Chemical Bonding

Chapter 8 Concepts of Chemical

Bonding

Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

John D. Bookstaver St. Charles Community College

St. Peters, MO © 2006, Prentice Hall, Inc.

Chemical Bonding

What is a Chemical Bond? •  a strong attraction between two atoms or

polyatomic ions that holds them near each other in space

•  Atoms and polyatomic ions form bonds by giving e- or taking valence e- from another atom or ion, or by sharing valence e- with another atom

•  Octet rule -- atoms usually lose, gain or share e- to have full s and p sublevels in their outermost energy level

•  3 types – ionic, metallic, covalent (label and annotate the drawings!)

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

Ø Electrostatic attraction between cations and anions forms an 3-D alternating array (a lattice) of many ions

Ø PROPERTIES OF IONIC COMPOUNDS:

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

Ø Electrostatic attraction between cations and anions forms an 3-D alternating array (a lattice) of many ions

Ø Produces solid compounds that generally are water-soluble

Ø Conduct electricity only when aqueous

Ø Contain a metal and a nonmetal

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

•  Electrons shared between nonmetal or metalloid atoms

•  Sharing usually occurs according to the octet rule

•  PROPERTIES OF COVALENT COMPOUNDS:

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

•  Electrons shared between nonmetal or metalloid atoms

•  Sharing usually occurs according to the octet rule

•  Forms distinct units (molecules) which are separate from each other

•  May or may not be soluble, do not conduct heat or electricity well

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

•  Metal atoms share e- with several other metal atoms

•  Shared e- move freely throughout the structure in an electron “sea”

•  PROPERTIES OF METALLIC SUBSTANCES:

Chemical Bonding

3 Types of Bonds Ionic, Covalent, Metallic

•  Metal atoms share e- with several other metal atoms

•  Occurs in pure metals and mixtures (alloys, layers)

•  Shared e- move freely throughout the structure in an electron “sea”

•  Produces insoluble solids which are malleable and lustrous; good conductors of heat and e-

Chemical Bonding

Lewis structures •  using dots to represent the valence e- is a Lewis

dot symbol. •  Each dot represents one valence e-. Start on

top of the symbol and place one dot. Then continue around placing one dot on the left, bottom, and right. Repeat until you run out of valence e-.

•  EXAMPLE Nitrogen, with 5 valence e- PRACTICE Draw the Lewis dot symbol for Oxygen

N N N N Nstart here

Chemical Bonding

Lewis structures

•  When nonmetals bond to each other they usually form covalent bonds, where the atoms share e-.

•  When two or more atoms join together with covalent bond(s) they form a molecule.

•  Each atom in a molecule will form enough bonds so that it has 8 valence e- around it (valence e- + bonds =8 ) except H, which wants 2 e- (always forms one bond). This is the octet rule or the Rule of Eight

EXAMPLE: Nitrogen is in Group 15 (5A) so it has 5 valence e-. It will form 3 bonds when it is in a molecule because 5e- + 3 bonds = 8.

Chemical Bonding

Lewis structures

•  Bonded pair refers to a pair of electrons that are involved in bonding between two different atoms (one valence from each atom).

•  Lone pair refers to a pair of electrons that are not involved in bonding but are paired up within an atom.

•  When we draw a molecule using dots to represent the valence e- it is called a Lewis dot structure.

•  PRACTICE – Draw a Lewis dot for OCl2

N H

H

H

lone pair of e-

bond pairs of e- NH3 +

Chemical Bonding

Lewis Structures

Lewis structures are drawings of molecules showing all electrons, bonding and nonbonding (“lone”). Bonding electrons are shown as………..

Chemical Bonding

Writing Lewis Structures 1.  Find the sum of valence

electrons of all atoms in the polyatomic ion or molecule. Ø  If it is an anion, add

one electron for each negative charge.

Ø  If it is a cation, subtract one electron for each positive charge.

