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Molecular Geometry and Polarity

Part A: Chemical Bonding Review

Dr. Chin Chu

River Dell Regional High School

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Nature of Chemical Bond• Atoms are held together by electrostatic attraction

between positively charged nuclei and negatively charged electron clouds.

• Chemical Bond: a link between atoms that result from mutual attraction of their nuclei for electrons.

• Bond energy: the energy required to break a bond.

• Forces in substances:– Attractive: between electron clouds and respective bonding

nuclei (of the two atoms that bond).– Repulsive: between all the electron clouds in the bonding

atoms; between positively charged nucleus.

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Formation of Bonds• Bonding involves only the valence electrons (those in the

highest energy level).

• Use the periodic chart to guide determination of valence electrons

• WHEN BONDING OCCURS:

– Atoms attain an OCTET: a stable Noble Gas configuration.

– the resulting system is at the lowest possible potential energy level.

– The process of bonding is, therefore, exothermic: energy is being released. If the energy released is Large we get a strong bond; small ΔE bond is weak

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Types of Bonds

• Ionic bond: formed by transfer of electrons from the valence energy level of one atom to another’s

• Covalent bond: formed when atoms share electrons.

• Metallic bond: ions of metals are surrounded by sea of electrons that bind all ions together.

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Lewis Theory: An Overview• Valence e- play a

fundamental role in chemical bonding.

• e- transfer leads to ionic bonds.

• Sharing of e- leads to covalent bonds.

• e- are transferred or shared to give each atom a noble gas configuration – the octet.

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Lewis Symbols (Structures)

• A chemical symbol represents the nucleus and the core e-.

• Dots around the symbol represent valence e-.

Si

N••

••

• P••

••

• As••

••

• Sb••

••

• Bi••

••

•

••Al••

Se••

•••

Ar••

••

••I •••

••

•••

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Metallic Bond

• Metals consist of crystalline lattice in which positive ions (kernels) are arranged in fixed patterns.

• The valence electrons are free to move and they belong to the entire crystal.

• “Electron Sea” model

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Metallic Bond

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Ionic and Molecular Bonds

• Formation of sodium chloride (ionic):

• Formation of hydrogen chloride (covalent):

A metal and a nonmetal transfer electrons to form an ionic bond. Two nonmetals share electrons to form a covalent bond.

Na + Na+ [ ]Cl

Cl

H + Cl

Cl

H

We know aCOVALENT bondcomes from sharing thebonding pair of electrons.

F FShared pair

(bonding pair)

F FThe nucleus of each atompulls on the bonding pair.

F FBoth atoms have equal pull,

so the bonding pair is shared equally.

H ClIf two different atoms share a bond,

one will pull more stronglyon the bonding electrons.

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H ClThe bonding electrons carry negative charge.

H ClThe closer they get to the chlorine atom,

the more negative it gets.The farther they get from the hydrogen,

the more positive it gets.

H Cl+

_

But the charge is only partial.Hydrogen has not lost the electronsas in the formation of an ion.

H ClThere is an unequal sharing of electrons.

H ClThe partial charge is denoted by a + or – and the Greek letter delta,

–+

The partial charge is denoted by a + or – and the Greek letter delta,

–+

The degree of sharing (equal to unequal)is determined by the electronegativitydifference between the two atoms.

Two atoms of equal electronegativitywill share the bond equally

F F

H ClTwo atoms with a small difference in electronegativity will share unequally,

resulting in partial charge.

3.02.1

H ClTwo atoms with a small difference in electronegativity will share unequally,

resulting in partial charge.

–+

H ClThis is a polar bond:

The bonding pair is, on average,closer to one atom.

–+

H Cl–+

Is a p o la r b o n d a co v a len t b o n d ?

FKTwo atoms with a large difference in electronegativity will result in a loss

of an electron,resulting in a full charge.

4.00.8

FK

FK

FK

FK

FK

FK

+ _

FKpositive ion negative ion

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Electronegativity

• A measure of how strongly the atoms attract electrons in a bond.

• The bigger the electronegativity difference the more polar the bond.

• 0.0 - 0.4 Covalent nonpolar• 0.4 - 1.0 Covalent moderately polar• 1.0 -1.7 Covalent polar• >1.7 Ionic

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Pg 335

Table 8-1Representative Electronegativity

Differences

Covalent: = 0

Polar:0.4 < < 1.7

Ionic > 1.7