UNIT 6 - CHEMICAL BONDING
INTRODUCTION TO CHEMICAL BONDING
I.  Types of Chemical Bonding
    A.  ____________________: mutual electrical attraction between the nuclei and valence e- of different atoms
         that binds the atoms together
    B.  Why do atoms bond together?
        1.  Atoms by themselves have _______ potential energy
        2.  Want to have _______ P.E. (this is what happens in nature)
        3.  Atoms are ______ stable alone than combined with other atoms
    C.  Electrons are ____________________ during bonding
    D.  Types of chemical bonding
        1.  ____________________: results from attraction between large numbers of cations and anions; involves
             the transfer of electrons
            a.  ____________________: positive ion;  atom that has lost e-
            b.  ____________________: negative ion;  atom that has gained e-
        2.  ____________________: results from sharing of e- pairs between two atoms
    E.  Determining Ionic or Covalent Bond (Official Way)
        1.  Remember __________________________________? (how much an atom wants to gain an e-)
        2.  Difference in EN tells whether bond is ionic or covalent
            a.  EN difference ____________________ (50% or less ionic character) = COVALENT
                1.  _______________COVALENT:  EN difference ______ to ______  (5 - 50% ionic character)
                2.  _______________ COVALENT: EN difference ______ to ______  (< 5% ionic character)
            b.  EN difference greater than ______ (> than 50% ionic character) = IONIC
    F.  Determining Ionic or Covalent Bond (Easy Way)
        1.  ____________________: metal & nonmetal in formula
        2.  ____________________: 2 or more nonmetals
            a.  ____________________:  2 different nonmetals
            b.  ____________________:  2 of the same nonmetal

COVALENT BONDING AND MOLECULAR COMPOUNDS
I.  Molecules & Molecular Formulas
    A.  ____________________: neutral group of atoms held together by covalent bonds
    B.  _________________Molecule: molecule that contains only two atoms (of the same element)
         -->  there are only 7 of them;  they are 

II.  Formation of a Covalent Bond
    A.  2 separate atoms far enough away ____________________________________________ on each other
    B.  As they move ______________, the _______________ (+ chrg.) attracts the ______________ from the
         other atom & vice versa (________________ P.E.)
    C.  When atoms get too close, the + chrgs. in nucleus repel each other;  + chrgs. from other nucleus & e-
         repel each other also (increase in P.E.)

III.  Characteristics of the Covalent Bond
    A.  ____________________:  distance between 2 bonded atoms at their minimum P.E.
        1.  In other words, avg. distance between 2 bonded atoms
        2.  Measured in _______________ (pm) = 1 x 10-   m
    B.  Amt. of energy released when a bond is formed is ______________ to the difference in P.E. from separate
         atoms to the minimum P.E.
    C.  Same amt. of energy must be added to _________________________________
    D.  _____________________________: energy required to break a chemical bond & form isolated atoms
        1.  measured in _______________
        2.  kJ = _________ J (J = Joule, measure of energy)
-----------------------------------------------------------------------------------------------------------------------------------------

 

LEWIS STRUCTURES:  formulas which symbols represent nucleus & inner shell electrons;  dot pairs or dashes represent electron pairs in covalent bonds
    1. ______________________________: (lone pair)  pair of electrons that is not involved in bonding &
        belongs to only one atom
    2. _______________________________: shows atoms, bonds, & arrangement, but not lone pairs of e-
    3. _______________________________: produced when one pair of e- is shared b/n two atoms
    4. MULTIPLE BONDS:  stronger & shorter than single bonds
        a. Double Bond:  produced when ______ pairs of e- are shared b/n two atoms
            1) shown by either 2 side-by-side pairs of dots
            2) or by 2 parallel dashes
        b. _______________ Bond: produced when 3 pairs of e- are shared b/n two atoms
            1) shown by either 3 side-by-side pairs of dots
            2) or by 3 parallel dashes

STEPS FOR DRAWING LEWIS STRUCTURES

1.  Count the total number of valence electrons in the compound.
        EXAMPLE:  CCl4
                    C:  1 x 4 e- =  4
                    Cl: 4 x 7 e- = 28
                                        32

2.  Draw the symbols for the elements with the central atom in the middle.
     (The central atom is the one that is written first, except hydrogen will never be central.)
                       

3.  Connect the surrounding atoms to the central atom with a pair of electrons.
                 Had 32 e- to start (step 1)
                 Used 8 e- to connect
                 Have 24 e- left

4.  Put electrons around surrounding atoms until all have 8 electrons around them.
     (Exception:  Hydrogen will only have 2 e- around it.)
Have 24 e- left
Used 24 e- to complete surrounding
Have 0 e- left to use

5.  If there are any electrons left over, put them on the central atom.

6.  Check to make sure that all atoms have 8 electrons around them.

7.  If there are not enough electrons to give all atoms 8 electrons around them, try multiple
     bonds.


VSEPR THEORY

VSEPR Theory: (Valence Shell Electron Pair Repulsion) 
valence electrons in an atom will orient themselves to be as far apart as possible

* CLASSES OF MOLECULES

A = central atom
B = surrounding atoms
E = unshared electron pairs on central atom
*** subscript tells how many of each you have ***

EXAMPLE 1:    CH4       

1 central atom, 4 surrounding atoms, no unshared e- prs. on central atom
CLASS = AB4

EXAMPLE 2:    NH3   

1 central atom, 3 surrounding atoms, 1 unshared e- pr. on central atom
CLASS = AB3E


* SHAPES OF MOLECULES

CLASS                    SHAPE  
AB2                        linear
AB2E                      bent
AB2E2                     bent
AB3                        trigonal planar
AB3E                      trigonal pyramid
AB4                        tetrahedral

MOLECULAR POLARITY vs.         BOND POLARITY

 

BOND POLARITY

~ refers to the equal (______________) or unequal (______________) sharing of electrons

 

~ What makes a bond polar (covalent)?

