Chemical Bonds

CHEMICAL BONDS

Rachel Patricia B. Ramirez

Why bind?

Bonding lowers the potential energy between positive and negative particles

Chemical Bonds

Attractive forces that hold atoms together in compounds.

The electrons involved in bonding are usually those in the outermost (valence) shell.

Types of Chemical Bonding

IONIC bonding results from electrostatic attractions among ions.Transfer of one or more electrons

Between atoms with large differences in their tendency to lose or gain electrons

Interaction of metals (Grps 1 and 2) with nonmetals (Grps 17 and 16)


COVALENT bonding results from sharing one or more pairs between two atoms

Each nonmetal atom holds onto its own electrons tightly and tends to attract other electrons as well

Attraction of each nucleus for the valence electrons of the other draws the atomstogether


METALLIC bonds

All the metal atoms in a sample pool their valence electrons into an evenly distributed “sea” of electrons that “flows” between and around the metal-ion cores and attracts them together.

Bonding electrons are free to move (delocalized) throughout the three-dimensional structure

Lewis Electron-Dot Symbols

Gilbert Newton Lewis (1875 – 1946)

Lewis electron-dot symbols

LiNucleus andinner electrons

Valence electron


Elements that are in the same group have the same Lewis electron dot symbols.

The specific positions of the paired and unpaired dots are arbitrary.

Lewis Symbols and Bonding

Metaltotal # of dots = # of electrons it loses

to form a cation

Nonmetal# of unpaired dots = # of electrons that

become paired (gaining or sharing)

Lewis Symbols and Bonding

Simple way of showing the valence electrons of atoms and tracking them in the course of bond formation

The Lewis electron-dot symbol of an atom depicts the number of valence electrons for a main-group element.


Element Electron Lewis Configuration Symbol

Li [He] 2s1 Li

Be [He] 2s2 Be

B [He] 2s2 2p1 B

C [He] 2s2 2p2 C

Check-Up

Write Lewis symbols for the following elements: N, P, As, Sb

Al, Se, I, Ar

Writing Lewis Symbols

N, P, As, Sb, Bi

These are the elements of group 15.

Their atoms have all have five valence electrons (ns2 np3).

The Lewis symbols all have five dots.


Al, Se, I, Ar Al is in group 13 Se in group 16 I is in group 17 Ar in group 18


For main-group elements, the number of valence electrons, and hence the number of dots appearing in a Lewis symbol, is equal to

The group number for the s-block elements

The “group number minus 10” for the p-block elements

Group 1 2 13 14 15 16 17 18

Lewis Dot Formulas for Representative Elements

The Ionic Bonding Model

Transfer of electrons from metal to nonmetal to form ions that come together in a solid ionic compound. NaCl crystals consist of Na+

and Cl- ions held together by electrostatic attractions

Cl-

Na+

Formation of Ionic Compounds

An ion is an atom or a group of atoms possessing a net electrical charge

Types: Positive (+) ions or CATIONS

Atoms have lost 1 or more electrons

Negative (-) ions or ANIONS Atoms have gained 1 or more electrons

Formation of Ionic Compounds

Monatomic ions (one atom). Na+, Ca2+, Al3+ (cations) Cl‾, O2‾, N3‾ (anions)

Polyatomic ions (more than one atom) NH4

+ , H3O+ (cation)

NO2‾, CO32‾, SO4

2‾ (anions)

Reaction of Group 1 metals with Group 17 Nonmetals

Grp1 Grp17

Metal Nonmetal

2 Li (s) + F2(g) → 2 LiF(s)

silver yellow white solid

solid gas which melt at

842°C

Bonding and Electron Configuration

Atom 1s 2s 2p

Li F

Ion 1s 2s 2p

Li+ [He]

F‾ [Ne]

valence electrons

Bonding and Lewis Symbols

Lewis symbols can be used to represent the neutral atoms and the ions they form

Li F FLi++


The Li+ ion contains 2 electrons, same as the helium atom. Li+ ions are isoelectronic with helium

The F‾ ion contains 10 electrons, same as the neon atom F‾ ions are isoelectronic with neon

NOTE: Isoelectronic species contain the same number of electrons

Check-Up

The reaction of potassium with bromine is another example of a Group 1 metal reacting with a Group 17 nonmetal. Write the reaction equation. Write the electron configuration of the atoms

and the ions. Write the Lewis symbols for the reaction.

