Bonding theories

Types of chemical bonds Bond: Force that holds groups of two or more atoms together and makes the atoms function as a unit. Example: H-O-H Bond Energy: Energy required to break a bond. Ionic Bond: Attractions between oppositely charged ions. Example: Na+ Cl-

Types of chemical bonds Ionic Compound: A compound resulting from a positive ion (usually a metal) combining with a negative ion (usually a non-metal). Example: M+ + X-  MX

Types of chemical bonds Covalent Bond: Electrons are shared by nuclei. Example: H-H

Types of chemical bonds Polar Covalent Bond: Unequal sharing of electrons by nuclei. Example: H-F Hydrogen fluoride is an example of a molecule that has bond polarity.

Lewis structures Lewis Structure: Representation of a molecule that shows how the valence electrons are arranged among the atoms in the molecule. Bonding involves the valence electrons of atoms. Example: water:

Lewis structures of elements Dots around elemental symbol Symbolize valence electrons Thus, one must know valence electron configuration

Valence electrons In the table in your notes, write the number of valence electrons in each group.

Lewis dot structure The electron dot notation that we use in chemistry is called the Lewis dot structure (after American chemist G. N. Lewis). When you draw the Lewis dot structure for an atom, start by placing a single dot at the top of the element symbol. Then proceed clockwise around the element until you have a single dot on all four sides (top, right, bottom, left). If you have more valence electrons to write, you should go clockwise around the atom again, placing a second dot right next to the first dot.

Lewis dot Structure Draw the Lewis dot structures for the elements listed in your notes, following the rules on the previous slide.

Lithium and beryllium are metals. Do they form covalent bonds? NO!

Valence Electrons and Bond Number For each element, you can find out how many bonds they form by subtracting the number of VALENCE ELECTRONS from the number 8. Fill out the table in your notes using this information.

OCTET RULE We define the OCTET RULE as follows: Atoms tend to gain, lose, or share electrons in order to achieve a full NOBLE GAS configuration containing 8 valence electrons

Lewis Structures of molecules Single Bond: Two atoms sharing one electron pair. Example: H2

Lewis Structures of molecules Double Bond: Two atoms sharing two pairs of electrons. Example: O2

Lewis Structures of molecules Triple Bond: Two atoms sharing three pairs of electrons. Example: N2

Lewis Structures of molecules Resonance Structures: More than one Lewis Structure can be drawn for a molecule. Example: O3

DRAWING LEWIS DOT STRUCTURES (covalent bonding)

Steps for Drawing the Lewis dot structure for a molecule: Calculate the total number of valence electrons in the molecule.

Steps for Drawing the Lewis dot structure for a molecule: Draw a skeleton structure for the molecule. Put the atom that normally forms the most bonds in the center.

Steps for Drawing the Lewis dot structure for a molecule: Draw a line to connect each atom with a single bond. Each single bond represents 2 electrons.

Steps for Drawing the Lewis dot structure for a molecule: Subtract the number of electrons you have used so far. If you have any electrons left over, place them in PAIRS around any atoms that need them.

Molecules consisting of only single bonds

H2O 8 e- total Calculate the total number of valence electrons for all the atoms. H = 1 x 2 = 2 O = 6 x 1 = 6 8

H2O 8 e- total O H H Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. O H H

H2O 8 e- total Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. O H H

H2O 8 e- total - 4 e- 4 e- O H H Subtract the number of electrons that you have used so far. O H H

H2O 8 e- total - 4 e- 4 e- If you have any electrons left over, place them IN PAIRS around any atoms that need them. O H H

NH3 8 e- total Calculate the total number of valence electrons for all the atoms. N = 5 x 1 = 5 H = 1 x 3 = 3 8

NH3 8 e- total H N H H Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. H N H H

NH3 8 e- total H N H H Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. H N H H

NH3 8 e- total - 6 e- 2 e- H N H H Subtract the number of electrons that you have used so far. H N H H

NH3 8 e- total - 6 e- 2 e- If you have any electrons left over, place them IN PAIRS around any atoms that need them. H N H H

CH4O 14 e- total Calculate the total number of valence electrons for all the atoms. C = 4 x 1 = 4 H = 1 x 4 = 4 O = 6 x 1 = 6

CH4O 14 e- total Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. H H C O H H

CH4O 14 e- total Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. H H C O H H

CH4O 14 e- total - 10 e- 4 e- H H C O H H Subtract the number of electrons that you have used so far. H H C O H H

CH4O 14 e- total - 10 e- 4 e- H H C O H H If you have any electrons left over, place them IN PAIRS around any atoms that need them. H H C O H H

We will do the rest in class

Molecules that contain a double or a triple bond

O2 12 e- total O = 6 x 2 = 12 Calculate the total number of valence electrons for all the atoms. O = 6 x 2 = 12

O2 12 e- total O O Draw a line to connect the atoms with a single bond. Each single bond represents 2 electrons. O O

O2 12 e- total - 2 e- 10 e- Subtract the number of electrons that you have used so far. O O

