Chapter 10Chapter 10ChemicalChemicalBondingBonding

Chapter 10Chapter 10ChemicalChemicalBondingBonding

Tro, 2Tro, 2ndnd ed. ed.

Metals are found to the left of the metalloidsNonmetals are found to the right of the metalloids.

LEWIS STRUCTURES OF ATOMS

Metals form cations and nonmetals form anions to attain a stable valence electron structure.

These rearrangements occur by losing, gaining or sharing electrons.

The Lewis structure of an atom is a representation that shows the valence electrons for that atom.

Valence electrons: the electrons that occupy the outermost energy level of an atom.

Valence electrons are responsible for the electron activity that occurs to form chemical bonds.

The Lewis structure of an atom uses dots to show the valence electrons of

atoms.

The number of dots equals the number of s and p electrons in the atom’s outermost shell.

B2s22p1

Lewis Structures of the first 20 elements

Notice that C’s e- config is 2s22p2.

CHEMICAL BONDINGAtoms will do one of three “things” to get to a noble

gas electron configuration:1. Take electrons from another atom2. Give electrons to another atom3. Share electrons with atom(s)In choices 1 & 2: cause ions to form, then ionic

bondsIn choice 3: sharing electrons results in covalent

bondsWith the exception of hydrogen & helium, this

structure consists of eight electrons in the outermost energy level (The Octet Rule)

The Ionic Bond

Cations (+) have given up e-s and anions (-) have gained e-s, and now have opposite electrical charges

Results in strong electrostatic force of attraction

All cations and anions exist in crystal lattices, defined geometric structures with repeating 3-D pattern

The 3s electron of sodium transfers to the 3p orbital of chlorine.

Lewis representation of sodium chloride formation.

A sodium ion (Na+) and a chloride ion (Cl-) are formed.

The force holding Na+ and Cl- together is an ionic bond.

Two 3s electrons of magnesium transfer to the 3p orbitals of two chlorine atoms.

A magnesium ion (Mg2+) and two chloride ions (Cl-) are formed.

The forces holding Mg2+ and two Cl- together are ionic bonds.

NaCl is made up of cubic crystals.In the crystal each sodium ion is surrounded by six chloride ions.

In the crystal each chloride ion is surrounded by six sodium ions.

The ratio of Na+ to Cl- is 1:1

There is no molecule of NaCl

The Ionic Bond

Using your Periodic Table, determine the cation and anion each atom is likely to form, then write the Lewis structures and made the compounds. Finish by writing the compound’s chemical formula.

Practice: Al & F, Mg & O, Na & O, Na & N, Al & O

A sodium ion is smaller than a sodium atom because:(1) the sodium atom has lost its

outermost electron.(2) the 10 remaining electrons are now attracted by 11 protons and are drawn closer to the nucleus.

A chloride ion is larger than a chloride atom because:(1) the chlorine atom has gained an

electron and now has 18 electrons and 17 protons.

(2) The nuclear attraction on each electron has decreased, allowing the chlorine to expand.

Transition Metals form cations – a little different

Transition metals lose their “s” electrons first, because they are in the highest principle energy level, then they lose their “d” electrons.

Zn Zn2+ + 2 e- Cu Cu+ + 1e-

[Ar]4s23d10[Ar]3d10[Ar]4s13d10[Ar]3d10

COVALENT BONDINGA covalent bond consists of a pair of

electrons shared between two atoms.In the millions of chemical compounds

that exist, the covalent bond is the predominant chemical bond.

Substances which covalently bond exist as molecules.

Carbon dioxide bonds covalently. It exists as individually bonded covalent molecules containing one carbon and two oxygen atoms.

The term molecule is not used when referring to ionic substances.

Sodium chloride bonds ionically. It consists of a large aggregate of positive and negative ions. No molecules of NaCl exist.

Instead they are called Formula Units.

