The Nature of Chemical Bonds Chapter 12

Homework Assignment Chap 12 Review Questions (p 262): 1 – 24 Multiple Choice Questions: 1 - 10

Electron Shells and Chemical Bonding Valence electrons Full capacity of a shell = 2n2 Group 1A Group 7A

Show one dot for each valence electron in the atom Fig 12.3 Show one dot for each valence electron in the atom

Fig 8.20 Electron dot notation for representative elements 1 8 3 4 5 6 7 Valence electrons – outermost electrons in an atom Octet 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 in 4 nonbonding pairs Octet rule: An atom will gain. lose, or share valence electrons until it is surrounded by 8 valence electrons.

Example:

Three Types of Chemical Bonds 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 Metallic bonds Metal atoms share many electrons in a “sea” that is free to move throughout the metal

The Ionic Bonds and Ionic Compounds Na Na+ + e- e- + Cl - Cl - Na + Cl Na+ NaCl Opposite charges hold ions together Form between metals and nonmetals

This energy is called the heat of formation Formation of an ionic bond Na 11 protons 11 electrons Na+ 11 protons 10 electrons + e− Cl- 17 protons 18 electrons Cl 17 protons 17 electrons 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! Formula unit + − Na+ ions Cl− ions

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

Ionic compounds consist of a combination of 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?