O2 12 e- total - 2 e- 10 e- O O If you have any electrons left over, place them IN PAIRS around any atoms that need them. O O

O2 12 e- total - 2 e- 10 e- If there are still atoms that do not have an octet, you can move electron pairs to form double bonds or triple bonds to satisfy the octet rule. MOVE THIS PAIR OF ELECTRONS INTO THE CENTER AREA BETWEEN THE ATOMS TO FORM A DOUBLE BOND. O O

O2 12 e- total - 2 e- 10 e- Now each oxygen atom has access to an octet. O O

O2 12 e- total - 2 e- 10 e- Note that it doesn’t really matter which side you decide to place the lone pairs. O O O O O O

N2 10 e- total N = 5 x 2 = 10 Calculate the total number of valence electrons for all the atoms. N = 5 x 2 = 10

N2 10 e- total N N Draw a line to connect the atoms with a single bond. Each single bond represents 2 electrons. N N

N2 10 e- total - 2 e- 8 e- N N Subtract the number of electrons that you have used so far. N N

N2 10 e- total - 2 e- 8 e- N N If you have any electrons left over, place them IN PAIRS around any atoms that need them. N N

N2 10 e- total - 2 e- 8 e- If there are still atoms that do not have an octet, you can move electron pairs to form double bonds or triple bonds to satisfy the octet rule. MOVE THIS PAIR OF ELECTRONS INTO THE CENTER AREA BETWEEN THE ATOMS TO FORM A DOUBLE BOND. N N

N2 10 e- total - 2 e- 8 e- If there are still atoms that do not have an octet, you can move electron pairs to form double bonds or triple bonds to satisfy the octet rule. MOVE THIS PAIR OF ELECTRONS INTO THE CENTER AREA BETWEEN THE ATOMS TO FORM A TRIPLE BOND. N N

N2 10 e- total - 2 e- 8 e- Now each nitrogen atom has access to an octet. N N

N2 10 e- total - 2 e- 8 e- Again, it doesn’t really matter which side you decide to place the lone pairs. N N N N N N

We will do the rest of these practice problems in class

exceptions to the octet rule:

BCl3 24 e- total Calculate the total number of valence electrons for all the atoms. B = 3 x 1 = 3 Cl = 7 x 3 = 21

BCl3 24 e- total Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. Cl B Cl Cl

BCl3 24 e- total Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. Cl B Cl Cl

BCl3 24 e- total - 6 e- 18 e- Cl B Cl Cl Subtract the number of electrons that you have used so far. Cl B Cl Cl

BCl3 24 e- total - 6 e- 18 e- Cl B Cl Cl If you have any electrons left over, place them IN PAIRS around any atoms that need them. You DON’T need to do anything to this structure. Boron will be an exception to the octet rule, with only six electrons around it. Cl B Cl Cl

We will complete the Lewis structures for the two further exceptions to the octet rule in class

Polyatomic Ions

Polyatomic ions When you are calculating the total number of valence electrons in a polyatomic ion, you should: SUBTRACT electrons for cations ADD electrons for anions

NH4+ 8 e- total Calculate the total number of valence electrons for all the atoms. N = 5 x 1 = 5 H = 1 x 4 = 4 The positive 1 charge means that you need to SUBTRACT one electron!

NH4+ 8 e- total H H N H H Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. H H N H H

NH4+ 8 e- total Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. + H A polyatomic ion will normally have brackets around it and the charge is indicated in the top right corner. H N H H

OH– 8 e- total O = 6 H = 1 Calculate the total number of valence electrons for all the atoms. O = 6 H = 1 The negative 1 charge means that you need to ADD one electron!

OH– 8 e- total – O H Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. If you have any electrons left over, place them IN PAIRS around any atoms that need them. – O H

NO3– 24 e- total Calculate the total number of valence electrons for all the atoms. N = 5 x 1 = 5 O = 6 x 3 = 18 The negative 1 charge means that you need to ADD one electron!

NO3– 24 e- total O N O O Draw a skeleton structure for your molecule. Put the atom that normally forms the most bonds in the center. O N O O

NO3– 24 e- total Draw a line to connect each atom to the central atom with a single bond. Each single bond represents 2 electrons. O N O O

NO3– 24 e- total - 6 e- 18 e- O N O O Subtract the number of electrons that you have used so far. O N O O

NO3– 24 e- total - 6 e- 18 e- If you have any electrons left over, place them IN PAIRS around any atoms that need them. MOVE THIS PAIR OF ELECTRONS INTO THE CENTER AREA BETWEEN THE ATOMS TO FORM A DOUBLE BOND. O N O O

NO3– 24 e- total - 6 e- 18 e- O N O O – If you have any electrons left over, place them IN PAIRS around any atoms that need them. – O N O O

O N O O N O O O O N O O – – – These three Lewis structures represent RESONANCE STRUCTURES of each other. – – O N O O N O O O – O N O O

We will finish the practice in class