COVALENT BONDINGNonmetal Atoms have deficiency of electrons in

outermost shell and want to gain electrons to get full shell

Since two nonmetal atoms both want more electrons, they will share electrons to get full shell

H has 1 e- and wants 2 Cl has 7 e-s and wants 8

Both satisfied if they share a pair of electrons between them

Each contributes 1 e- to the pair and each gets to share the 2 e-s in the pair

H has 2 e-s and Cl has 8 e-s and they are “HAPPY”

COVALENT BONDINGIMPORTANT: in giving e-s to be shared, atom

actually gains e-s The number of e-s an atom contributes to be

shared is equal to the number of e-s it needs to have an octet! (or a full shell)

A pair of shared e-s is called a covalent bond1 pair of e-s between two atoms = single bond2 pairs of e-s betwn two atoms = double bond3 pairs “ “ “ “ “ = triple bond

LEWIS STRUCTURES OF COMPOUNDS

In writing Lewis structures, the most important consideration for forming a stable compound is that the atoms attain a noble gas configuration.

The most difficult part of writing Lewis structures is determining the arrangement of the atoms in a molecule or an ion.

In simple molecules with more than two atoms, one atom will be the central atom surrounded by the other atoms.

Cl2O has two possible arrangements.

Cl-Cl-OThe two chlorines can be bonded to each other.

Cl-O-ClThe two chlorines can be bonded to oxygen.

Usually the single atom will be the central atom. (also usually the “leftist or lowest” on the Periodic Table)

Atom GroupValence

Electrons

Cl VIIA 7

H IA 1

C IVA 4

N VA 5

S VIA 6

P VA 5

I VIIA 7

Valence Electrons of Group A Elements

Procedures for WritingLewis Structures

Step 1. Obtain the total number of valence electrons to be used in the structure by adding the number of valence electrons in all the atoms in the molecule or ion.If you are writing the structure of an ion, add one electron

for each negative charge or subtract one electron for each positive charge on the ion.

Step 2. Write the skeletal arrangement of the atoms and connect them with a single covalent bond (two dots or one dash). Choose the “leftist or lowest” element as the central atom. Arrange terminal atoms symmetrically around the central atom.Hydrogen, which contains only one bonding electron, can

form only one covalent bond. Oxygen atoms usually have a maximum of two covalent

bonds (two single bonds, or one double bond).

Procedures for WritingLewis Structures

Step 3. Subtract two electrons for each single bond you used in Step 2 from the total number of electrons calculated in Step 1.

This gives you the net number of electrons available for completing the structure by adding lone pairs of electrons to the terminal atoms until they have an octet. Any remaining electrons become lone pairs on the central atom.

Step 4. Check that each atom is satisfied. If one atom doesn’t have an octet, move lone pairs of electrons in as bond pairs to make multiple covalent bonds. Do this symmetrically.Step 5. Check the total number of electrons in the structure and make sure it matches the number of valence electrons in step 1.(Also learn the number of bonds an atom prefers to make:H and F always 1 bond and terminal atom; C mostly 4 (and usually a central atom); halogens mostly 1; O and S mostly 2; N and P mostly 3)

Procedures for WritingLewis Structures

Write the Lewis structure for H2O.

The total number of valence electrons is eight, two from the two hydrogen atoms and six from the oxygen atom.

The two hydrogen atoms are connected to the oxygen atom which is central. Write the skeletal structure:

Place two dots between the hydrogen and oxygen atoms to form the covalent bonds.Subtract the four electrons used from eight valence electrons to obtain four electrons yet to be used around the oxygen. (Why not the H?)

H O HorH OH

: : : :

Distribute the four remaining electrons in lone pairs around the oxygen atom. (Hydrogen atoms cannot accommodate any more electrons. NEVER have more than 1 bond to H or have lone pairs around H.)

These arrangements are Lewis structures because each atom has a noble gas electron structure.

H O HorH OH

: : : ::: ::

The shape of the molecule is not shown by the Lewis structure.

hydrogen chlorine iodine nitrogen

Covalent bonding with equal sharing of electrons occurs in diatomic molecules formed from one element.

A dash may replace a pair of dots that represent a bond:

H-H

Practice Lewis Structures

Look in packet for practice sheet and work with one partner to draw the Lewis structures on separate paper.

