Section 6.2 Covalent Bonding and Molecular Compounds Sharing is caring! 1

Chapter 6 Vocabulary Reading Guide molecule (molecular compound) molecular formula bond energy electron-dot notation Lewis structure structural formula single bond resonance Laughing Gas Reading Guide Section 2 write answers on same paper as Section 1 due at end of hour 2

Section 6.2 Covalent Bonding Main Ideas: Covalent bonds form from shared electrons Bond lengths and energy vary from molecule to molecule Atoms tend to form bonds to follow the octet rule

Section 6.2 Covalent Bonding Main Ideas: Dots placed around an element’s symbol can represent valence electrons Electron-dot notations can represent compounds. Some atoms can share multiple pairs of electrons.

Section 6.2 Covalent Bonding Main Ideas: Resonance structures show hybrid bonds Some compounds are networks of bonded atoms

Covalent Bonds valere - “to be strong” The word covalent is a combination of : co- (latin) “together” valere - “to be strong” Two electrons shared together have the strength to hold two atoms together in a bond.

Covalent bonds Nonmetals hold on to their valence electrons. They can’t give away electrons to bond. But still want noble gas configuration. Get it by sharing valence electrons with each other = covalent bonding By sharing, both atoms get to count the electrons toward a noble gas configuration.

Molecules Many elements found in nature are in the form of molecules: a neutral group of atoms joined together by covalent bonds. Example: air contains oxygen molecules, consisting of two oxygen atoms joined covalently Called a “diatomic molecule” (O2) 7 diatomics: H2, N2, O2, F2, Cl2, Br2, I2

Molecule

Molecular Compounds Compounds that are bonded covalently (like in water, or carbon dioxide) are called molecular compounds Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds

Molecular Compounds Thus, molecular compounds tend to be gases or liquids at room temperature Ionic compounds are solids (crystals) A molecular compound has a molecular formula: Shows how many atoms of each element a molecule contains

Molecular Formulas The formula for water is written as H2O The subscript “2” behind hydrogen means there are 2 atoms of hydrogen; if there is only one atom, the subscript 1 is omitted Molecular formulas do not tell any information about the structure (the arrangement of the various atoms).

These are some of the different ways to represent ammonia: 3. The ball and stick model is the BEST, because it shows a 3-dimensional arrangement. 1. The molecular formula shows how many atoms of each element are present 2. The structural formula ALSO shows the arrangement of these atoms!

(diatomic hydrogen molecule) The nuclei repel each other, since they both have a positive charge (like charges repel). How does H2 form? (diatomic hydrogen molecule) + +

How does H2 form? But, the nuclei are attracted to the electrons They share the electrons, and this is called a “covalent bond”, and involves only NONMETALS! + +

Bond Dissociation Energies... The total energy required to break the bond between 2 covalently bonded atoms High bond energy = strong bond High bond energy usually means the chemical is relatively unreactive, because it takes a lot of energy to break it down.

Section 6.2 Covalent Bonding Main Ideas: Covalent bonds form from shared electrons Atoms tend to form bonds to follow the octet rule

Section 6.2 Covalent Bonding Main Ideas: Dots placed around an element’s symbol can represent valence electrons Electron-dot notations can represent compounds. Some atoms can share multiple pairs of electrons.

OBJECTIVES: Determine the number of valence electrons in an atom of a representative element. Explain how atoms tend to form bonds to follow the octet rule. Use electron-dot notation to represent valence electrons and represent compounds

Valence Electrons are…? The electrons responsible for the chemical properties of atoms, and are those in the outer energy level. Valence electrons - The s and p electrons in the outer energy level the highest occupied energy level Core electrons – are those in the energy levels below.

Determining Valence Electrons Atoms in the same column... Have the same outer electron configuration. Have the same valence electrons. The number of valence electrons are easily determined. It is the group number for a representative element Group 2: Be, Mg, Ca, etc. have 2 valence electrons

Valence electron – electron in the highest occupied energy level of an atom. *To find the number of valence e- in an atom of a representative element, simply look at its group number

Electron Dot diagrams… shows valence e-’s symbol represents nucleus & core e-’s Each side = orbital (s or p) dot = valence e- (8 max) don’t pair up until they have to (Hund’s rule) (px) X (py) (s) (pz)

The Electron Dot diagram for Nitrogen Nitrogen has 5 valence electrons to show. First we write the symbol. Then add 1 electron at a time to each side CCW. Now they are forced to pair up. We have now written the electron dot diagram for Nitrogen.

