Chapter 9: Chemical Bonding I: Lewis Theory
What is the difference of Electron Affinity and Electronegativity.
EA- an enthalpy change. isolated gas phase atoms gain an electron. (ATOMS ARE IN GAS PHASE) EN- the ability of an atom to compete for electrons with other atoms to which it is bonded, or the ability of an atom to attract electrons to itself in a covalent chemical bond. (greater EN an atom is, greater its electron-withdrawing power is.) PROPERTY OF A BONDED ATOM (COVALENT)
what forms a triple bond?
three shared electron pairs.
which atoms follow the "duet rule"? why?
H, Li, Be stable with electron configuration of He (ns^2) no available p shell
what do we use the relative EN of atoms for?
to predict the skeletal structure of molecules and covalently bonded (polyatomic) ions
what is the result of a slight negative charge (s-) on fluorine and the slight positive charge (s+) on hydrogen?
uneven sharing of electrons between the two atoms
what happens with polar covalent bond
uneven sharing of electrons so there is a partial positive charge and a partial negative charge
what's the "octet rule"
usually eight electrons in the valence shell (nobel gas configuration)
what do dots around the symbol represent?
valence electrons
What are resonance structures
different plausible lewis structures.
if there is a SMALL EN difference (∆EN= 0-0.4)
non-polar covalent
what would happen if an electron were completely transferred?
-bond would have 100% ionic character -no bond is 100% ionic (no such thing as a completely transfer of electrons) -we consider a bond "ionic" if its ionic character > 50%
whats formal charge used for?
to determine which is the best structure -use the option that gives as little formal charge as possible)
What's metallic bonding between?
to metals
how to form N2 if the nitrogen atom has only five valence electrons?
two nitrogen atoms must share 3 electron pairs.
Why are there 2 double bonds in CO2?
-all three atoms will have a complete octet if carbon shares one more with each oxygen. -carbon needs to acquire four electrons - if it shares one electron with each oxygen, carbon now has six and each oxygen has seven
what is considered the "best" structure? (what are the guidelines)
-if formal charges of all the atoms are zero -if zero formal charge is not possible. formal charges should be as small as possible. -negative formal charges should appear on the most electronegative atoms -adjacent atoms in a structure should not carry formal charges of the same sign) - the sum of the formal charges must be zero for a neutral molecule and must be equal to the overall charge on a polyatomic ion.
What does a lewis structure do?
-indicates how atoms combine in a molecule -usually shows the bonded atoms joined in a way that gives all atoms a filled valence shell (i.e. obeys the octet rule) -does not predict anything about the shape of molecules
what is the skeletal structure comprised of
-one or more central atoms (atoms bonded to two or more other atoms) - plus TERMINAL ATOMS (bonded to only one other atom)
steps to writing skeletal structures
1) the least EN in the center 2) hydrogen atoms are always terminal (with one electron to share, hydrogen can form only one bond with another atom) 3) atoms with lower EN values are central while atoms with higher EN values are terminal. (F is always terminal) 4) molecules and polyatomic ions usually have compact, symmetrical structures
What are the rules/steps for drawing lewis structures?
1. Determine the total # of valence electrons (this is the sum of the available valence electrons in all of the atoms. For a polyatomic ion, add one electron for each unit of negative charge; subtract one electron for each unit of positive charge. THIS NUMBER MUST APPEAR IN THE LEWIS STRUCTURE! NEVER ADD ANY OTHER ELECTRONS! ) do this at least 3 times. if # is odd, something went wrong 2. write the skeletal structure (connect atoms with dashes to represent single bonds) 3. Place pairs of electrons (dots) around the terminal atoms to give each terminal atom (except hydrogen) an octet 4. Assign any remaining electrons as lone pairs around the central atoms. (do not use "extra" electrons to make multiple bonds! it would give the terminal atoms too many electrons!) 5. If central atom is still lacking an octet, move one or more lone pairs from a terminal atom (or atoms) to form a multiple bond to the central atom(s).
what forms a double bond?
2 shared electron pairs
what's the formula for determining the number of bonds formed by the non-metals?
8 - group #
What's formal charge?