General Trend

Cations become isoelectronic with the preceding noble gas.

Anions become isoelectronic with the following noble gas.

General Trend

In nearly every main-group element that forms a monatomic ion, the configuration has a filled outer level of electrons (either two or eight), the same number as in the nearest noble gas.

Octet Rule

When atoms bond, they lose, gain, or share electrons to attain a filled outer shell of eight (or two) electrons.

The octet rule holds for all the compounds of Period 2 elements and a large number of others as well.

An octet of electrons consists of full s and p subshells on an atom


Grp1 Grp17

Metal Nonmetal

2 Li (s) + F2(g) → 2 LiF(s)

silver yellow white solid

solid gas which melt at

842°C

General Representation

Reaction equation2 M(s) + X2 → 2 M+ X‾ (s)

where:M is a metal (Li to Cs)

X is a nonmetal (F to I)

ns np ns np

M → M+

X → X‾


Grp2 Grp17

Metal Nonmetal

Be(s) + F2(g) → BeF2(s)

2s 2p 2s 2p

Be [He] → Be2+

F [He] → F ‾


Be Be2++

F

F

F

F


Reaction equation

M(s) + X2 → M2+ X2‾

where:M is a metal (Be to Ba)

X is a nonmetal (F to Cl)

ns np ns np

M → M+

X → X‾

Check-Up

Barium (Grp 2) reacts with chlorine (Grp 17).Write the reaction equationDraw the electron configuration for Ba and

Cl, and their appropriate ions.Draw the Lewis symbols for this reaction.

Simple Binary Ionic Compounds

Reacting

Groups

General Formula

Example

1 + 17

2 + 17

3 + 17

MX

MX2

MX3

LiF

BeF2

AlF3


Grp1 Grp16

Metal Nonmetal

4 Li(s) + O2(g) → 2 Li2+ O2‾(s)

2s 2p 2s 2p

Li [He] → Li+

O [He] → O2‾

Check-Up

Draw the Lewis symbols for the reaction of Li and O2.


Reaction equation

2 M(s) + X → M2+ X‾

where:M is a Grp 1 metal (Li to Cs)

X is a Grp 16 nonmetal (O to Te)


Reacting

Groups

General Formula

Example

1 + 16

2 + 16

3 + 16

M2X

MX

M2X3

Na2O

BaO

Al2S3


Grp1 Grp15

Metal Nonmetal

6 Na(s) + N2(g) → 2 Na3+N3‾(s)

Check-Up

Draw the Lewis symbols for the reaction of Na and N2.


Reacting

Groups

General Formula

Example

1 + 15

2 + 15

3 + 15

M3X

M3X2

MX

Na3N

Mg3P2

AlN

Characteristics of Ionic Compounds

Form extended three-dimensional arrays of oppositely charged ions

Na+

Cl-


In A, no current flows in the solid because ions are immobile.


In the molten compound, B mobile ions flow toward the oppositely charged electrodes and carry a current.


In an aqueous solution of the compound, C mobile solvated ions carry a current.

The Covalent Bonding Model

Covalent bonds form when atoms share electrons If 2 electrons are shared – single bond If 4 electrons are shared – double bond If 6 electrons are shared – triple bond

Electrons have a lower potential energy when bound

Potential energy curve for H2

Too far apart, weak attractive force: no bond


Each nucleus attractsthe other atom’s electron


The combination ofnucleus-electron attractions and electron-electron and nucleus-nucleus repulsions gives the minimum energy of the system.


If the atoms move closer, repulsions increase the system’s energy and force the atoms apart.


For any covalent bond, there is an internuclear distance where the attractive forces are maximized in the presence of the repulsive forces.

BOND LENGTH


At the bond length, the combination of bonded atoms is more stable than the separated atoms by an amount of energy.

BOND ENERGY

Covalent H–H Bond

Net result of attractive and repulsive electrostatic forces.

Nucleus-electron attractions and nucleus-nucleus and electron-electron repulsions occur simultaneously.

Change in electron density as two hydrogen atoms approach each other.

Formation of Covalent Bonds

Lengths of Covalent Bonds

Bond Order, Bond Length and Bond Energy

Bond Bond Order

Average Bond Length (pm)

Average Bond Energy (kJ/mol)

C−O

CO

CO

1

2

3

143

123

113

358

745

1070

C−C

CC

CC

1

2

3

154

134

121

347

614

839

N−N

NN

NN

1

2

3

146

122

110

160

418

945

Some Common Covalent Compounds

Octet Rule

When atoms bond, they lose, gain, or share electrons to attain a filled outer shell of eight (or two) electrons.

