Chapter 8: Periodic Properties of the Elements Chemical Bonds ionic bond covalent bond metallic...

Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements

Chemical Bonds

• ionic bond

• covalent bond

• metallic bond

The formation of chemical bonds involves valence electrons

Lewis Symbols: “shorthand” for depicting valence electrons

Gilbert Lewis (1875-1946)

Lewis dot structures:

Be B C N O F Ne

Na Mg Al Si P S Cl Ar

For representative (main group) elements:group number = number of valence electrons

1A 2A 3A 4A 5A 6A 7A 8A

Noble gases, except helium, have

When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence

electrons (an “octet” of electrons)

K Cl+ K+ + Cl

Na+ Cl-

Cl- Na+

Na+ Cl-

Formation of an ionic lattice

Na+ (s) + Cl- (g) NaCl (s) very exothermicH<0

Na+ Cl-

charge on ions

distance between charges

Strength of ionic bond depends on Eel

• the larger Eel, the stronger the bond

The stronger the ionic bond the the melting point

SrF2 +2, -2

66, 133

66, 140

113, 133

1261oC

Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding

Covalent Bonds …

H H+ H H

F F+ F F

Covalent Bonds

H H+ H H

F F+ F F

• A shared electron pair is drawn as a dash (two electrons!)

• Unshared electrons are drawn as dots

Single and Multiple Bonds

F F+ F F

O O+ C + O C O

N NN + N

Single and Multiple Bonds

Bond Polarity and Electronegativity

Cl Cl non-polar covalent bond:equal sharing of electrons

• When both atoms attract bond electrons equally, electrons are shared equally

H Cl polar covalent bond:unequal sharing of electrons

• If one of the atoms attracts bond electrons more strongly, electrons are shared unequally

H Cl polar covalent bond:unequal sharing of electrons

For comparison:

ionic bond:electrons are not sharedNa+ Cl -

How do we know when a non-polar, polar, or ionic bond is formed?

The tendency of atom to attract electrons when forming a molecule is

summarized in the concept of

electronegativity

lectro

tivity

Linus Carl Pauling (1901-1994)

• developed concept of electronegativity

• the electronegativity scale assigns avalue of 4 to Fluorine and 0.7 to Cesium, the least electronegative element.

• least electronegative atoms are thosethat give up electrons most easily.

How do we know when a non-polar, polar, or ionic bond is formed?

If the difference in Electronegativity, EN, is…

…smaller than 0.5EN < 0.5 => nonpolar bond

…greater than or equal to 0.5 AND smaller than 2.00.5 EN < 2.0 => polar bond

…greater than or equal to 2.0EN 2.0 => ionic bond

Among the following examples, which bond is most polar?

Among the following examples, which bond is shortest?

Bond length depends on (a) radii of the bonded atoms

(b) the number of bonds between atoms

remember that the atomic radii decrease along a period in the P.T.

If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break?

Drawing Lewis Structures of Molecules

If the compound contains more than 2 atoms:

• how are the atoms bonded and,

• if there are nonbonding electron, where are they?

Molecules with a central atom : NH3, PCl3, CHCl3

central atom is generally the first in the molecular formula

…unless the first element is Hydrogen :

HCN C NH

(same order as in formula)

(1) sum valence electrons from all atoms: these are the ones that need to be distributed

(3) complete "octets" of atoms bound tocentral atom

(4) place any leftovers from (1) on the central atomcheck that central atom has octet

(2) connect atoms by covalent bonds: count electrons used up

(5) If there are not enough electrons to give the central atom an octet, try multiple bonds

Rules for Drawing Lewis Structures

for ions, the charge is generally indicated by

square brackets and the sign

for ions, the charge is generally indicated by

square brackets and the sign

Exceptions to the Octet rule

On occasion, an atom in a molecule does not have an octet of valence electrons:

• If the molecule has an odd number of valence electrons

• an atom may have less than an octet [mainly Be, B]

• an atom may have more than an octet [period (n) 3]

Exceptions to the Octet rule: odd number of electrons

Exceptions to the Octet rule: less than an octet

Exceptions to the Octet rule: more than an octet

Resonance

Which of the following molecules exhibits resonance?

NO3- HCN SO2

Draw Lewis structures!

If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break?

Bond Enthalpy… …is the energy required to break a covalent bond…is always positive

C (g) + 4 H (g) H = 1660 kJ/mol

C H (g) C (g) + H (g) D (C-H) = 1660/4 kJ/mol = 415 kJ/mol

per C-H bond:

Bond Enthalpy is the energy required to break a covalent bond

… and it offers yet another way to estimate the Horxn

Horxn = Σ bond enthalpies of bonds broken – Σ bond enthalpies of bonds formed

Horxn = Σ n x Dbroken – Σ m x Dformed

Estimate Ho for the following reaction:

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

Estimate Ho for the following reaction:

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

Horxn = Σ n x Dbroken – Σ m x Dformed

Horxn = [4 x 414 + 2 x 498] – [2 x 803 + 4 x 463] = - 806 kJ/mol

Chapter 8: Periodic Properties of the Elements Chemical Bonds ionic bond covalent bond metallic...

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