CHAPTER 7Chemical Bonding

Attractive forces that hold atoms together in compounds. The electrons involved in bonding are usually those in the outermost (valence) shell.

Most elements in compounds want to gain noble gas configuration. They will do so by either losing or gaining electrons (ionic compounds) or by sharing electrons (covalent compounds)

Chemical bonds

Chemical bonds are classified into two types:

Ionic bonding results from electrostatic attractions among ions; which are formed by the transfer of one or more electrons from one atom to another. (metals low with nonmetals high )χ χ

Covalent bonding results from sharing one or more electron pairs between two atoms.

(nonmetals only similar )χ

Ionic and Covalent bonding

Comparison of Ionic & Covalent Compounds

Ionic Covalent Melting Pt

Solubility◦ (polar solvents)

Solubility◦ (nonpolar solvents)

Conductivity◦ (molten & aqueous

solutions)

High Low

Soluble Insoluble

Insoluble Soluble

High Low

2 extremes in bonding

pure covalent bonds◦ electrons equally shared by the atoms

pure ionic bonds ◦ electrons are completely lost or gained by one of

the atoms

most compounds fall somewhere between these two extremes

Ionic vs. Covalent bonding

# of atoms in the molecule◦ Monatomic = 1 atom Ex. He◦ Diatomic = 2 atoms Ex. O2 ◦ Triatomic = 3 atoms Ex. O3 ◦ Polyatomic = many Ex. H2SO4 or S8

Homonuclear: the mlcl is composed of only 1 kind of atom: O2, H2, P4

Heteronuclear: the mlcl is made up of more than 1 kind of atom: H2O

Terminology

Lewis Dot Representations of Atoms

Li Be B C N O F Ne.... .. ..

..HeH

.

.. . .

.. ....

...

..

.. .. .. .... ..

.

or Lewis dot formulas, a convenient bookkeeping method for valence electrons (electrons that are transferred or involved in chemical bonding) Only the electrons in the outermost s and p orbitals are shown as dots.

Li & Na. .

N & P.. ..

..

. ..

. F & Cl...

....

.

... ..

.

elements in the same group have same Lewis dot structures

For groups IA – VIIIA, the group number equals the # of valence electrons

Valence electrons determine the chemical and physical properties of the elements as well as the kinds of bonds they form.

metals react with nonmetals to form ionic compounds

cations or positive (+) ions (metals)◦atoms have lost 1 or more electrons

anions or negative (-) ions (nonmetals)◦atoms have gained 1 or more electrons

Ionic Bonding

C842 MPsolid gas metal

white yellow silver

LiF2 F Li2 - VIIA IA

o

(s)2(g)(s)

We can use Lewis formulas to represent the neutral atoms and the ions they form.

Li + F.....

.. . Li+ F[ ]..

.... ..

underlying reasons for LiF formation

1s 2s 2pLi ­¯ ­F ­¯ ­¯ ­¯­¯­

becomesLi+ ­¯ [He]F- ­¯ ­¯ ­¯ ­¯ ­¯ [Ne]

Li+ ions contain two electrons◦ same number as helium

F- ions contain ten electrons ◦ same number as neon

Li+ ions are isoelectronic with heliumF- ions are isoelectronic with neon

Isoelectronic species contain the same number of electrons.

cations become isoelectronic with preceding noble gas

anions become isoelectronic with following noble gas

....

... F..

. F....

Be .. Be2+

2 F

.... ....

IIA metals with VIIA nonmetals, mostly ionic compounds ~ exceptions - BeCl2, BeBr2, BeI2 these are covalent compounds

Be(s) + F2(g) BeF2(s) electronically this is happening

similarly for all of the IIA & VIIAM(s) + X2 M2+ X2

-

IA + VIIA MXIIA + VIIA MX2

IIIA + VIIA MX3

IA + VIA M2X IIA + VIA MXIIIA + VIA M2X3

NaF

BaCl2

AlF3

Na2O

BaO

Al2S3

SUMMARIZING TABLE Groups Gen. Form. Example

IA + VA M3X IIA + VA M3X2

IIIA + VA MX

Na3N

Mg3P2

AlN

SUMMARIZING TABLE Groups Gen. Form. Example

H forms ionic compounds with IA and IIA metalsLiH, KH, CaH2, BaH2,, etc.

other H compounds are covalent

extended three dimensional arrays of oppositely charged ions

high melting points because coulomb force is strong

Structures of Ionic Compounds

Coulomb’s Law

ions ofcenter between distance dionson charge of magnitude q

ionsbetween attraction of force F

whered

qqF 2

~ ions with high charges F is large

~ ions with small charges F is small

arrange these compounds in order of increasing attractions among ions

KCl, Al2O3, CaOK+Cl- < Ca2+O2- <Al23+O32-

covalent bonds formed when atoms share electrons

share 2 electrons - single covalent bondshare 4 electrons - double covalent bondshare 6 electrons - triple covalent bond

attraction is electrostatic in nature◦ lower potential energy when bonded

Covalent Bonding

Covalent bonding may be explained by 2 different theories

◦ Valence bond (VB) theory: each atom has electrons in atomic orbitals which overlap to form bonds (Ch. 8)