= (the # of valence e- in the uncombined atom)- (the # of valence e- assigned to the bound atom in the Lewis structure) = (the # of valence e- in the uncombined atom) -[(the # of lone pairs on the bonded atom) +1/2(the # of bonding e-)]
percent ionic character
=(measured dipole moment)/(dipole moment if the electron were completely transferred) * 100%
B lewis structure
B - l
Be Lewis structure
Be - l
incomplete octets are seen most often in molecules that have which as the central atom?
Be, B, or Al
How do opposite charges achieve lower potential energy?
By getting closer
What are electrical forces?
Chemical bonds
Li lewis structure
Li -
How does sodium chloride form? (NaCl)
Na loses its electron Cl gains one electron to fill its 3p shell
Why do chemical bonds form?
Oppositely charged particles that comprise atoms can lower their potential energy by interacting with one another.
what does the degree of a bond's polarity depend on?
difference in EN between 2 bonded atoms
what is the Born-Haber cycle
a hypothetical multi-step process that culminates in the formation of an ionic compound from its constituent elements in their standard states.
How do you represent polar bonds?
a vector wit ha + at one end and the head of the arrow directed toward the negative end of the molecule indicates that this bond is polar +---> H---F the lower case Greek letter S indicates an atom with partial positive (s+) or negative (s-) charge H is (s+) and f is (S-)
Energy is ___ and the bonds of reactant molecules are broken
absorbed so ∆H > 0
What do chemical symbols represent?
an atom's nucleus and core electrons
Bonding is a balance between...
attractive and repulsive
how do you distinguish between bonding pair and lone pair?
bonding pair with a - lone pair with a pair by itself.
how is bond strength assessed?
by how difficult it is to break a covalent bond in a gas phase molecule (energy must be absorbed)
in lewis theory, how do you represent ionic bonding?
by moving dots from the metal to the non-metal
How do identical charges achieve lower potential energy?
by separating
what happens when atoms bond to forma solid?
each atom donates one or more electrons to an "Electron see" (The metal is an array of positive ions held together by their attraction to the negative sea of delocalized [pooled] electrons)
whats pure covalent bond
electrons shared evenly
what happens in ionic bonds
electrons transferred completely
electron affinity
energy change that occurs when an electron is added to an atom or ion in the gas phase
Bond Energy
energy required to break one mole of covalent bonds between two atoms in a molecule in the gas phase. (breaking=endothermic)
ionization
energy required to remove an electron from an atom or ion IN THE GAS PHASE
enthalpy of formation
enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states.
How you should draw acids that contain oxygens?
exactly how they are written. acid w/ oxygen are called "oxyacid" and have the acidic hydrogen (or H's) attached directly to oxygens. ins H2SO4, S is not in the middle.
how do you determine the main-groups # of valence electrons?
group #
explain covalent bonding
in lewis theory, neighboring atoms in a molecule are depicted as sharing some (or all) of their valence electrons.
Whats the EN trend
increases across a period EN decreases down a group
if there is a LARGE electronegativity difference (∆EN > 2.0)
ionic bond (metal and non-metal)
what are the strong, on-directional electrostatic forces that hold the crystal lattice together?
ionic bonds
Describe what happens when an atom forms a bond with another atom
it shares or transfers one or more electrons in such a way that both atoms in the bond attain a more stable electron configuration.
what happens if oxygen formed three covalent bonds with hydrogen atoms?
it would violate the octet rule.
C lewis structure
l - C - l
N lewis structure
l -N= l
O lewis Structure
l -O= ll
F lewis structure
l =F= l
Trends in lattice energy
less negative going down because size increases so charge/size ratio decreases less negative with increasing ionic radius and more negative with increasing magnitude of the charge
Ne lewis Structure
ll =Ne= ll
properties of metals explained
malleable and ductile because they contain no specific bonds. conduct electricity because in metal electrons are not held by the atoms on which they originate; they are free to move and conduct electrical current.