Bring them up to show on the document camera.

Complex Lewis Structures

Do the Lewis structures for the following with a partner:

HCN, CH4, SO3, CH3OH, SF6, PCl3, NO2

Some will have EXCEPTIONS to the Octet Rule.

Complex Lewis Structures

Exceptions to Octet Rule:Expanded valence shell: any central atom

with outermost e-s in period 3 or below has d orbitals available for bonding and can hold 10 or 12 e-s

Electron deficient or free radical structures: have less than 8 e-s and will be very reactive compounds

Complex Lewis Structures

There are some molecules and polyatomic ions for which no single Lewis structure consistent with all characteristics and bonding information can be written.

When more than one structure satisfies the rules, we call them resonance structures.

Real molecule is a hybrid of all possible Lewis structures.

Resonance stabilizes the molecule.Try O3.

DRAWING LEWIS STRUCTURES

Multiple Bonds: O2 and N2

Multiple Central Atoms: C2H6, N2H4, C3H8, C6H6, CH3NH2, CH3COOH

Compounds ContainingPolyatomic Ions

A polyatomic ion is a stable group of atoms that has either a positive or negative charge and behaves as a single unit in many chemical reactions.

Practice: NH4+, SO3

2-, NO2-, NO3

-, I3-,

A scale of relative electronegativities was developed by Linus Pauling. Electronegativity decreases down a group for representative elements.

Electronegativity generally increases left to right across a period.

Metals are low in EN and nonmetals are high.

ELECTRONEGATIVITY

Electronegativity: The relative attraction that an atom has for a pair of shared electrons in a covalent bond.

If the two atoms that constitute a covalent bond are identical then there is equal sharing of electrons.

This is called nonpolar covalent bonding.Ionic bonding and nonpolar covalent bonding

represent two extremes.

ELECTRONEGATIVITYIf the two atoms that constitute a

covalent bond are not identical then there is unequal sharing of electrons.

This is called polar covalent bonding.One atom assumes a partial positive

charge and the other atom assumes a partial negative charge.This charge difference is a result of the

unequal attractions the atoms have for their shared electron pair.

ELECTRONEGATIVITY

H-H are the same atom, and have the same “greediness,” so the two atoms are forced to share equally.

F-F same - forced to share equally.If two atoms have diff EN, the one with higher EN

will “take” the e-s in the pair more often than the other atom.

H-F are not the same atoms, and are not equal in greediness, F is far greedier, takes the e-s more than half the time.

:H F+ -

Shared electron pair.

:The shared electron pair is closer to fluorine than to hydrogen.

Partial positive charge on hydrogen.

Partial negative charge on fluorine.

Fluorine has a greater attraction for the shared electron pair than hydrogen.

Polar Covalent Bonding in HF

Types of Covalent Bonding

The polarity of a bond is determined by the difference in electronegativity values of the atoms forming the bond.

If the electronegativity difference between two bonded atoms is greater than 1.9 to 2.0, the bond will be more ionic than covalent.

If the electronegativity difference is greater than 2, the bond is strongly ionic.

If the electronegativity difference is less than 1.9 but greater than 0.5, the bond is polar covalent.

If the electronegativity differences is 0.5 or less, the bond in nonpolar covalent.

Types of Covalent Bonding

Estimate whether a bond is polar cov, mostly pure or nonpolar cov or ionic by finding the absolute value of the difference in EN between the two atoms in the bond.

__________________________________________| | | | | | | | | | | | | | 0 .2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6nonpolar | polar cov | mostly ioniccovalent

Practice: H-Cl, C-Cl, C-O and C=O, N-Cl, Ca-NH-Cl C-Cl C-O C=O N-Cl Ca-N0.9 0.5 1.0 1.0 0.0 2.0pol cov “ “ “ nonpol ionic

Molecular Geometry

= bent(bent)

One more geometry is trigonal pyramidal.

