SYMBOLS FOR MOLECULES:

You can use the octet rule to write the Lewis Dot symbols for molecules. To do this, you need to know the following information:

1) How many of each kind of atom are in the molecule? Determine this from the chemical formula.

2) How many valence electrons are available? Determine this from looking at which "Group" each element is within - example: sulfur is in group 6A, therefore there are 6 valence electrons.

3) What is the skeleton structure? The skeleton structure shows which atoms are bonded to each other. The skeleton structure can be proven experimentally. GENERALLY, the MORE electronegative element is located as the central atom - so another GENERAL rule is that if you have ONE atom type make it your central atom.

4) Where do the "dots" go in the structure? Place the dots around the EXTERNAL atoms first so that each atom has 8 electrons - an octet structure. THEN place the remaining dots around (on) the central atom.


EXAMPLE: Write the Lewis Dot Symbol for H2O, NH3, and CH4.

Answer:

Molecule

H2O

 NH3

CH4 
Number of each kind of atom in molecule

O = 1

H = 2

N = 1

H = 3

C = 1

H = 4
Valence electrons for each atom

O = 6

H = 1

N = 5

H = 1

C = 4

H = 1
Total number of valence electrons

O = 1 x 6 = 6

H = 2 x 1 = 2

6 + 2 = 8

 N = 1 x 5 = 5

H = 3 x 1 = 3

5 + 3 = 8

 C = 1 x 4 = 4

H = 4 x 1 = 4

4 + 4 = 8
Skeleton Structure

Arrangement of dots

6) Practice Problems: Draw the Lewis Dot Symbols for:

a) H2S

 

 

b) CF4

 

 

c) PBr3

 

 


DOUBLE AND TRIPLE BONDS:

Try to draw the Lewis Dot symbol for carbon dioxide, CO2. When you use the method described above - you will encounter a problem. Carbon hhas four valence electrons, and the two oxygen together have 12 ( 2 x 6 ), for a total of 16 electrons. The two possible structures that can be drawn for carbon dioxide can be these:

In either case, carbon or oxygen will not have eight electrons represented. However, if each oxygen atoms shares two pairs of electrons with the carbon atom, double bonds would be formed, and octets around both carbon and oxygen can be achieved.

The circles are drawn to represent the octet for each atom.

A Double Bond is a covalent bond in which four electrons (two pairs) are shared by the bonding atoms.

A Triple Bond is a covalent bond in which two atoms share three pairs of electrons. Nitrogen gas is an example of a triple bond.

EXAMPLE PROBLEM:

Draw the Lewis Dot Symbol for HCN.

Answer:

7) PRACTICE PROBLEMS: Draw the Lewis Dot Symbol for:

a) C2H4

 

 

b) CO

 

 


LEWIS DOT SYMBOLS FOR POLYATOMIC IONS:

There are a large number of ionic compounds made of more than two elements. In these compounds, at least one of the ions consists of two or more atoms which are polar covalently bonded. However, the particle as a whole possesses an overall charge.

For example, sulfate, SO4-2,

Each oxygen has a polar covalent bond to the sulfur - because the oxygen and sulfur atoms only have 6 electrons in their outer orbitals, all the atoms CANNOT have 8 electrons in their respective outer orbital at any one point in time ( where the "X" is located on the two oxygen representing the missing electron for the octet. )

Since there are two ( total ) missing electrons for this polyatomic ion - when the sulfate ion reacts with a metal(s), the electrons will occupy the missing spots giving the polyatomic ion its charge. Theoretically, the above polyatomic ion does not have a charge - yet (until the reaction occurs!)

Also, for a positive polyatomic ion, the positive charge is designated because the ion will be donating electrons.

8) PRACTICE PROBLEMS: Write the Lewis Dot symbol for:

a) ClO4-1

 

 

b) NH4+1

 

 

c) CO3-2

 

 


EQUIVALENT LEWIS DOT SYMBOLS (RESONANCE STRUCTURES):

Some molecules and polyatomic ions have properties that cannot be adequately explained by a single Lewis Dot symbol. An example is the carbonate ion, CO3-2. One Lewis Dot Symbol that fulfills the octet rule is:

However, the double bond could be on any oxygen atom (not just the oxygen atom that is pictured). Therefore, there are three possible Lewis dot symbols possible for carbonate.

However, experimental studies show that all three of the carbon-oxygen bonds are identical; there is no evidence of both single and double bonds. In fact, the bonds are stronger than a carbon-oxygen single bond and weaker than a carbon-oxygen double bond. This phenomenon is called RESONANCE.

