CHEMICAL BONDINGChapter 3

Pages 78-84

INTRODUCTION TO BONDING

Definition The forces that hold atoms and molecules

together are called chemical bonds. Bonds are responsible for the structure and

many properties of matter. Patterns result from similar bonding relationships.

PATTERNS OF THE PERIODIC TABLE- ELECTRONEGATIVITY

Describes the relative ability of an atom’s nucleus to attract a pair of bonding electrons in the valence level.

Metals want to lose electrons so the more reactive metals will have a lower electronegativity

Nonmetals want to gain electrons so the more reactive nonmetals will have a higher electronegativity

When one atom loses an electron (or electrons) and the other atom gains an electron (or electrons), and ionic bond is created!

Electronegativity increases as you move from left to right.

Electronegativity decreases as you m

ove down each

column.

Two factors influence electronegativitysize of the atom: larger smaller electronegativity

# of valence electrons: more valence e- larger electronegativit

NONMETAL EXAMPLE:

The valence e- are close to the nucleus, so the protons pull strongly to complete the octet.

The electronegativity = 4.0

7e-

2e-

F9p+

METAL EXAMPLE:

Since the valence e- are farther from the nucleus, the protons do not pull as strongly and the electrons can move away easily.

The electronegativity = 1.0

2 e-

8 e-

8 e-

2 e-

Ca20p+

Li F

Li F

A Li Atom An F Atom

A Li+ Ion

-

+

An F- Ion

VALENCE ELECTRONS

The periodicity is related to the number of valence electrons in the atom (valence # = last digit of group #). Chemical reactions are thought to only involve the valence electrons.

Electrons occupy orbitals within the valence level. Orbitals are regions of space where electrons are most likely to be found.

There are 4 orbitals in the valence level and each orbital can contain 0, 1, or 2 electrons.

Electrons occupy any empty orbital before pairing up.

A maximum of 8 electrons can occupy the entire valence level. This is called the octet rule.

X1 5

84

2 6

7 3

This can be drawn as a Lewis dot diagram such as: Mg has 2 valence e-

Mg

N has 5 valence e-

 

C has 4 valence e-

N

C

A SINGLE ELECTRON IN AN ORBITAL IS A BONDING ELECTRON. A LONE PAIR IS A PAIR OF ELECTRONS FILLING ON ORBITAL. THESE ELECTRONS DO NOT FORM BONDS.

iv. O has 6 valence e-

O

IONIC BONDS

If the electronegativities of two nuclei are very different, the atom with the stronger electronegativity may remove the bonding electron from the other atom.

An electron transfer occurs and positive and negative ions are formed. The ionic bond results from the attraction of positive ions (cations) and negative ions (anions) in a 3 dimensional crystal array.

The formula of an ionic compound represents the simplest unit of the crystal lattice.

BECAUSE OF THE CRYSTAL ARRAY OF IONIC COMPOUNDS, THE SOLIDS ARE VERY HARD, AND HAVE HIGH MELTING AND BOILING POINTS.

Na+ Cl-

Na Cl

• When Electronegativities of bonding atoms are the same (as they are in diatomic molecules) or close to the same, they SHARE electrons. Bonds formed when atoms share electrons are

called Covalent Bonds.

• In diatomic molecules (like H2 or Cl2), the electronegativities of both atoms are exactly the same so electrons are shared equally!

In Covalent bonds, electrons are

Shared

• Covalent bonds in large networks (Network Bonding) gives rise to substances with very high melting points.

Diamonds are “forever”!

Some melting points of Network Solids:

Diamond (Carbon) 3550 oCSilicon Carbide (SiC) 2700 oCBoron Nitride (BN) 3000 oC

Covalent bonds are very strong!

In Class Assignment

Page 82 questions 2-4 We will check our answers before the end of

class

Homework Handout: Lewis Dot Diagrams