bonding notes

Chemistry 100

Bettelheim, Brown & March

Seventh Edition

Introduction to General,

Organic and Biochemistry

Chapter 3

Chemical Bonds

Electronic Structure & Chemical Valence

An atom that loses one or more electrons becomes a positively charged ion called a cation.

An atom that gains one or more electrons becomes a negatively charged ion called an anion.

Na Na+ + 1

e-1s

_{, 2s}

_{, 2p}

_{, 3s}

_1s

_{, 2s}

_{, 2p}

_{= [Ne]+}

Mg Mg 2+ + 2

e-1s

_{, 2s}

_{, 2p}

_{, 3s}

_1s

_{, 2s}

_{, 2p}

_{= [Ne]2+}

F + 1 e-

F-1s

_{, 2s}

_{, 2p}

_1s

_{, 2s}

_{, 2p}

_{= [Ne]-}

The Octet Rule – Some Limitations

•Ions of period 1 and 2 elements with charges greater than +2 are unstable. For example, boron does not lose its three valence electrons to become B3+_{, nor does carbon lose its four valence}

electrons to become C4+_{. Below period 2, +3 ions are stable, e.g.}

Al3+_{, Sc}3+_{, Fe}3+_{, etc. form.}

•Ions of period 1 and 2 elements with charges greater than -3 are also unstable. For example, carbon does not gain four valence electrons to become C4-, _{and -3 ions only form in period 2 (N}3-_).

Ionic Bond- An atom may lose or gain enough electrons to acquire a filled valence shell and become an ion. When two ions of opposite charges are brought together an

ionic bond

Covalent Bond- An atom may share electrons with one or more other atoms so as to acquire a filled valence shell. A

covalent bond

Forming Chemical Bonds

The Lewis Model

Electronegativity & Chemical Bonds

In forming a covalent compoundthe less electronegative element shares one or more electrons with the more electronegative element. The resulting moleculeis polar because the more electronegative element gets more than an equal share.

1s

_{, 2s}

_{, 2p}

_[Ne]-

Li

+

F

Li+

F-1s

_{, 2s}

_[He]+

H

+

F

H

F

Naming Ionic Compounds

lithium ion and fluoride ion form LiF barium ion and bromide ion form BaBr₂

aluminum ion and oxide ion form Al₂O₃ potassium ion and bicarbonate ion form KHCO₃

sodium ion and phosphate ion form Na₃PO₄

Naming Ionic Compounds

Iron (III) chloride is FeCl₃ Iron(II) chloride is FeCl₂ Manganese(IV) oxide is MnO₂

Manganese(III) oxide is Mn₂O₃ Manganese(II) oxide is MnO Manganese(VII) oxide is Mn₂O₇

In every case the chemist must know the charges of ions which are constant.

For ionic compounds that contain polyatomic

ions name the positive ion first followed by the

name of the negative ion:

NH₄NO₃is ammonium nitrate CaCO₃is calcium carbonate NaH₂PO₄is sodium dihydrogen phosphate

NaOH is sodium hydroxide CuCO₃is copper(II) carbonate Fe₂(CO₃)₃is iron(III) carbonate

FeSO₄is iron(II) sulfate

Naming Ionic Compounds

Lewis Dot Electron Configurations

in putting together covalently bonded compounds.

1A

2A

3A

4A 5A

6A

7A 8A

O

Be

B

C

F

Ne

Ar

Cl

S

Si

Al

Mg

Li

Na

H

He

P

N

Drawing Lewis Dot Structures

C

H

H

F

F

H

C

F

H

F

Hydrogen and fluorine form only one bond each, they must always be on the outer perimeter of the molecule.

Drawing Lewis Dot Structures

Carbon is nearest the center, Oxygen has two unpaired electrons, hydrogen has only one.

Connect Carbon and Oxygen, then add hydrogens.

H C

H

H H

O

C

H

H

H

O

H

Always connect the higher valence elements before adding monovalent elements.

Drawing Lewis Dot Structures

Connect the Carbon and Oxygen, since there are only two hydrogens, connect a second pair of C & O electrons.

C

H

H

O

H C

H

O

a double bond

When two atoms share two pairs of electrons a double bond is formed.

Drawing Lewis Dot Structures

a triple bond

C

H

N

H C N

Draw the structure of acetylene: H₂C₂

Connect the two Carbons so the remaining unpaired electrons match the number of hydrogens.

a triple bond

H C

C

H

H

C

C

H

Drawing Lewis Dot Structures

Write the structure of Phosphorus(V) Fluoride: PF₅

P

_F

F

F

F

F

_F

P

F

F

_F

F

We split the pair on phosphorus and bonded fluorine atoms to each unpaired electron. The result was a compound with 10 electron pairs around phosphorus.

Drawing Lewis Dot Structures

Connect the oxygen atoms to sulfur by using two models of oxygen from the table of electron configurations.

H

S

O

O

_O

O

H

H

S

O

O

O

O

H

A molecular compoundis a compound in which all bonds are covalent. When naming binarymolecular compounds the less electronegative element is named first (it is generally written first in the formula). The prefixes “mono-”, “di-”, tri-”, “tetr-”, etc. are used to show the number of atoms of each element; “mono-” is commonly omitted when it refers to the first atom, and is rarely used with the second atom: CO is carbon monoxide; NO is nitrogen oxide

SF₂is sulfur difluoride, N₂O is dinitrogen oxide N₂O₄is dinitrogen tetroxide, NO₂is nitrogen dioxide

Naming Molecular Compounds

The Geometric Structures of Molecules

Each involvement creates a negatively charged region of electron density around the nucleus.

Because like charges repel each other, the various regions of electron density around an atom spread so that each is as far away from the others as possible while remaining connected to the atomic core.

The shape of a molecule reflects this distribution of electron density about each atom.

The Geometric Structures of Molecules

Predicting the shape of a molecule of CH₂F₂ : Count the regions of electron density about the element(s) nearest the center of the Periodic Chart.

Then apply the following criteria: 2 = linear, angles 180E;

3 = trigonal, angles 120E

4 = tetrahedral, angles 109.5E

5 = trigonal bipyramid, angles 120Eand 90E

The Geometric Structures of Molecules

Predicting the shape of a molecule of CH₂F₂ : Count the regions of electron density about the element nearest the center of the Periodic Chart. 4 = tetrahedral, angles 109.5E

H

C

F

H

F

4

C

H

H

F

F

The Geometric Structures of Molecules

molecule of CH₃OH

Predicting the shape of a molecule of H₂O

H C

H

H H

O

4 +4

H

C

H

H

O

H

H

H

O

H

O

H

4

The Geometric Structures of Molecules

H C

H

O

a double bond

3

_C

H

H

O

Predicting the shape of a molecule of formaldehyde: H₂CO

Predicting the shape of a molecule of acetylene: H₂C₂

a triple bond

The Geometric Structures of Molecules

a molecule of PF₅

P F

F F _F

F

5 P

F

F

F _F

F

Predicting the shape of a molecule of SF₆

S

F

F

F

F

F

_F

S

F

F

F

F

F

F

6

Geometry and Polarity of Molecules

This is depicted by:

_δ

₊

_δ

_-

When the effect of all the arrows along bonds are added vectorally the result is a “dipole moment”. This means the molecule is polar. In some molecules the bond dipoles cancel and the molecule is non-polar.

Geometry and Polarity of Molecules

Water is polar:

H O

H Carbon dioxide is not polar:

O C O

zero dipole moment

C H

H F

F CH₂F₂ is polar: