Regents Chemistry

• Agenda

– Start Chapter 12 - Chemical Bonding

– Bonding worksheet

– HW: Finish worksheet

What is bonding?

• Bonding is the “glue” that hold two or more elements together

• This “glue” is most likely formed as a result of a chemical reaction

• Bonding and molecular structure play a central role in determining the course of chemical reactions

What is a bond?

• A bond can be thought of as a force that holds groups of two or more atoms together and makes them function as a unit

• Example : water

OH H

Bonds require energy to break and release energy when made

Bonds and Energy

• BREAKING BONDS requires energy – You Must Heat Wood to get it to react with

oxygen (ENDOTHERMIC)

• MAKING BONDS releases energy– Once the bonds are broken, they form new

products and this process releases heat (EXOTHERMIC)

Types of bonds

• Ionic bonds - typically formed between metals and nonmetals

• Covalent bonds - typically formed between nonmetals

• Metallic bonds - formed between metals

Ionic Bonds

• Ionic substances are formed when an atom that loses electrons relatively easily reacts with an atom that “wants” electrons

• We call these substances ionic compounds and result when a metal reacts with a nonmetal

• Ionic bonds are strong

Ionic Bonds

• Na and Cl– Na is a metal and likes to lose one electron– Cl is a nonmetal and likes to gain one electron– the final ionic compounds is NaCl

Na+ Cl-+ NaClThe electrostatic interaction keeps them together!

Ionic Bonds

• Na looses an electron and chlorine gains it!

• They do this to achieve an octet!

Na Cl

Covalent Bonds

• Covalent Bonds– exist between nonmetals bonded together– form when atoms of nonmetals share

electrons– electrons can be shared equally or unequally

Covalent Bonds Con’t

• Covalent bonds that have elements that share electrons equally are simply said to have a covalent type of bond

• Covalent bonds that share electrons unequally are said to have a polar covalent type of bond

• The unequal sharing results in a bond polarity

FH+ -

Metallic Bonds

• Metallic bonds exist between metals

• Occur when two metals, usually the same metal, are bonded together

Regents Chemistry

• Electronegativity

How can we tell really tell which type of bond we have?

• Electronegativity – is the relative ability of an atom in a molecule to attract shared electrons to itself

• This tells us what type of bond we have;– Covalent, polar covalent or ionic

• Electronegativity values are determined by measuring the polarities of bonds between various elements to determine a specific value for each element

Electronegativity

• Electronegativity values for each element are obtained by using the Periodic Table

• In fact, there is a general trend in electronegativity we observe in the Periodic Table

• Electronegativity values increase across and up the Periodic Table– See table on pg. 332

• We take the difference between the electronegativity values to determine exactly what type of bond exists, in essence the polarity of the bond

See table 12.1

Electronegativity

Determining Bond Polarity

• If the difference between the electronegativity values is:– 0.0 – 0.5: covalent bond (equal sharing)– 0.6 – 1.6: polar covalent bond (unequal

sharing)– 1.7 – up: ionic bond (transferring electrons)

Examples

• Use your Reference Tables to determine the difference in electronegativity values and the type of bond for each of the following:– H-H – H-Cl– H-O– H-S

• H-F• NaCl

• O2

• KBrWorksheet

Regents Chemistry

• Intro to valence electrons

Electrons in an atom

• Electrons surround the nucleus of an atom in specific energy levels or shells

• Each level can hold only a certain amount of electrons

• It is an atoms ability to the lose, gain or share electrons from its outer shell that determine its reactivity

The outer shell

• The outer shell in an atom contains the valence electrons

• Valence electrons can be lost, gained or shared to have eight electrons in the outer shell

• Each group on the table tells the number of valence electrons

Periodic Table

• Groups 1, 2, 13, 14, 15, 16, 17, 18 have 1,2,3,4,5,6,7,8 valence electrons, respectively

• We will not consider the transition metals

• See periodic table

Sharing to reach the Octet Rule

• The octet rule states that an atom cannot have more than 8 electrons in its outer shell

• Valence electrons are lost, gained or shared with other atoms to attain 8 electrons in the outer shell

• Eight valence electrons means a filled and happy shell - like the noble gases

Nonmetals share

• Nonmetals share electrons to reach eight valence electrons

• Single, double and triple bonds can be formed by sharing electrons

Metals + non-metals = lose/gain e-

• metals and nonmetals interact by losing and gaining electrons to reach 8 electrons (filled outer shell)

• The oxidation states on the periodic table represent this desire to move electrons

• ex: K+ want to lose 1 electron to reach noble gas configuration of eight electrons

Lewis structures: your assignment

• The reading and problems focus on drawing Lewis structures

• Lewis structures are a means to represent bond formation between atoms

• Covalent bonded compounds have different Lewis structures than Ionic bonded compounds

Example of a Lewis Structure

CH H

H

HC

H

CH4 Covalent bonds

Regents Chemistry

• Lewis Structures

Lewis StructuresLewis Structures The Lewis Structure is a The Lewis Structure is a

representation of a molecule that representation of a molecule that shows how the valence electrons shows how the valence electrons are arranged among the atoms in are arranged among the atoms in a moleculea molecule