What's ionic bonding between? why?
metal + non-metal metals--low ionization energies non-metals--large negative electron affinities electrons are transferred from metal to non-metal
The Pauling Scale -where/what are the most electronegative elements -where are the least EN elements?
most electronegative elements (F, O, N, Cl, Br, I, C,S) are in the upper right corner of the periodic table. least EN are on he lower left corner of the periodic table
Do the exact positions of the dots matter?
no but no more than 2 dots can appear on any side of a symbol
whats the skeletal structure
no electron dots or multiple bonds shows the order in which atoms are bonded
do Lewis dots reflect electron pairing?
no. Lewis theory predates quantum mechanics
do ionic solids conduct electricity?
no. aqueous soln of ionic compounds conduct electricity but ionic solids are non-conductive. (electrons are localized. Ions are separated from the crystal lattice and are free to roam through the solution and carry electrical current.)
What's covalent bonding between? why?
non-metal + non-metal non-metals--high ionization energies electrons are shared between atoms shared electrons are stabilized by interaction with nuclei of both atoms
What are the exceptions to the Octet Rule?
odd number of valence electrons make it so that it is unstable and highly reactive, making it impossible to write a Lewis structure that obeys the octet rule. You can use a single dot to represent the unpaired electron (i.e. NO 11 electrons ClO2 19 valence electrons NO2 17 valence electrons)
why is water H2O?
oxygen needs 2 e- to fill its electron shell forming covalent bonds w/2 H+ toms gives each hydrogen a duet.
if there is an INTERMEDIATE EN different (∆EN= 0.4- 2.0)
polar covalent
Atoms are rearranged, product bonds are formed, and energy is ___
released ∆H <0
what happens when two atoms share one electron pair
result is a single bond
if there's NO difference in EN... (∆EN = O)
same nucleus/identical atoms it is pure covalent
order bond strength from weakest to strongest
single bond double bond triple bond
what influences lattice energy?
size and charge greater the charge density (charge-to-size ratio) of the ions, the more negative the lattice energy.
what does EN vary inversely with
size--larger the atom, the less electronegative it is.
is an expanded valence shell needed to accommodate lone electron pairs around a central atom?
sometimes
melting point of ionic solids
they have very high melting points--enough heat has to be added to the solid to overcome the very strong coulombic forces of attraction that hold the ions in the crystal lattice.
the stronger the bond is, the more ...
stable and less chemically reactive it is
what makes a reaction endothermic
stronger bonds are broken and weaker bonds form
how do you write an ionic bond?
symbol of ion in square brackets with charge indicated as a superscript outside the brackets. ll [Na]+ + [=Cl=] - ll
what happens in HF?
the F- aligns with the + pole the H+ toward the - pole
whats a resonance hybrid
the actual molecule or ion that is a composite (an average of them)
what is bond energy in a O3 atom
the average of the energy of an O-O bond and an O=O bond
What is length in a O3 atom
the average of the length of the single bond and the length of a double bond
what does bond energy depend on?
the bonds immediate environment within a molecule.
def of bond length
the distance between the nuclei of two atoms joined by a covalent bond
lattice energy
the energy released when cations and anions come together in the gas phase to form one mole of an ionic solid
what happens when two or three electron pairs are shared?
the formation of a multiple bond
What happens when we have too many terminal atoms around a central atom to write a structure that obeys the octet rule?
the octet rule allows an atom to make as many as four bonds... but you can with 2nd and 3rd + period elements. 2nd period elements have only 2 and p orbitals, and they can accommodate a maximum of 8 electrons in their valence shell. 3rd period have d orbitals that are accessible for bonding.
What does the Lewis model focus on?
the role of valence electrons in forming chemical bonds
Why do halogens exist as diatomic molecules rather than as isolated atoms?
the shared electrons are "doublt counted"--they belong to both atoms. i.e. Cl2
what makes a reaction exothermic
weaker bonds are broken and stronger bonds are formed
how do you represent resonance structures in lewis theory?
writing all the plausible forms joined by double-headed arrows.
how do you estimate ∆H (Rxn) using bond energies? (the equation)
∆Hrxn = ∑BE(bonds broken) ∑BE(bonds formed)
which is more accurate. ∆Hrxn calculated using ∆H°f or using bond energies?
∆H°f because tabulated bond energy values represent average values for each bond type, but the bond energies offer a good estimate when ∆H°f data are not available.