PCl3

5 + 3(7) = 26

Chemical Bonding

1. Writing Lewis Structures •  Find the sum of …

•  If it is an anion… •  If it is a cation…

PCl3

5 + 3(7) = 26

Chemical Bonding

2. Writing Lewis Structures

The central atom is …. usually the atom the

needs the most bonds (fewest valence e-)

Connect the outer atoms to it by single bonds.

Keep track of the electrons: 26 - 6 = 20

Chemical Bonding

Writing Lewis Structures

3.  Fill the octets of the outer atoms.

Keep track of the electrons: 26 - 6 = 20 - 18 = 2

Chemical Bonding

Writing Lewis Structures

4.  Fill the octet of the central atom.

Keep track of the electrons: 26 - 6 = 20 - 18 = 2 - 2 = 0

Chemical Bonding

Writing Lewis Structures 5.  If you run out of electrons

before the central atom has an octet…atoms can share 2 pairs (a double bond) or 3 pairs (a triple bond) of e- in order to fill their valence

Consider a multiple bond only if no structure with only single bonds gives all atoms a full valence shell.

Chemical Bonding

HONC if you like molecules!

•  The four elements most commonly found in molecules are O, H, N and C

•  HONC 1234 is an easy to remember rule that tells how many bonds H, O, N, and C form. H=1 bond, O-2bonds, N = 3 bonds, C = 4 bonds

EXAMPLE: C3H9N PRACTICE: C2H6O •  Isomers are molecules that have the same

molecular formula but different structural formulas. EXAMPLE: C3H9N continued. PRACTICE: C2H6O continued

Chemical Bonding

Resonance This is the Lewis structure we would draw for ozone, O3.

What is resonance? What are the

resonance structures of ozone?

-

+

Chemical Bonding

Resonance

•  In the true, observed structure of ozone, in which… Ø …both O—O bonds

are the same length. Ø …both outer

oxygens have a charge of -1/2.

Chemical Bonding

Resonance •  One Lewis structure cannot

accurately represent a molecule like ozone.

•  We use multiple structures, resonance structures, to describe the molecule.

•  Just as green is a blend of blue and yellow… …ozone is a blend of these two resonance structures.

•  Draw the resonance structures for HCO2

-

Chemical Bonding

Exceptions to the Octet Rule •  There are 3 types of ions or molecules that

do not follow the octet rule: Ø Those with an odd number of electrons. NOTES: Ø Those with less than an octet. NOTES: Ø Those with more than 8 valence e- NOTES:

Chemical Bonding

Odd Number of Electrons

Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of e- (e.g. NO, NO2, O2

-). In this case the Lewis structure(s) used to

represent the molecule should obey the other principles discussed.

Draw a Lewis structure for NO2 (including resonance if needed)

Chemical Bonding

Fewer Than Eight Electrons

•  Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.

•  BUT Boron is much less electronegative than fluorine SO

•  This would not be an accurate picture of the distribution of electrons in BF3.

•  What would be better?

Chemical Bonding

Fewer Than Eight Electrons Leaving B with only 6 valence e- is actually better.

The lesson is: If filling the octet of the central atom results in a - charge on the central atom and a + charge on the more electronegative outer atom, don’t fill the octet of the central atom.

Most often this occurs in compounds of B and Be.

Chemical Bonding

More Than Eight Electrons

•  The only way PCl5 can exist is if phosphorus has 10 e- around it.

•  It is allowed to expand the octet of atoms on the 3rd row or below.

•  Presumably d orbitals in these atoms participate in bonding.

Chemical Bonding

More Than Eight Electrons •  the better structure puts a double bond between

the P and one of the Os. •  This eliminates the charge on the P and the

charge on one of the Os. •  The lesson is: When the central atom is on the

3rd row or below and expanding its octet eliminates some formal charges, do so.

