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Barbara A. Gage PGCC CHM 1010
Why Do Atoms Bond?• Chemical bonds form because they
lower the potential energy between the charged particles that compose atoms
• A chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms
Barbara A. Gage PGCC CHM 1010
Types of Bonds
Types of Atoms
Type of BondBond
Characteristic
metals to nonmetals
Ionicelectronstransferred
nonmetals tononmetals
Covalentelectrons shared
metals tometals
Metallicelectronspooled
• We can classify bonds based on the kinds of atoms that are bonded together
Barbara A. Gage PGCC CHM 1010
Ionic Bonds• When a metal atom loses electrons it
becomes a cation– metals have low ionization energy,
making it relatively easy to remove electrons from them
• When a nonmetal atom gains electrons it becomes an anion – nonmetals have high electron affinities,
making it advantageous to add electrons to these atoms
• The oppositely charged ions are then attracted to each other, resulting in an ionic bond
Barbara A. Gage PGCC CHM 1010
Covalent Bonds• Nonmetal atoms have relatively high
ionization energies, so it is difficult to remove electrons from them
• When nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons– potential energy lowest when the electrons are
between the nuclei• Shared electrons hold the atoms together
by attracting nuclei of both atoms
Barbara A. Gage PGCC CHM 1010
Metallic Bonds
• The relatively low ionization energy of metals allows them to lose electrons easily
• The simplest theory of metallic bonding involves the metal atoms releasing their valence electrons to be shared as a pool by all the atoms/ions in the metal– an organization of metal cation islands in
a sea of electrons– electrons delocalized throughout the
metal structure• Bonding results from attraction of cation for
the delocalized electrons
Barbara A. Gage PGCC CHM 1010
Lewis Electron-Dot Symbols
For main group elements -
Example:
Nitrogen, N, is in Group 5A and therefore has 5 valence electrons.
N:.
..
:
N .. ..N :.
. :N ...
The A group number gives the number of valence electrons.Place one dot per valence electron on each of the four sides of the element symbol.
Pair the dots (electrons) until all of the valence electrons are used.
Barbara A. Gage PGCC CHM 1010
Determining the Number of Valence Electrons in an Atom• The column number on the Periodic Table
will tell you how many valence electrons a main group atom has– Transition Elements all have two valence
electrons. Why?
9
Barbara A. Gage PGCC CHM 1010
Lewis Structures of Ions
• Cations have Lewis symbols without valence electrons– lost in the cation formation
• Anions have Lewis symbols with eight valence electrons– electrons gained in the formation of the
anion
10
Barbara A. Gage PGCC CHM 1010
Ionic Bonding & the Crystal Lattice
• Ionically bonded substances form a structure in which every cation is surrounded by anions, and vice versa
• This structure is called a crystal lattice• The crystal lattice is held together by the
electrostatic attraction of the cations for all the surrounding anions
• The crystal lattice maximizes the attractions between cations and anions, leading to the most stable arrangement
Barbara A. Gage PGCC CHM 1010
Crystal Lattice• Electrostatic attraction is
nondirectional!!– no direct anion–cation pair
• Therefore, there is no ionic molecule– the chemical formula is an empirical
formula, simply giving the ratio of ions based on charge balance
– One unit of the empirical formula is called a formula unit
Barbara A. Gage PGCC CHM 1010
Electrical conductance and ion mobility.
Solid ionic compound
Molten ionic compound
Ionic compound dissolved in
water
Barbara A. Gage PGCC CHM 1010
Lewis Theory of Covalent Bonding
• Lewis theory implies that another way atoms can achieve an octet of valence electrons is to share their valence electrons with other atoms
• The shared electrons would then count toward each atom’s octet
• The sharing of valence electrons is called covalent bonding
Covalent Bonding
• Atoms with incomplete octets can share rather than transfer electrons.
• Each pair of shared electrons = 1 bond
• Shared electrons move around the nuclei of both atoms in the bond so both atoms have possession of the shared electrons.
Barbara A. Gage PGCC CHM 1010
Lewis Dot Structures for Covalent Compounds
• Sum the valence electrons of all atoms.
• Determine the central atom.• Position the central atom and place
the additional atoms equally around it.• Place the required number of electrons
around the outside atoms first and then around the central atom to be each one meets the octet rule (or the number needed if it is an exception).
Barbara A. Gage PGCC CHM 1010
Lewis Dot Structures for Covalent Compounds
CCl4 Total electrons = 1(4) + 4(7) = 32
ClCl C Cl
Cl
ClCl C Cl
Cl
Barbara A. Gage PGCC CHM 1010
Lewis Dot Structures for Covalent Compounds
SO3 Total electrons = 1(6) + 3(6) = 24
OO S O
O OO S O O S O
Barbara A. Gage PGCC CHM 1010
Barbara A. Gage PGCC CHM 1010
....OSO.. .... ..
.. ....
Covalent Bonding:Bonding and Lone Pair Electrons
Bonding pairs Lone pairs
• Electrons that are shared by atoms are called bonding pairs
• Electrons that are not shared by atoms but belong to a particular atom are called lone pairsaka nonbonding pairs
Barbara A. Gage PGCC CHM 1010
Single Covalent Bonds
F••
••
•• • F•••••••
F••
••
••
••
••F
•••• HH O•••••
•
••
H•H• O••
•
•
••
F F
• When two atoms share one pair of electrons it is called a single covalent bond 2 electrons
• One atom may use more than one single bond to fulfill its octet to different atoms H only duet
Barbara A. Gage PGCC CHM 1010
Double Covalent Bond
• When two atoms share two pairs of electrons the result is called a double covalent bond– four electrons
O•••• O
••
••••••
O••
•
•
••O
•••
•
••
Octet Rule Exceptions
• Some elements are stable with fewer or more than 8 e-.