In cases where resonance occurs, more than one acceptable Lewis dot symbol can be written without changing the arrangement of atoms.

Resonance is often represented by writing each of the different Lewis Dot Symbols and including double-headed arrows between the possible symbols.

GO TO LEWIS DOT SYMBOL WORKSHEET:


VALENCE-SHELL ELECTRON-PAIR REPULSION THEORY


Video: H.W.: WORKSHEET: VSEPR WORKSHEET

LIMITATIONS OF THE OCTET RULE:

While the octet rule is a useful model that allows you to picture the structure of molecules, it is important to realize that all MOLECULES do not obey the octet rule. The concept serves only as a rule of thumb.

MOLECULES WITH MORE THAN AN OCTET:

Compounds also exist in which the central atom has more than an octet of electrons. All of the compounds formed from the noble gas elements (Argon on down) are examples.

A very important concept to remember: ONLY Carbon, Nitrogen, Oxygen, and Fluorine, MUST have an octet !

The existence of compounds of noble gas elements was thought to be impossible - because the noble gas atoms already have complete octets. One of the first noble gas compounds to be synthesized was xenon tetrafluoride, XeF4. The electron dot structure for XeF4 has twelve electrons in the valence orbitals of xenon.


- PREDICTING THE SHAPES OF MOLECULES:

There is no direct relationship between the formula of a compound and the shape of its molecules. The shapes of these molecules can be predicted from their Lewis structures, however, with a model developed about 30 years ago, known as the valence-shell electron-pair repulsion (VSEPR) theory.

The VSEPR theory assumes that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of that atom. The five compounds shown below can be used to demonstrate how the VSEPR theory can be applied to simple molecules.

LEWIS STRUCTURE: 

VSEPR STRUCTURE:

GEOMETRY: 

 

 LINEAR

 

 

TRIGONAL PLANAR 

 

TETRAHEDRAL 

 

 

TRIGONAL BIPYRMIDAL 

 

 

 OCTAHEDRAL

- LINEAR MOLECULES:
There are only two places in the valence shell of the central atom in CO2 where electrons can be found. Repulsion between these pairs of electrons can be minimized by arranging them so that they point in opposite directions. Thus, the VSEPR theory predicts that CO2 should be a linear molecule, with a 1800 angle between the two C - O double bonds.

- TRIGONAL PLANAR MOLECULES:
There are three places on the central atom in boron trifluoride (BF3) where valence electrons can be found. Repulsion between these electrons can be minimized by arranging them toward the corners of the equilateral triangle. The VSEPR theory, therefore, predicts a trigonal planar geometry for the BF3 molecule, with a F - B - F bond angle of 1200. Also, it is important to note that boron is VERY HAPPY with only six electrons and not eight. This is a "tricky" element that is overlooked easily.

- TETRAHEDRAL MOLECULES:
CO2 and BeF3 are both two-dimensional molecules, in which the atoms lie in the same plane. If we place the same restriction on methane (CH4), we should get a square-planer geometry in which the H - C - H bond angle is 900. If we let this system expand into three dimensions - we end up with a tetrahedral molecule in which the H - C - H bond is approximately 1090.

- TRIGONAL BIPYRAMID MOLECULES:
Repulsion between the five pairs of valence electrons on the phosphorus atom PF5 can be minimized by distributing these electrons toward the corners of a trigonal bipyramid. Three of the positions in a trigonal bipyramid are labeled equatorial because they lie along the equator of the molecule. The other two are axial because the they lie along an axis perpendicular to the equatorial plane. The angle between the three equatorial positions is 1200, while the angle between an axial and an equatorial position is 900.

- OCTAHEDRON MOLECULES:
There are six places on the central atom in SF6 where valence electrons can be found. The repulsion between these electrons can be minimized by distributing them toward the corners of an octahedron. The term octahedron literally means "eight sides," but it is the six corners, or vertices, that interest us. To imagine the geometry of an SF6 molecule, locate fluorine atoms on opposite sides of the sulfur atom along the X, Y, and Z axes of an XYZ coordinate system.


THE ROLE OF NONBONDING ELECTRONS IN VSEPR (BENT STRUCTURES)


The valence electrons on the central atom in both NH3 and H2O should be distributed toward the corners of a tetrahedron, as shown in the figure below. Our goal, however, is not predicting the distribution of valence electrons. It is to use this distribution of electrons to predict the shape of the molecule. Until now, the two have been the same. Once we include nonbonding electrons, that is no longer true.

