© prentice hall 2001chapter 11 bonding outer shell = valence electrons octet rule - an atom is most...

© Prentice Hall 2001 Chapter 1 1

Bonding Outer shell = valence electrons Octet rule - An atom is most stable if it has an outer

shell of eight electrons and no electrons of higher energy

Elements at the left of the periodic table generally have one or two electrons in excess of a stable noble gas structure (octet of electrons)

These electrons are easily removed - ionization energy is low

Such elements are electropositive

Li Li + e


Bonding Elements at the right of the periodic

table generally are just one or two electrons short of a noble gas structure (octet of electrons)

These elements easily add electrons

F + e F


Ionic Bonds Opposite charges attract Attractions between ions hold a crystal

together and are called ionic bonds


Covalent Bonds Instead of giving up or acquiring

electrons, an atom can also achieve eight electrons by sharing

covalent bond

F + F F F


Covalent Bonds Other covalent bonds

H OH

2 H + O


Polar Covalent Bonds In the fluorine-fluorine bond as well as in the hydrogen-

hydrogen bond, electrons are shared equally

In hydrogen fluoride and in water, electrons are attracted more toward one atom


Polar Covalent Bonds This tendency of atoms to attract electrons is

known as electronegativity There is a continuum of bonding types

ionic bond polar covalent bond nonpolar covalent bond


Electronegativity


Lewis Structures The chemical symbols we have been

using in which valence electrons are shown as dots are called Lewis structures


Drawing Lewis Structures

Write the symbols for the elements in the correct structural order

Consider nitric acid, HNO3



Calculate the number of valence electrons for all atoms in the compound

1 H @ 1 electron = 1

3 O @ 6 electrons = 18

1 N @ 5 electrons = 5

Total = 24



Put a pair of electrons between each symbol - at least one bond needed between each atom and its neighbor



Beginning with the outer atoms, place remaining electrons in pairs around atoms until each has eight (except for hydrogen)



If you run out of electrons before each atom (other than hydrogen) has eight electrons, move unshared pairs to form multiple bonds


Formal Charges

A bookkeeping system for electrons

Used to show the approximate distribution of electron density in a molecule or polyatomic ion


Formal Charges

Assign each atom half of the electrons in each pair it shares


Formal Charges Also give each atom all electrons from its

unshared pairs6

7

46

1


Formal Charges Subtract the number of assigned electrons from

the number of valence electrons for an uncombined atom of the same element

6 - 6 = 0

6 - 7 = -1

5 - 4 = +16 - 6 = 0

1 - 1 = 0


Formal Charges The algebraic sum of all formal charges

on a species (molecule or ion) must equal the actual charge on the species Zero for molecules Positive for cations Negative for anions


Kekulé Structures In most cases we show electron pairs between

atoms as bonds and represent them with a dash

Also, unless there is a particular need to show unpaired electrons, we generally do not show them

becomes

H O N O

H O N O


Condensed Structural Formulas

Kekulé formulas also are called structural formulas

Often, structural formulas are condensed

C C

H

H

C

H

H

H

H

H

H

becomes

CH3CH2CH3

© prentice hall 2001chapter 11 bonding outer shell = valence electrons octet rule - an atom is most...

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pair of electrons

electrons short

unpaired electrons

number of valence electrons

number of assigned electrons

place remaining electrons

drawing lewis structureswrite

drawing lewis structuresput