◦ Molecular orbital (MO) theory: the electrons belong to the molecule as a whole and are in molecular orbitals instead of belonging to each atom (Ch. 9)

The element needing the most electrons to fill its octet is usually the central atom

The most symmetrical skeleton is usually correct

Halogens and H always share one electron to complete outer shell

In ternary acids, H are bonded to O (ternary acids are oxy-acids: they contain H, O, and another nonmetal)

General rules for Lewis Dot Diagrams for Covalent bonds

Carbon always obeys the octet rule

Carbon rarely has lone pairs of electrons. Exception: If it’s at the end of a molecule or ion. Ex. CN- , CO, CNO

When forming multiple bonds between atoms, both atoms donate the same number of electrons

Oxygen atoms normally bond to other nonmetals, not to each other

Oxygen can do several things depending on the mlcl.◦ Single bond by sharing an electron

◦ Single bond by accepting 2 electrons from another atom and not sharing at all

◦ Double bonds by sharing 2 of its electrons

homonuclear diatomic molecules◦ hydrogen, H2

◦ fluorine, F2

◦ nitrogen, N2

Pure covalent bonds - Nonpolar Covalent Bonds

H HorH H..

F F.. .. ....

..

.. .. F F.. .... ..

.. ..or

N N········ ·· N N·· ··or

nonpolar covalent bonds - electrons are shared equally

symmetrical charge distribution - must be the same element to

share exactly equally

+H. H . H H.. or H2

Lewis dot representation H2 molecule formation

heteronuclear diatomic molecules

hydrogen halides◦ hydrogen fluoride, HF

◦ hydrogen chloride, HCl

◦ hydrogen bromide, HBr

Polar Covalent bonds - Unequal sharing of electrons

or ··H F··

··H F..

·· ····

or ··H Cl··

··H Cl..

·· ····

or ··H Br··

··H Br..

·· ····

polar covalent bonds - unequally shared electrons

• assymmetrical charge distribution

• different electronegativities

Some bonds are very polar, Ex. HF

bondpolar very 1.9 Difference

4.0 2.1 ativitiesElectroneg F H

1.9

Polar Covalent Bonds Electron density map of

HF◦ blue areas - low electron

density◦ red areas - high electron

density

polar molecules have separation of centers of negative and positive charge

bondpolar slightly 0.4 Difference

2.5 2.1 ativitiesElectronegI H

0.4

Some bonds are only slightly polar, ex. HI

Polar Covalent BondsElectron density map of

HI◦ blue areas - low electron

density◦ red areas - high electron

densitynotice that the charge

separation is not as big as for HF◦ HI is only slightly polar

Representative elements achieve noble gas configurations in most of their compounds.

Lewis dot formulas are based on the octet rule.

H needs two electrons to have Helium's noble gas configuration, everything else wants 8

The Octet Rule

water, H2O ammonia molecule , NH3

ammonium ion , NH4+ hydrogen cyanide, HCN

sulfite ion, SO32-

Lewis Dot Formulas for Molecules and Polyatomic Ions

Two or more Lewis dot diagrams are needed to describe the bonding in a molecule or ion.

LDD for sulfur trioxide, SO3

Resonance

Resonance

three possible structures for SO3

invoke resonance◦ Double-headed arrows are used to indicate

resonance formulas.

O S

O

O·· ···· ·· ··

······

OS

O

O·· ···· ·· ··

··

······

O S

O

O·· ···· ·· ··

····

Resonanceflaw in our representations of molecules

no single or double bonds in SO3

all bonds are the same

best picture OSO

O

--- ---

The concept of formal charges helps us choose the correct Lewis structure for a molecule. If a resonance structure has a high formal charge it’s not a very good one.

Formal charge = group # - e- you can assign

to that atom

Or F.C. = (valence e- ) – (# of bonds + # of

unshared e- )pg 289

Formal Charges

Sigma bonds (σ) : result of head-on (end to end overlap, there is a free rotation around σ bonds.

Pi bonds (π) : result of side-on overlap of p orbitals. There is no free rotation around a π bond. The side –on overlap locks the molecule into place.

All single bonds are sigma bonds: 1σ bondAll double bonds: 1 σ bond, 1 π bondAll triple bonds: 1 σ bond, 2 π bonds

Sigma and Pi bonds

Limitations of the Octet Rule for Lewis Formulas species in which the central element must

have a share of more or less than 8 valence electrons to accommodate all substituents

compounds of the d- and f-transition metals

In cases where the octet rule does not apply, the elements attached to the central atom nearly always attain noble gas configurations. ◦ The central atom does not

Write LDD for BBr3

Write LDD for AsF5

Write LDD for XeF4

Limitations of the Octet Rule for Lewis Formulas

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