Sec 6.2 Practice problems 19. Page 200 Li e. C Ca f. P Cl g. Al O h. S

The Octet Rule In Chapter 5, we learned that noble gases are unreactive in chemical reactions In 1916, Gilbert Lewis used this fact to explain why atoms form certain kinds of ions and molecules The Octet Rule: in forming compounds, atoms tend to achieve a noble gas configuration; 8 in the outer level is stable Each noble gas (except He, which has 2) has 8 electrons in the outer level

Electron dot structure– a notation that depicts Electron dot structure– a notation that depicts valence electrons as dots around the atomic symbol of the element. Octet rule – atoms react by gaining or losing electrons so as to acquire stable e- structure of a noble gas, usually 8 valence e-

Covalent bonding Fluorine has seven valence electrons (but would like to have 8) F

F F Covalent bonding Fluorine has seven valence electrons A second atom also has seven F F

F F Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… F F

F Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… F

F Covalent bonding …both end with full orbitals Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals F

F F Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals F F 8 Valence electrons

F Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals 8 Valence electrons F

Lewis Structure Replace the two bonded electrons with a straight line… F F

A Single Covalent Bond is... A sharing of two valence electrons. Only nonmetals and hydrogen. Different from an ionic bond because they actually form molecules. Two specific atoms are joined. In an ionic solid, you can’t tell which atom the electrons moved from or to

Carbon covalent bonding C ….. special example - can it really share 4 electrons: 1s22s22p2? 2p 1s 2s C Yes, due to e- promotion!

Carbon covalent bonding C ….. special example - can it really share 4 electrons: 1s22s22p2? 2p 1s 2s C Yes, due to e- promotion!

How to show the formation… It’s like a jigsaw puzzle. You put the pieces together to end up with the right formula. Carbon is a special example - can it really share 4 electrons: 1s22s22p2? Yes, due to electron promotion! Another example: lets show how water is formed with covalent bonds, by using an electron dot diagram

H O Water Each hydrogen has 1 valence electron - Each hydrogen wants 1 more The oxygen has 6 valence electrons - The oxygen wants 2 more They share to make each other complete H O

H O Water Put the pieces together The first hydrogen is happy The oxygen still needs one more H O

H O H Water So, a second hydrogen attaches Every atom has full energy levels Note the two “unshared” pairs of electrons H O H

Another way of indicating bonds Often use a line to indicate a bond Called a structural formula Each line is 2 valence electrons H O H H O H =

Electron Dot/Lewis Structure Rules Central atom is least electronegative (furthest left and down) Count up the total number of electrons to make all atoms happy=8 except H=2 (Need). Add up all the valence electrons (Have). Subtract; then Divide by 2 = number of bonds Fill in the rest of the valence electrons so each atom, except for hydrogen, beryllium, and boron, satisfies the octet rule. Check that the number of valence electrons used is the same as added in step 3.

Lewis Structure for Water

N H Example NH3, which is ammonia N – central atom; has 5 valence electrons, wants 8 H - has 1(x3) = 3 valence electrons, wants 2(x3) = 6 NH3 has 5+3 = 8 NH3 wants 8+6 = 14 (14-8)/2= 3 bonds 4 atoms with 3 bonds N H

H H N H Examples Draw in the bonds; start with singles All 8 electrons are accounted for Everything is full – done with this one. H H N H

Lewis Structural Example H N H

Sec 6.2 Practice problems Set C p.176 #1-4 1. Draw the Lewis structure for ammonia, NH3 2. Draw the Lewis structure for hydrogen sulfide, H2S

3. Draw the Lewis structure for silane(silicon tetrahydride), SiH4 4. Draw the Lewis structure for phosphorus trifluoride, PF3

Multiple Bonds Sometimes atoms share more than one pair of valence electrons. A double covalent bond is when atoms share two pairs of electrons (4 total) A triple covalent bond is when atoms share three pairs of electrons (6 total)

Diatomic Elements F2 Cl2 Br2 I2 O2 N2 H2 Know these 7 elements as diatomic: F2 Cl2 Br2 I2 O2 N2 H2 What’s the deal with the oxygen dot diagram? halogens 52

Lewis Structures for Diatomics Single, Double, and Triple Covalent Bonds

Electron Dot/Lewis Structure Rules Central atom is least electronegative (furthest left and down) Count up the total number of electrons to make all atoms happy=8 except H=2 (Need). Add up all the valence electrons (Have). Subtract; then Divide by 2 = number of bonds Fill in the rest of the valence electrons so each atom, except for hydrogen, beryllium, and boron, satisfies the octet rule. Check that the number of valence electrons used is the same as added in step 3.