H 2e- Be 4e- B 6e-
P, Cl, Br (and more) 10e-
S, Se, Xe (and more) 12e-
Barbara A. Gage PGCC CHM 1010
Barbara A. Gage PGCC CHM 1010
Resonance: Delocalized Electron-Pair Bonding
Resonance structures have the same relative atom placement but a difference in the locations of bonding and nonbonding electron pairs.
OO O
A
B
C
OO O
A
B
C
O3 can be drawn in 2 ways -
OO O
OO O
Neither structure is actually correct but can be drawn to represent a structure which is a hybrid of the two - a resonance structure.
is used to indicate that resonance occurs.
O
O O
Barbara A. Gage PGCC CHM 1010
Writing Resonance Structures
SOLUTION:
PROBLEM: Write resonance structures for the nitrate ion, NO3-.
Nitrate has 1(5) + 3(6) + 1 = 24 valence e-
N
O
O O
N
O
O O
N
O
O O
N does not have an octet; a pair of e- will move in to form a double bond.
N
O
O O
N
O
O O
N
O
O O
Barbara A. Gage PGCC CHM 1010
Bond Lengths
• The distance between the nuclei of bonded atoms is called the bond length
• Because the actual bond length depends on the other atoms around the bond we often use the average bond length– averaged for similar bonds
from many compounds
Barbara A. Gage PGCC CHM 1010
Bond Energies• Chemical reactions involve breaking bonds
in reactant molecules and making new bonds to create the products
• The H°reaction can be estimated by comparing the cost of breaking old bonds to the income from making new bonds
• The amount of energy it takes to break one mole of a bond in a compound is called the bond energy– in the gas state– homolytically – each atom gets ½ bonding
electrons
Barbara A. Gage PGCC CHM 1010
Silberberg, Principles of Chemistry
What is the relationship between bond order and bond length for bondsbetween the same two elements?
What is the relationship between bond length and bond energy for bondsbetween the same two elements?
Barbara A. Gage PGCC CHM 1010
Comparing Bond Length and Bond Strength
SOLUTION:
PROBLEM: Using the periodic table, rank the bonds in each set in order of decreasing bond length and bond strength:
(a) S - F, S - Br, S - Cl
(b) C = O, C - O, C O
(a) The bond order is one for all and sulfur is bonded to halogens; bond length should increase and bond strength should decrease with increasing atomic radius. (b) The same two atoms are bonded but the bond order changes; bond length decreases as bond order increases while bond strength increases as bond order increases.
(a) Atomic size increases going down a group.
Bond length: S - Br > S - Cl > S - F
Bond strength: S - F > S - Cl > S - Br
(b) Using bond orders we get
Bond length: C - O > C = O > C OBond strength: C O > C = O > C - O
Barbara A. Gage PGCC CHM 1010
Break1 mol C─H +414 kJ1 mol Cl─Cl +243 kJ
Make1 mol C─Cl −339 kJ1 mol H─Cl −431 kJ
Electron Distribution in a Covalent Bond
• Are electrons shared equally in a covalent bond?
• If not, why not?• Distance of electrons from nucleus
and number of protons in the nucleus• Electronegativity – attraction of one
atom in a bond for the electrons in that bond
Barbara A. Gage PGCC CHM 1010
Barbara A. Gage PGCC CHM 1010
Polar Covalent Bonding• Covalent bonding between unlike atoms results in
unequal sharing of the electrons– one atom pulls the electrons in the bond closer
to its side– one end of the bond has larger electron density
than the other• The result is a polar covalent bond
– bond polarity– the end with the larger electron density gets a
partial negative charge– the end that is electron deficient gets a partial
positive charge
Polarity
• When atoms in a bond have different electronegativities, the electron sharing is unequal.
• As the ΔEN increases, the electron distribution becomes more uneven and the molecule becomes polar.
Barbara A. Gage PGCC CHM 1010
Polarity
• HCl
• ENH = 2.1 ENCl = 3.0 ΔEN = 0.9
• The end with the higher EN will be slightly negative and the other will be slightly positive
δ+H – Clδ- H – Cl
Barbara A. Gage PGCC CHM 1010
Barbara A. Gage PGCC CHM 1010
Electronegativity Difference and Bond Type
• If difference in electronegativity between bonded atoms is 0, the bond is pure covalent– equal sharing
• If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent
• If difference in electronegativity between bonded atoms is 0.5 to 1.9, the bond is polar covalent
• If difference in electronegativity between bonded atoms is larger than or equal to 2.0, the bond is ionic
“100%”
0 0.4 2.0 4.0
4% 51%Percent Ionic Character
Electronegativity Difference
Barbara A. Gage PGCC CHM 1010
Bond Dipole Moments
• Dipole moment, , is a measure of bond polarity– a dipole is a material with a + and − end– it is directly proportional to the size of the
partial charges and directly proportional to the distance between them
• = (q)(r)• not Coulomb’s Law• measured in Debyes, D
• Generally, the more electrons two atoms share and the larger the atoms are, the larger the dipole moment
Barbara A. Gage PGCC CHM 1010
Determining Bond Polarity from EN Values
SOLUTION:
PROBLEM: (a) Use a polar arrow to indicate the polarity of each bond: N-H, F-N, I-Cl.
(b) Rank the following bonds in order of increasing polarity: H-N, H-O, H-C.
(a) Find EN values; the arrow should point toward the negative end.
(b) Polarity increases across a period.
(a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0
N - H F - N I - Cl
(b) The order of increasing EN is C < N < O; all have an EN larger than that of H.
H-C < H-N < H-O