The octet rule holds for all the compounds of Period 2 elements and a large number of others as well.

An octet of electrons consists of full s and p subshells on an atom

Octet Rule

The representative elements usually attain noble gas electron configuration in most of their compounds

Distinguish between bonding (shared) electrons and nonbonding (unshared or lone pairs) of electrons

Writing Lewis Formulas

Add up the valence electrons for each atom in the molecule. Use the periodic table to determine the number of valence electrons for each atom.

Write the symbols for the atoms in the molecule. In simple molecules, one atom will be the central atom surrounded by the other atoms.


The central atom is often the first atom in the formula.

Central atom is determined by:the atom that requires the largest number

of electrons to complete its octet goes in the center

for two atoms in the same periodic group, the less electronegative element goes in the center


Draw a dash (single bond, representing 2 electrons) between each pair of atoms covalently bonded together.

For each dash you drew, subtract 2 from your total number of valence electrons. Then draw the remaining electrons as dots around the atoms. Arrange the dots so that most atoms have eight valence electrons, and hydrogen has two.


If there are not enough electrons to give the central atom an octet, try multiple bonds.

REMEMBER: In achieving an octet, the bonding electrons are counted for BOTH atoms.

Molecules with an odd number of electrons

NO has 5 + 6 = 11 valence electrons

Molecules in which an

atom has less than an octet.

Some Exceptions to the Octet Rule

Molecules in which an atom has more than an octet.

Some Exceptions to the Octet Rule

Check Up

Write the Lewis formula forBBr3

AsF5

Between the Extremes

EITHER complete electron transfer OR complete electron sharing

In pure ionic bonds, electrons are completely lost or gained by one of the atoms

In pure covalent bonds, electrons are equally shared by the atoms

Most compounds fall somewhere between these two extremes.

Electronegativity (EN)

The relative ability of a bonded atom to attract the shared electrons.

Most common scale of relative EN values was developed by Linus Pauling.

EN values are not measured quantities but are based on Pauling’s assignment of the highest EN value, 4.0 to fluorine.

Electronegativities of the Elements

Nonpolar Covalent Bonds

Covalent bonds in which the electrons are shared equally are designated as nonpolar covalent bonds.

Nonpolar covalent bonds have a symmetrical charge distribution.

To be nonpolar, the two atoms involved in the bond must be the same element to share equally.

Nonpolar Covalent Bonds

H2

N2

H H or H H

N N or N N

Polar Covalent Bonds

Covalent bonds in which the electrons are NOT shared equally are designated as polar covalent bonds.

Polar covalent bonds have an asymmetrical charge distribution.

To be polar, the two atoms involved in the bond must have different electronegativities.


When atoms with different electronegativities form a bond, the bonding pair is shared unequally.

A + indicates that the atom that is less electronegative has a partial positive charge. A - indicates the atom that is more electronegativehas a partial negative charge.

H F FH

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron richregion

electron poorregion e- riche- poor

+ -

9.5


The existence of partial charges means that a polar covalent bond behaves as if it were partially ionic.

The partial ionic character of a polar bond is related directly to the electronegativity difference (EN).

EN is the difference between the EN values of the bonded atoms


LiF EN = 4.0 – 1.0= 3.0

HF EN = 4.0 – 2.1= 1.9

F2 EN = 4.0 – 4.0= 0

The bond in LiF has more ionic character than the H−F bond, which has more than the F−F bond.

A greater EN results in larger partial charges and a higher partial ionic character.

Covalent

share e-

Polar Covalent

partial transfer of e-

Ionic

transfer e-

Increasing difference in electronegativity

Classifying ionic character of chemical bonds

EN Ionic Character

> 1.7 Mostly ionic

0.4 – 1.7 Polar covalent

< 0.4 Non Polar Covalent

Percent Ionic Character

Calculated by comparing the actual behavior of a polar molecule in an electric field with the behavior it would have if the electron were transferred completely (pure ionic)

A value of 50% ionic character is often chosen to divide substances we recognize as “ionic” from those we recognize as “covalent.”


Percent ionic character generally increases with EN.


H—F = 43% ionic character

H—Cl = 19%

H—Br = 11%

H—I = 4%

As EN becomes smaller,the bond becomes more covalent.

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Chemical Bonds