forming chemical bonds - mister chemistrys o se te po cl f br i at he ar ne kr xe rn 1a 2a 3a 4a 5a...

Forming Chemical Bonds

Basic Concepts of Chemical

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© 2012 Pearson Education, Inc.

Chemical Bonds• Three basic types of

bonds•

Covalent• Sharing of

electrons.

Metallic• Metal atoms bonded to

several other atoms.

Ionic• Electrostatic attraction

between ions.

Types of Chemical Bonds

Why do atoms form chemical bonds ?

so that the system can achieve the lowest possible potential energy

H • H•

Example covalent bonding in H2

Distance of separation between atoms

0

Pote

ntia

l ene

rgy

Ionic Bonding

especially prevalent in compounds formed between group 1A and 2A elements with group 6A and 7A elements.

between Elements with the biggest difference in electronegativity

Ionic compounds

Ionic compounds are usually formed between metals and nonmetals.

Ionic and Molecular Compounds

Molecular compounds are usually formed between two nonmetals.

When electrons are removed from or added to a neutral atom or molecule, a

charged particle called an ion is formed.

Ions

Positively charged ions are calledcations

Negatively charged ions are calledanions

Atoms vs Ions

Na: 11 protons; 11 electrons

Cl: 17 protons; 17 electrons

Cl–: 17 protons; 18 electrons

–

Na+: 11 protons; 10 electrons

+

Lewis Dot Symbols

A Lewis dot symbol consists of the symbol of an element and one dot for each valence electron in an atom of the element.

H

Na

Li

K

Rb

Cs

Fr

Mg

Be

Ca

Sr

Ba

Ra

Al

B

Ga

In

Tl

Si

C

Ge

Sn

Pb

P

N

As

Sb

Bi

S

O

Se

Te

Po

Cl

F

Br

I

At

He

Ar

Ne

Kr

Xe

Rn

1A 2A 3A 4A 5A 6A 7A 8A

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Formation of LiF

Li(s) + 1/2 F2(g) LiF(s)

1s2 2s1 1s2 2s22p5 1s2 1s2 2s22p6

. .. : . .

.Li F+ Li+ . .: : . .

F-

the ions are packed together to maximize (+) (-) attractions and minimize (+) (+) and (-) (-) repulsions

Solid structure of LiF

Crystal lattice

Coulomb’s Law

Energy of electrostatic attraction is directly proportional to product of charge and inversely proportional to distance

therefore, strong lattices are favored when the ions have a high charge to size ratio

k = (2.31 x 10-19 J nm)

Coulombic attraction between

two ions

Q1Q2

r= k

Coulomb’s Law

Energy of electrostatic attraction is directly proportional to product of charge and inversely proportional to distance

= k Q1Q2

r

therefore, strong lattices are favored when the ions have a high charge to size ratio

small ion size

highly chargedCoulombic

attraction between two ions

Coulomb’s Law Example:

sodium-chlorides energy of interaction is equal to


two ions

(Na+)(Cl-)r( )= (2.31 x 10-19 J nm)



two ions

(+1)(-1)r( )= (2.31 x 10-19 J nm)


0.276 nm= - 8.37 x 10-19 J

The negative sign indicates an attractive force

i.e. the ion pair has a lower potential energy



two ions

(+1)(-1)( )= (2.31 x 10-19 J nm)


Lattice Energy of IonicCompounds


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Energetics of Ionic Bonding

As we saw in the last chapter, it takes 496 kJ/mol to remove electrons from sodium.


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We get 349 kJ/mol back by giving electrons to chlorine.


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Na+496

more energy required to remove the electron than is released


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Energetics of Ionic Bonding the numbers don’t explain why the reaction of

sodium metal and chlorine gas to form sodium chloride is so exothermic!


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• There must be a third piece to the puzzle.

• What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.

Na+ Cl-


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Lattice Energy• This third piece of the puzzle is the lattice

energy:

The energy required to completely separate a mole of a solid ionic compound into its gaseous ions.

Eel = κ Q1Q2d

• The energy associated with electrostatic interactions is governed by Coulomb’s law:

the energy released when an ionic solid forms from its ions

Lattice energy

negative sign (-)


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Lattice Energy

• Lattice energy, then, increases with the charge on the ions.

• It also increases with decreasing size of ions.

Lattice energies of some ionic compounds

compound lattice energy (kJ/mol

lattice energy (kJ/molcompound

-632KI KF

KBr

RbF

NaBr

NaCl

NaI

-671

-774

-681

-732

-769

AgCl

NaF

LiF

SrCl2

MgO

-808

-910

-910

-1030

-2142

-3795

K+ I- Mg2+ O2-

larger ions smaller ionssmaller L.E. = larger L.E. =

Li(s) + 1/2F2(g) LiF(s)

Li(g)

+155 kJ

Li+(g)

+520 kJ

F(g)

+75 kJ

F–(g)

–333 kJ

–594 kJ

–1012 kJ

Formation of ionic compounds

requires the lattice energy to be sufficiently large to overcome ionization energy of the element that forms the cation.

balance between energy input (ionization energies) and stability gained from formation of the solid

Formation of ionic compounds

The main impetus for the formation of an ionic compound rather than a covalent compound results from the strong mutual attraction among the ions

Example

Ionization energies ( I.E.)

Mg(g) Mg2+(g) ΔH = I.E.1 +I.E.2+ 2e-

= 753 +1435 = +2180 kJ/mol

ΔH = I.E.1 + I.E. 2 + I.E. 3

bonds in AlCl3 are polar covalent

Al (g) Al3+(g) + 3e-

= 580 +1815 + 2740 = +5135 kJ/mol

bonding in MgCl2 is ionic;

Example

bonds in AlCl3 are polar covalent

Al (g) Al3+(g) + 3e-

energy input (ionization energies) out weighs stability gained from formation of an ionic solid

ΔH = I.E.1 + I.E. 2 + I.E. 3= 580 +1815 + 2740 = +5135 kJ/mol

Ionic compounds

Any compound when melted that conducts electricity is considered ionic

NaCl( s) is made up of Na+ and Cl- ions

Example : NaCl(s) NaCl( l)

forming chemical bonds - mister chemistrys o se te po cl f br i at he ar ne kr xe rn 1a 2a 3a 4a 5a...

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