Chemical Bonding

Do Now -- March 22 1.  Draw a Lewis structure for HOBr. Label the lone and

bond pairs. Using the text and table on p.305-6 to answer #2-5, 2.  explain what lattice energy is and why it is important. 3.  What does the magnitude of the lattice energy

depend on? 4.  How do the lattice energies of LiF and LiCl

compare? What is the explanation for this difference?

5.  How do the lattice energies of NaCl and MgCl2 compare? What is the explanation for this difference?

Chemical Bonding

Ionic Bonding

Chemical Bonding

Energetics of Ionic Bonding

As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium.

Chemical Bonding

Energetics of Ionic Bonding

We get 349 kJ/mol back by giving electrons to chlorine.

Chemical Bonding

Energetics of Ionic Bonding

•  But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!

Chemical Bonding

Energetics of Ionic Bonding

•  This is where the lattice energy comes in….

•  What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.

Chemical Bonding

Lattice Energy

The energy required to completely separate a mole of a solid ionic compound into its gaseous ions.

•  The energy associated with electrostatic interactions is governed by Coulomb’s law:

•  What do these variables symbolize?

Eel = κ Q1Q2 d

Chemical Bonding

Lattice Energy •  Lattice energy, increases with the charge on the

ions. WHY? •  It also increases with

decreasing size of ions. WHY?

•  EXPLAIN WHY LiF has a larger lattice energy than LiCl?

•  Why does MgO have a larger lattice energy than MgCl2?

Chemical Bonding

Energetics of Ionic Bonding Forming an ionic compound

from elements is exothermic OVERALL

It involves several steps, some endo some exo.

Label each step in the diagram of the reaction:

Na(s) + 1/2Cl2(g) àNaCl (s) If we add up the energy

change of each step we get the overall delta H of reaction.

Chemical Bonding

Energetics of Ionic Bonding

•  This is WHY the “octet rule.” is true

•  Metals tend to stop losing electrons once they attain a “noble gas” configuration because more energy is required to remove the electrons than the lattice energies.

Chemical Bonding

Covalent Bonding

•  In these bonds atoms share electrons.

•  There are 3 kinds of electric forces in these bonds: Ø Attractions between electrons

and nuclei Ø Repulsions between electrons Ø Repulsions between nuclei

Chemical Bonding

Polar Covalent Bonds

•  Although atoms often form compounds by sharing electrons, the electrons are not always shared equally.

•  Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does.

•  Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. SO WHAT?

Chemical Bonding

Electronegativity:

•  The ability of atoms in a molecule to attract electrons to itself.

•  On the periodic chart, electronegativity increases as you go… Ø …from left to right

across a row. Ø …from the bottom to

the top of a column.

•  WHY?

Chemical Bonding

Electronegativity:

•  The ability of atoms in a molecule to attract electrons to itself.

•  On the periodic chart, electronegativity increases as you go…

Chemical Bonding

Polar Covalent Bonds

The greater the difference in electronegativity…

Chemical Bonding

Polar Covalent Bonds

•  When two atoms share electrons unequally, a bond dipole results.

•  The dipole moment, µ, produced by two equal but opposite charges separated by a distance, r, is calculated:

µ = Qr •  It is measured in debyes (D).

Chemical Bonding

Writing Lewis Structures •  Then assign formal charges.

Ø For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms.

Ø Subtract that from the number of valence electrons for that atom: The difference is its formal charge.

Chemical Bonding

Writing Lewis Structures

•  The best Lewis structure… Ø …is the one with… Ø puts a negative charge….

Ø Which of these is best???

Chemical Bonding

Writing Lewis Structures

•  The best Lewis structure… Ø …is the one with… Ø puts a negative charge….

Chemical Bonding

Resonance

•  In truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.

•  They are not localized, but rather are delocalized (shared amongst the two O atoms and one C atom).

Chemical Bonding

Resonance

•  The organic compound benzene, C6H6, has two resonance structures.

•  It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.