            * If the bond occurs between two _____________________ nonmetals

 

MOLECULAR POLARITY

~ refers to the symmetry (______________) or asymmetry (______________) of a molecule         

 

~ What makes a molecule asymmetrical?

            * If there are ______________________________________ around the central atom.

            and/or

            * If the surrounding atoms are different ____________________________________

 

 

POLAR MOLECULE WITH POLAR COVALENT BONDS


unshared e- pair on central atom makes the molecule polar


bond is between 2 different nonmetals – so bond is polar covalent

 

          

 

NONPOLAR MOLECULE WITH POLAR COVALENT BONDS
 

no unshared e- pairs on central atom

 

                                                 bond is between 2        
AND surrounding atoms are same
different nonmetals, so bond
element, so molecule is NONPOLAR. is polar covalent                              

 



Special Cases Regarding Molecular Polarity:
~ When there are only 2 atoms in the compound or ion…
            * examples:  F2 or HF

 

         

* POLARITY OF MOLECULES (symmetry of molecules) – different from bond polarity

A MOLECULE is nonpolar if...
    - there are no unshared e- pairs around the central atom
        AND
    - the surrounding atoms are all the same element

A BOND is nonpolar (covalent) if…
    - it occurs between two atoms of the same element

IT IS POSSIBLE, EVEN LIKELY, THAT A NONPOLAR MOLECULE HAS POLAR COVALENT BONDS!

Ionic Bonding & Ionic Compounds
    A.  ____________________: composed of + and - ions whose charges cancel each other
         out;  electrons are _______________
        1.  Most exist as ____________________
        2.  __________ network of + and - ions;  not composed of individual uunits
    B.  Formation of Ionic Compounds
                           . .
        1.  Na .  +  : Cl :  --> 
                            .
   
      2.  Characteristics of Ionic Bonding
            a.  minimize P.E. by organizing into _______________
            b.  ____________________: energy released when 1 mole of ionic crystalline cmpd.
                 is formed from gaseous ions  

IONIC VS. COVALENT (MOLECULAR) COMPOUNDS

Ionic Compounds                                                   Covalent (Molecular) Compounds
~ formed by transferring electrons                          ~ formed by sharing electrons
~ occurs between positive and negative                   ~ occurs between nonmetals
    ions (metal & nonmetal or metal &   
    polyatomic ion)
~ smallest unit = "formula unit"                              ~ smallest unit = "molecule"
~ exist as solids, usually crystal shape                      ~ exist as liquids, gases, or non-crystal
                                                                                solids
~ have high melting points                                      ~ have low melting points
~ most dissolve in water                                         ~ few dissolve in water
~ melted or dissolved in water conduct                     ~ melted or dissolved in water does not
    electricity                                                               conduct electricity
~ fixed + and - parts;  anions & cations                    ~ may have slightly + and - parts or
                                                                                totally neutral
~ EXAMPLES:  KCl, NaF, KNO3                                ~ EXAMPLES:  NH3, CO2, CH4

Polyatomic Ions
    1.  ____________________: charged group of covalently bonded atoms
    2.  Combine with ions of _______________ charge to form ionic compounds
    3.  Compounds with polyatomic ions have ____________________________ bonding
    4.  Lewis Structures
        a.  Negatively charged polyatomic ion = _____ e- to total number of valence e- for cmpd
        b.  Positively charged polyatomic ion = ________ e- from total # of valence e- for cmpd
-------------------------------------------------------------------------------------------------------------------------------------------INTERMOLECULAR (IM) FORCES
*without them, all matter would be gases*

intermolecular forces:  forces of attraction between molecules

4 TYPES OF IM FORCES (in order from strongest to weakest)

1.    HYDROGEN "BONDING"
        * occurs between molecules that have hydrogen & nitrogen, hydrogen & oxygen, and
           hydrogen & fluorine
        * accounts for the unusually high boiling point of NH3, H2O, and HF

2.    DIPOLE-DIPOLE FORCES
        * occurs between polar molecules (dipoles)
        * the slightly positive end of one polar molecule is attracted to the slightly negative end
           of another polar molecule

3.    DIPOLE-INDUCED DIPOLE FORCES
        * occurs between a polar molecule (dipole) and a nonpolar molecule or between a polar
           molecule and a noble gas
        * the slightly positive end of a polar molecule temporarily attracts the electrons from a
           nonpolar molecule or noble gas

4.    LONDON DISPERSION FORCES
        * occurs between all molecules
        * only force that acts between nonpolar molecules and noble gases
        * moving electrons (in electron cloud of an atom) temporarily attract the electrons from
           another atom

** COVALENT BONDING IS STRONGER THAN ANY OF THESE IM FORCES.  **

Back to the Unit 6 Notes Page

Back to the Unit 6 Page

Back to the Main Page