The VSEPR theory predicts that the valence electrons on the central atoms in ammonia and water will point toward the corners of a tetrahedron. Because we cannot locate the nonbonding electrons with any precision, this prediction cannot be tested directly. But the results of the VSEPR theory can be used to predict the positions of the nuclei in these molecules, which can be tested experimentally. If we focus on the positions of the nuclei in ammonia, we predict that the NH3 molecule should have a shape best described as trigonal pyramidal, with the nitrogen at the top of the pyramid. Water, on the other hand, should have a shape that can be best descried as bent, or angular. Both of these predictions have been shown to be correct, which reinforces our faith in the VSEPR theory.


EXAMPLE:
Predict the shape of the following molecules.


H2S

 

 

HCN

 

 

CO3-2

 

 


HOMEWORK PROBLEMS:
Draw the Lewis Dot Structure and then name the shape of the following molecules, :

2a. HOCl

 

 

2b. CCl4

 

 

2c. H2CO

 

 

2d. SeO2

 

 

2e. SF6

 

 

2f. PO4-3

 

 


Video:

- INCORPORATING DOUBLE AND TRIPLE BONDS INTO VSEPR:
Compounds that contain double and triple bonds raise an important point: The geometry around an atom is determined by the number of places in the valence shell of an atom where electrons can be found, not the number of pairs of valence electrons. Consider the Lewis structures of carbon dioxide (CO2) and carbonate (CO32-) ion, for example.

and

There are four pairs of bonding electrons on the carbon atom in CO2, but only two places where these electrons can be found. (There are electrons in the C=O double bond on the left and electrons in the double bond on the right.) The force of repulsion between these electrons is minimized when the two C=O double bonds are placed on opposite sides of the carbon atom . The VSEPR theory, therefore, predicts that CO2 will be a linear molecule, just like BeF2, with a bond angle of 1800.

The Lewis structure of the carbonate ion also suggests a total of four pairs of valence electrons on the central atom. But these electrons are concentrated in three places: The two C - O single bonds and the C=O double bond. Repulsion between these electrons are minimized when the three oxygen atoms are arranged toward the corners of an equilateral triangle. The CO3-2 ion should therefore have a trigonal-planer geometry, just like BF3, with a 1200 bond angle.


MOLECULAR POLARITY:

Bond polarities (Polar Bonds) arise from bonds between atoms of different electronegativity. When more complex molecules are considered we must consider the possiblility of molecular polarities that arise from the sums of all of the individual bond polarities.

To do full justice of molecular polarity - one must consider the concept of vectors (mathematical quantities that have both direction and magnitude).

Let's begin by thinking of a polar bond as a VECTOR pointed from the positively charged atom to the negatively charged atom. The size of the vector is proportional to the difference in electronegativity of the two atoms.

If the two atoms are identical, the magnitude ofthe vector is ZERO, and the molecule has a nonpolar bond.

CO2

Let's consider molecules with three atoms. We can establish from the Lewis dot symbols and VSEPR that CO2 is a linear molecule. Each of the C - O bonds will have a vector arrow pointing from the carbon to the oxygen. The two vectors should be identical and pointed in exactly opposite directions.

The sum of these two vectors must be zero because the vectors must cancel one another out. Even though the C - O bonds must be polar, the CO2 MOLECULE is NONPOLAR.

- - - -

HCN

HCN, hydrogen cyanide, is linear. Since carbon is more electronegative than hydrogen one would expect a vector pointing from H to C. In addition, nitrogen is more electronegative than C so one should expect a bond vector pointing from C to N. The H-C and C-N vectors add to give a total vector pointing from the H to the N.

HCN is a POLAR MOLECULE with the vector moving from the hydrogen to the nitrogen - making the hydrogen end somewhat positive and the nitrogen end is somewhat negative.

- - - -

SO2

In contrast, let's examine the case of SO2. We know from the Lewis dot symbol and from VSEPR that this molecule is "bent." Its overall geometry would be considered to be trigonal planar if we considered the lone pair electrons on the Sulfur.

Lone pair electrons are NOT considered when we examine polarity since they have already been taken into account in the electronegativity.

We would predict that there should be polarity vectors pointing from the sulfur to the two oxygens. Since the molecule is bent the vectors will NOT cancel out. Instead they should be added together to give a combined vector that bisects the O-S-O angle and points from the S to a point in-between the two oxygens.

- - - - - - - -