CHEMICAL BONDSCHEMICAL BONDS

CHAP 9CHAP 9

Three Types of Chemical BondsThree Types of Chemical Bonds(1) Ionic bonds

• Electrons are transferred from one atom to another

• Occur between a metal atom and a nonmetal atom

(2) Covalent bonds

• Electrons are shared between two atoms• Occur between two nonmetal atoms

(3) Metallic bonds

• Metal atoms share many electrons in a “sea” that is free to move throughout the metal

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 electrons

Formation of an ionic bond

+ e−

Net reaction: Na + Cl → Na+Cl− + energy

This energy is called the heat of formation

Fig. 9.6 Model of the sodium chloride crystal

−+

No molecules in an ionic compound!No molecules in an ionic compound!

Na+ ionsCl− ions

NaCl is a“formula unit”

Fig. 9.7 The cubic crystal structure of sodium chloride

• Ionic compounds consist of a combination of positive ions and negative ions

• Ions are formed as metal atoms lose and nonmetal atoms gain electrons to achieve stable noble gas structure

• The sum of the charges on the positive ions and negative ions in each formula unit must equal zero

• How do we determine the formula for an ionic compound?

• Key: All ionic formulas must be neutral!

5 6 7

8

3 4

1

2

Fig 8.19 The Periodic Table

8

Formula of Ionic Compounds

Al2O3

2 x +3 = +6 3 x -2 = -6

Al3+ O2-

CaBr2

1 x +2 = +2 2 x -1 = -2

Ca2+ Br-

Al and O

Ca and Br

9

Covalent bond - 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

electronic structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

Fig. 9.8 Overlap of two atomic orbitals to form amolecular orbital

11

8e-

H HO+ + OH H O HHor

2e- 2e-

Electronic 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 N8e-8e-

N N

triple bondtriple bond

or

How do we know if a bond between two atoms will be ionic or covalent?

Electronegativity - ability of an atom in a chemical bond to attract the electrons to itself

In general:

• metal and nonmetal → ionic bond• nonmetal and nonmetal → covalent bond

Specifically:

• we use:

Fig. 9.10 Electronegativities of the elements

Very activemetals

Very activenonmetals

14

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

0.5 ≥ Covalent

≥ 1.7 Ionic

0.5 - 1.7 Polar Covalent

Table 9.5

Fig. 9.11 Electron distribution and kinds of bonding

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

H F

electron richregion

electron poorregion

Box Fig 9.1 How a microwave oven cooks food

18

Metallic Bonds

• Held together by metallic bonds• Malleable and ductile• Good conductors of heat and electricity

Cross Section of a Metallic Crystal

nucleus &inner shell e-

mobile “sea”of e-

19

Composition of Ionic Compounds

Common names:

“salt”→ sodium chloride

“lye” → sodium hydroxide

“rust” → iron oxide

“baking soda” → sodium bicarbonate

“chalk” → calcium carbonate

Fig. 9.12 Three substances containing sodium and some form of the carbonate ion