We used dots around the We used dots around the elemental symbol to represent the elemental symbol to represent the valence electronsvalence electronsC

Single Lewis Structure - Single Lewis Structure - PracticePractice Draw four lone electrons first (if Draw four lone electrons first (if

necessary) them pair them upnecessary) them pair them up Draw Lewis Structures for the Draw Lewis Structures for the

following atomsfollowing atoms

Na Be Al Br

Lewis Structures for Ionic Lewis Structures for Ionic CompoundsCompounds

For Lewis Structures of ionic bonds For Lewis Structures of ionic bonds the atoms are not joined but draw the atoms are not joined but draw next to each othernext to each other

[ Br ]K+ -Bromine gains anelectron to achievethe noble gas configurationof Krypton

Potassium loses anelectron to achievethe noble gas configurationof Argon

example:KBr

Lewis Structures – Covalent Lewis Structures – Covalent BondsBonds

Hydrogen forms stable molecules when Hydrogen forms stable molecules when it shares two electronsit shares two electrons

Two electrons fill Hydrogen’s valence Two electrons fill Hydrogen’s valence shellshell

Helium does not form bonds because its Helium does not form bonds because its valence shell is already filled; it is a valence shell is already filled; it is a noble gasnoble gas

Second row non-metals Carbon through Second row non-metals Carbon through Fluorine from stable molecules when Fluorine from stable molecules when surrounded by eight electrons – surrounded by eight electrons – the the Octet RuleOctet Rule

Lewis Structures – Covalent Lewis Structures – Covalent BondsBonds

Valence electrons in covalent Valence electrons in covalent bonds can either be bonds can either be bonding pairsbonding pairs, , if involved directly in the bond or if involved directly in the bond or lone pairslone pairs if not involved in the if not involved in the bondbond

Writing Lewis Structures - Writing Lewis Structures - RulesRules Obtain the total sum of the valence Obtain the total sum of the valence

electrons from all of the atomselectrons from all of the atoms Use one pair of electrons to form a bond Use one pair of electrons to form a bond

between each pair of bound atoms. For between each pair of bound atoms. For convenience, a line (instead of a pair of convenience, a line (instead of a pair of dots) dots) cancan be used to indicate each pair be used to indicate each pair of bonding electronsof bonding electrons

Arrange the electrons to satisfy the duet Arrange the electrons to satisfy the duet rule for hydrogen and the octet rule for rule for hydrogen and the octet rule for second row second row

non metalsnon metals

Lewis Structures – Covalent Lewis Structures – Covalent BondsBonds

ExamplesExamples

PH3

P H

Step 1) 8 total valence e- total

Step 2) Draw one pair of electrons per bond 8-6 = 2 left

Step 3) Arrange the remaining electrons according to octet rule

H

H H lH– P –H ••

Lewis Structures – Covalent Lewis Structures – Covalent Bond Practice ExamplesBond Practice Examples

HBr ..

H:Br:

··

CF4

Worksheet

Regents Chemistry

– Ionic Lewis Structures– Multiple bonds in Lewis Structures– Polyatomic ion Lewis Structures and

Resonance

Lewis Structures for Ionic Lewis Structures for Ionic CompoundsCompounds

For Lewis Structures of ionic bonds For Lewis Structures of ionic bonds the atoms are not joined but draw the atoms are not joined but draw next to each othernext to each other

[ Br ]K+ -Bromine gains anelectron to achievethe noble gas configurationof Krypton

Potassium loses anelectron to achievethe noble gas configurationof Argon

example:KBr

Examples of Ionic Lewis Examples of Ionic Lewis StructuresStructures

Draw Lewis Structures for the Draw Lewis Structures for the following:following:

NaClNaCl

LiBrLiBr

KIKI

Multiple Bonds and Lewis Multiple Bonds and Lewis Structures…Structures…review firstreview first

We have seen how to draw Lewis We have seen how to draw Lewis Structures for molecules with Structures for molecules with single bondssingle bonds• For exampleFor example

NH3NH31. Sum the total valence e-

2. Subtract number of bonding e-3. Place remaining valence e-

8 total valence e-3 bonds x 2e- = 6 bonding2 e- left over

H

H HN

Multiple BondsMultiple Bonds

Between atoms of the same Between atoms of the same elementelement

ExampleExample• OxygenOxygen

O OO O

O = OAlso a Lewis Structure

Just O = O is called a structural model

Example of Multiple BondsExample of Multiple Bonds

NitrogenNitrogen

N N

N N

We now meet the octet rule!

Multiple BondsMultiple Bonds

Between atoms of different Between atoms of different elementselements

COCO22 C OO

O = C = O

We must use double bonds to meet the octet rule!

Lewis Structures for Lewis Structures for Polyatomic Ions and Polyatomic Ions and

Resonance StructuresResonance Structures Read pg. 344 (bottom) to 349 and Read pg. 344 (bottom) to 349 and

answer questions a-g in example answer questions a-g in example

12.4 (pg. 347) and a-i in the Self 12.4 (pg. 347) and a-i in the Self Check exercise 12.4 (pg. 348)Check exercise 12.4 (pg. 348)