CHEMICAL BONDING

VOCABULARY:

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

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

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

Crystal Lattices: The three dimensional arrangement of atoms.

Octet Rule: Each atom would then have 8 ( an octet of ) electrons in its valence orbitals.

Electronegativity: The relative attraction of an atom for a shared pair of electrons.

Ionic Bond: The electrostatic attraction between ions of opposite charge.

Nonpolar Covalent Bond: A bond characterized by the equal sharing of a pair of electrons between atoms.

Polar Covalent Bond: A bond characterized by the sharing of one of more pairs of electrons between two different atoms.


In this chapter, you begin looking at the differences between compounds by studying the concept of chemical bonding.

BONDING CONCEPTS:

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

Factors that affect bonding is: 1) The __________________________ 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 __________________________ 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 __________________________. - To know the proportion of a compound can only be determined from the lab.

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


SHAPE OF A MOLECULE AND WHY IT IS IMPORTANT:

Shape is another aspect of structure that influences __________________________.

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 __________________________ 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 __________________________ 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 __________________________ 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 __________________________ 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.


POTENTIAL ENERGY CHANGES and COVALANET BONDS:

- The most stable arrangement of the atoms is when the potential energy is __________________________ .

- The total potential energy of the __________________________ is less than the total potential energy of the __________________________ that form the molecule.

- Therefore, a molecule of hydrogen is more stable than two individual atoms of hydrogen.


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 __________________________. 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 __________________________.

- 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 __________________________ . A dipole has two separated, equal but opposite charges.

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

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 __________________________ 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 __________________________ 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 __________________________ electronegative than hydrogen.

In general, the Bonding Electrons will be __________________________ 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 ];


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

HBr

NaF

N2

GO TO "PREDICTING BOND TYPES" WORKSHEET


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 __________________________ .

Example: Write the electron dot symbol for phosphorus.

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


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

1. Iodine

 

2. Krypton

 


HOMEWORK PROBLEMS:

1a. Arsenic

1b. Oxygen

1c. Neon

1d. Beryllium

1e. Francium

1f. Lead

1g. Calcium

1h. Potassium


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.


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

1. sodium oxide

 

2. 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 __________________________ 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 __________________________ 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

Practice Problems: Write the Lewis Dot Symbols for:

H2S

 

SiF4

 

CCl4

 


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 haas 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:

PRACTICE PROBLEMS: Draw the Lewis Dot Symbol for:

 

C2H4

 

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 __________________________ 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 __________________________ electrons.

PRACTICE PROBLEMS: Write the Lewis Dot symbol for:

ClO4-1

 

NH4+1

 

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.

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