Some Geometric FiguresLinear

2 atoms on opposite sides of central atom

180° bond angles

Trigonal Planar3 atoms form a triangle around the

central atomPlanar120° bond angles

Tetrahedral4 surrounding atoms

form a tetrahedron around the central atom

109.5° bond angles

180°

120°

109.5°

Some Geometric Figures

Trigonal Pyramidal3 atoms form a triangular

pyramid beneath the central atom

Not planar~109° bond angles Derivative of tetrahedral

geometry

The Valence ShellElectron Pair Repulsion

(VSEPR) Model

The VSEPR model is based on the idea that electron pairs will repel each other electrically and will seek to minimize this repulsion.

To accomplish this minimization, the electron pairs will be arranged as far apart as possible around a central atom.

The 3-dimensional arrangement of the atoms within a molecule determines molecular interactions (physical properties and chemical reactions).

BeCl2 is a molecule with only two pairs of electrons around beryllium, its central atom.

Its electrons are arranged 180o

apart for maximum separation.LINEAR EP ARRANGEMENT & MOLECULAR GEOMETRY

BF3 is a molecule with three pairs of electrons around boron, its central atom.

Its electrons are arranged 120o apart for maximum separation.

This arrangement of atoms is called trigonal planar.

CH4 is a molecule with four pairs of electrons around carbon, its central atom.

An obvious choice for its atomic arrangement is a 90o angle between its atoms with all of its atoms in a single plane.

However, since the molecule is 3-dimensional the molecular structure is tetrahedral with a bond angle of 109.5o.

Ball and stick models of methane, CH4, and carbon tetrachloride, CCl4.

Tetrahedral ShapesTetrahedral

4 areas of electrons around the central atom

109.5° bond anglesAll Bonding = tetrahedral3 Bonding + 1 Lone Pair =

trigonal pyramid2 Bonding + 2 Lone Pair =

bent

H

HCH

H

——

Tetrahedral Derivatives

H

HNH

——

HOH ——

Ammonia, NH3, has four electron pairs around nitrogen.

The arrangement of electron pairs around nitrogen is tetrahedral.

The NH3 molecule is trigonal pyramidal.

NH3 has one lone pair of electrons.

Water has four electron pairs around oxygen.

The arrangement of electron pairs around oxygen is tetrahedral.

The H2O

molecule is bent.

H2O has two lone pairs of electrons.

The VSEPR Model

Summary: electron pair arrangement depends upon # of bonded atoms and lone pairs around the central atom. Lone pairs exert more repulsion that bond pairs. Count # of bonded atoms (B) and # of LPs (E).EP Arrangements: 2 = Linear; 3 = Trigonal Planar; 4 = Tetrahedral

Within these electron pair arrangements, the molecular geometry is based only on “seeing” the atoms. I call the molecular geometry the family members and the electron pair arrangement is the electron pair family the members are in.

Electron Pair Families and their Molecular Geometry members:Linear: AB2, linear onlyTrigonal Planar: AB3 trigonal planar; AB2E bentTetrahedral: AB4 tetrahedral; AB3E trigonal pyramidal, and AB2E2 bent

VSEPR PracticeUse practice sheets in packet to fill

in Electron Pair Arrangement and Molecular Geometry and Bond Angles.

Dipole MomentsA dipole is a molecule with positively and negatively

charged endsPolar covalent bonds or molecules have one end

slightly positive, +; and the other slightly negative, -

(not “full” charges, come from nonsymmetrical electron distribution)

Dipole Moment is a measure of the size of the polarity. (We are NOT going to worry about the Debye unit or actual numbers for dipole moment, just whether a molecule has a dipole or not.)

Polarity of Molecules

For a molecule to be polar it must- have polar bonds

electronegativity difference - theorybond dipole moments – measured values

- have an unsymmetrical shapevector addition

Polarity affects the intermolecular forces of attraction

:: OCO

Polar covalent bonds,but nonpolar molecule,because vectors cancel

OH H

Polar covalent bonds and unsymmetrical shape cause moleculeto be polar

DIPOLENO DIPOLE

64

Adding Dipole Moments

Table 10.3