The Nature ofThe Nature ofChemical BondsChemical Bonds

Chapter 12Chapter 12

Homework AssignmentHomework AssignmentChap 12Chap 12

Review Questions (p 262): 1 – 24Review Questions (p 262): 1 – 24

Multiple Choice Questions: 1 - 10Multiple Choice Questions: 1 - 10

Electron Shells and Chemical Bonding

Valence electrons

Group 1A Group 7A

Full capacity of a shell = 2n2

Show one dot for each valence electron in the atom

Fig 12.3

Fig 8.20 Electron dot notation for representative

elements1

5 6 7

8

3 42

Valence electronsValence electrons – outermost electrons in an atomOctet ruleOctet rule – atoms attempt to acquire 8 valence electrons

Closer look at the inert gases

• Inert gas atoms have filled valence shells

• Except for He, all have 8 valence electrons in4 nonbonding pairs

• Octet rule: An atom will gain. lose, or sharevalence electrons until it is surroundedby 8 valence electrons.

Example:

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 + Cl Na+ Cl -

The Ionic Bonds and Ionic Compounds

Na Na+ + e-

e- + Cl Cl -

NaCl• Opposite charges hold ions together

• Form between metals and nonmetals

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. 12.7(a) Model of the sodium chloride crystal

−+

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

Na+ ionsCl− ions

Formula unit

Fig. 12.7(b) The crystal structure of sodium chloride

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

• Ions are formed as nonmetal atoms gain and metal atoms lose 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?