Sec 6.2 Practice problems Set D p.178 #1-2 *Worked out on next slides 1. Draw the Lewis structure for carbon dioxide, CO2 2. Draw the Lewis structure for hydrogen cyanide, HCN *Worked out on next slides

Page 178 # 1.. Draw Lewis structure for CO2 - Carbon dioxide CO2 - Carbon is central atom ( least electronegative) Carbon has 4 valence electrons Wants 4 more Oxygen has 6 valence electrons Wants 2 more O

O C O Carbon dioxide The only solution is to share more Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short O C O The only solution is to share more

Carbon dioxide The only solution is to share more O O C

C O Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond O C

C O Carbon dioxide 8 valence electrons The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond O 8 valence electrons C

C O Carbon dioxide 8 valence electrons The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond O C 8 valence electrons

C O Carbon dioxide 8 valence electrons The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond O C 8 valence electrons

Carbon dioxide Write as Lewis structure: O C

p. 178 # 2.. Draw Lewis structure for hydrogen cyanide - HCN HCN: C is central atom N - has 5 valence electrons, wants 8 C - has 4 valence electrons, wants 8 H - has 1 valence electron, wants 2 HCN has 5+4+1 = 10 HCN wants 8+8+2 = 18 (18-10)/2= 4 bonds 3 atoms with 4 bonds – this will require multiple bonds - not to H however

N H C HCN Put single bond between each atom Need to add 2 more bonds Must go between C and N (Hydrogen is full) Uses 8 electrons – need 2 more to equal the 10 it has Must go on the N to fill its octet N H C

A Coordinate Covalent Bond... When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) is a good example: Both the carbon and oxygen give another single electron to share O C

Coordinate Covalent Bond When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) is a good example: Oxygen gives both of these electrons, since it has no more singles to share. This carbon electron moves to make a pair with the other single. C O

Coordinate Covalent Bond When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) The coordinate covalent bond is shown with an arrow as: O C C O

Coordinate covalent bond Most polyatomic cations and anions contain covalent and coordinate covalent bonds The ammonium ion (NH41+) can be shown as another example

Lewis Structure Examples H C N C O C O H

The 3 Exceptions to Octet rule For some molecules, it is impossible to satisfy the octet rule #1. usually when there is an odd number of valence electrons NO2 has 17 valence electrons, because the N has 5, and each O contributes 6. It is impossible to satisfy octet rule, yet the stable molecule does exist

# 1 – Odd number of valence electrons

Exceptions to Octet rule #2 –Not enough valence electrons Ex: B, Be Page 173 explains boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron But fluorine has a high electronegativity (it is greedy), so this coordinate bond does not form

#2 –Not enough valence electrons

Exceptions to Octet rule #3 – More than an octet Top page 229 examples exist because they are in period 3 or beyond Ex: sulfur hexafluoride

Resonance is... When more than one valid dot diagram is possible. Consider the two ways to draw ozone (O3) Which one is it? Does it go back and forth? It is a hybrid of both, like a mule; and shown by a double-headed arrow found in double-bond structures!

Resonance in Ozone Note the different location of the double bond Neither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow.

Polyatomic ions – note the different positions of the double bond. Resonance in a carbonate ion (CO32-): Resonance in an acetate ion (C2H3O21-):

Sec 6.2 Practice problems Page 201 #24 a. OH 1- b. H3C2O2 1- 21. Draw Lewis structures for each of the following polyatomic ions. Show resonance structures, if they exist. a. OH 1- b. H3C2O2 1- c. BrO3 1- add: d. H3O 1+

OBJECTIVES: Identify the reason why network solids have high melting points.

Attractions and properties Why are some chemicals gases, some liquids, some solids? Depends on the type of bonding! More in Section 6.5 and Chapter 7 Network solids – solids in which all the atoms are covalently bonded to each other

Attractions and properties Network solids melt at very high temperatures, or not at all (decomposes) Diamond does not really melt, but vaporizes to a gas at 3500 oC and beyond SiC, used in grinding, has a melting point of about 2700 oC

Covalent Network Compounds Some covalently bonded substances DO NOT form discrete molecules. Instead: 3-D networks of bonded atoms Network Solids Video Diamond, a network of covalently bonded carbon atoms Graphite, a network of covalently bonded carbon atoms

Section 6.2 Practice Problems Chapter 6 Notes 5 blocks (A-E) Chapter 6 Vocabulary 25 words Chapter 6 Reading Guide Section 2 Section 6.2 Practice Problems Page 200 # 6,7 Set C p.176 #1-4 Set D p.178 #1-2 Page 201 # 24 (add one) 6.2 Study Guide 84