BONDING Let’s get together… Barbara A. Gage PGCC CHM 1010.

BONDING

Let’s get together…

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


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


Types of Bonding


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


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


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


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.


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


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


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


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


Electrical conductance and ion mobility.

Solid ionic compound

Molten ionic compound

Ionic compound dissolved in

water


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.


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).



CCl4 Total electrons = 1(4) + 4(7) = 32

ClCl C Cl

Cl

ClCl C Cl

Cl



SO3 Total electrons = 1(6) + 3(6) = 24

OO S O

O OO S O O S O



....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


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


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-



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


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


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


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


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?


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


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



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


The Pauling electronegativity (EN) scale.

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.


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



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


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


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


Percent Ionic Character

• The percent ionic character is the percentage of a bond’s measured dipole moment compared to what it would be if the electrons were completely transferred

• The percent ionic character indicates the degree to which the electron is transferred

BONDING Let’s get together… Barbara A. Gage PGCC CHM 1010.

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