chem1- 4th q 1st lt

Chemical Bonding I:Basic Concepts

9.1

Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons thatparticpate in chemical bonding.

1A 1ns1

2A 2ns2

3A 3ns2np1

4A 4ns2np2

5A 5ns2np3

6A 6ns2np4

7A 7ns2np5

Group # of valence e-e- configuration

In the period 1916-1919, two Americans, G.N. Lewis and Irving Langmuir, and a German, Walther Kossel, advanced an important proposal about chemical bonding: Something unique in the electron configurations of noble gas atoms accounts for their inertness, and atoms of other elements combine with one another to acquire electron configurations like noble gas atoms.

Some fundamental ideas of Lewis theory •Electrons, especially those of the outermost (valence) electronic shell, play a fundamental role in chemical bonding.

•In some cased electrons are transferred from one atom to another. Positive and negative ions are formed and attract each other through electrostatic forces called ionic bonds.

•In other cases one or more pairs of electrons are shared between atoms, this sharing of electrons is called covalent bond.

•Electrons are transferred, or shared, in such a way that each atom acquires an especially stable electron configuration. Usually this is a noble gas configuration, one with eight outer shell electrons, or an octet.

Lewis symbol – consists of a chemical symbol to represent the nucleus and core (inner shell) electrons of an atom, together with dots placed around the symbol to represent the valence (outer shell) electrons. Write the Lewis symbol of the following elements1. Si 2. N 3. P 4. As 5. Sb 6. Bi 7. Al 8. Al 9. Al 10. I 11. Se 12. Ar Write the Lewis symbol of the following1. Sn 2. Br – 3. Na+ 4. S2-

Lewis structure – is a combination of Lewis symbols that represents either the transfer or sharing of electrons in a chemical bond.

Write Lewis structures for the following compounds(a) BaO (b) MgCl2 (c) Al2O3 (d) Na2S (e) Mg3N2 (f) calcium iodide (g)barium sulfide (h) lithium oxide

9.1

9.2

Li + F Li+ F -

The Ionic Bond

1s22s1 1s22s22p5 1s21s22s22p6[He] [Ne]

Li Li+ + e-

e- +F F -

F -Li+ + Li+ F -

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Why should two atoms share electrons?

F F+

7e- 7e-

F F

8e- 8e-

F F

F F

Lewis structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

9.4

8e-

H HO+ + OH H O HHor

2e- 2e-

Lewis structure of water

Double bond – two atoms share two pairs of electrons

single covalent bonds

O C O or O C O

8e- 8e-8e-double bonds double bonds

Triple bond – two atoms share three pairs of electrons

N N

8e- 8e-

N N

triple bondtriple bond

or

9.4

Bond Type

Bond Length(pm)

C-C 154

CC 133

CC 120

C-N 143

CN 138

CN 116

Lengths of Covalent Bonds

Bond LengthsTriple bond < Double Bond < Single Bond

9.4

9.4

H F FH

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron richregionelectron poor

region e- riche- poor

d+ d-

9.5

Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.

Electron Affinity - measurable, Cl is highest

Electronegativity - relative, F is highest

X (g) + e- X-(g)

9.5

9.5

9.5

Covalent

share e-

Polar Covalent

partial transfer of e-

Ionic

transfer e-

Increasing difference in electronegativity

Classification of bonds by difference in electronegativity

Difference Bond Type

0 Covalent

2 Ionic

0 < and <2 Polar Covalent

9.5