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CHEMICAL BONDING

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In a chemical bond between atoms, their valence electrons are redistributed in ways that make the atoms more stable.

The way that atoms are redistributed determines the type of bonding.

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CHEMICAL BONDING

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1. Ionic Bonding – chemical bonding that results from the electrical attraction between large numbers of cations (+, metals) and anions (-, nonmetals).

2. Covalent Bonding – results from the sharing of electron pairs between two atoms.

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TYPES OF CHEMICAL BONDING

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Electronegativity is a measure of an atom’s ability to attract electrons.

By calculating the diff erence in the bonded elements’ electronegativity, the type of bond can be determined(ionic or covalent)….generally bonds are not 100% ionic or covalent but a mixture.

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IONIC OR COVALENT???

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Type of Bond

Difference in electronegativities

Percentage of ionic character

Nonpolar-covalent bonds

0 to 0.3 0 to 5%

Polar-covalent bonds

0.3 to 1.7 5 to 50%

Ionic bonds 1.7 to 3.3 50 to 100%

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COVALENT OR IONIC???

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1. Nonpolar Covalent – a covalent bond in which the bonding electrons are shared equally by the bonded atoms, resulting in a balanced distribution of electrical charge.

2. Polar Covalent – is a covalent bond in which the bonded atoms have an unequal attraction for the shared electrons…..uneven distribution of charge.

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TYPES OF COVALENT BONDING

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Example: When Hydrogen and Chlorine combine the diff erence in their electronegativites is 3.0 – 2.1 = 0.9, indicating a polar covalent bond. The electrons in this bond are closer to the more electronegative chlorine atom that to the hydrogen atom. The chlorine end of the bond has a partial negative charge, indicated by the symbol δ - . The hydrogen end has a partial positive charge, δ+ .

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See periodic table on page 151 for values of electronegativity.

Classify the following bonds as either ionic, nonpolar covalent, or polar covalent.

Chlorine & CalciumChlorine & OxygenChlorine & Bromine

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PRACTICE PROBLEMS

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Bonding Between Chlorine and….

Electronegativity difference

Bond type More-negative atom

Calcium 3.0 – 1.0 = 2.0 Ionic Chlorine

Oxygen 3.5 -3.0 = 0.5 Polar-covalent Oxygen

Bromine 3.0 – 2.8 = 0.2 Nonpolar-covalent

Chlorine

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Page 163, 1 – 4Answer the questions in your composition book.Do not write the questions unless you feel that it is

necessary.

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CHAPTER 6 SECTION 1

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Chapter 6 Section 2

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COVALENT BONDING AND MOLECULAR

COMPOUNDS

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Many chemical compounds are composed of molecules.

Molecule – is a neutral group of atoms that are held together by covalent bonds….are capable of existing on its own.Example: Oxygen (O2), water (H2O), sugar (C12H22O11)

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COVALENT BONDING & MOLECULAR COMPOUNDS

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Molecular compound – is a chemical compound whose simplest units are molecules.

Chemical formulas give the composition of a compound

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Chemical formulas also indicate the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts….the chem. formula of a molecular compound is called a molecular formula.

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A molecular formula show the types and numbers of atoms combined in a single molecule of a molecular compound.

A diatomic molecules is a molecule containing only two atoms..ex: O2, N2, Cl2, I2, Br2, F2, H2

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Most atoms are at a lower potential energy when they are bonded to other atoms than they are at as independent particles….this is what makes them more stable.

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FORMATION OF COVALENT BONDS

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There are three different types of forces at work when atoms come together. (electron-electron, proton-proton, & electron-proton).

A bond length is the distance between two bonded atoms at their minimum potential energy….this occurs at a distance when the forces of attraction = the forces of repulsion.

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http://www.youtube.com/user/TTUchem1010#p/u/22/z3F7LjTvdX0

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Bond energy is the energy required to break a chemical bond and form neutral isolated atoms…this is the same amount of energy that is released as atoms change from isolated individual atoms to part of a molecule.

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Bond Bond length (pm)

Bond Energy (kJ/mol)

Bond Bond length (pm)

Bond Energy (kj/mol)

H-H 74 436 C-C 154 346

F-F 141 159 C-N 147 305

Cl-Cl 199 243 C-O 143 358

Br-Br 228 193 C-H 109 418

I-I 267 151 C-Cl 177 327

H-F 92 569 C-Br 194 285

H-Cl 127 432 N-N 145 163

H-Br 141 366 N-H 101 386

H-I 161 299 O-H 96 459

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TABLE 6-1, PAGE 168

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Bond lengths and energies vary with the type of atoms that have combined…and can vary with the same type of atoms bonding together. Bond length will decrease as bond energy or strength increases.

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Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level. (this rule applies to both ionic and covalent compounds)

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THE OCTET RULE

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Noble-gas atoms exist independently in nature….because of their electron configurations. This stability is a result of their outermost s and p orbitals being completely filled with 8 electrons….their octet is full.

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Other main-group elements (s and p block elements) are able to obtain a stable configuration through the use of the octet rule.

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F F F F 1s 2s 2p 1s 2s

2p Bonding

electron pair in overlapping

orbitals

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Hydrogen forms bonds in which it is only surrounded by two electrons.

Boron, has just three valence electrons, so it tends to form bonds in which it is only surrounded by 6 electrons. Example: BF3

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EXCEPTIONS TO THE OCTET RULE

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Shows the electron-configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol.

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ELECTRON-DOT NOTATION

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Uses dashes to show covalent bonds between atoms and uses dots to show the remaining valence or the unshared pairs of electrons.

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LEWIS STRUCTURES

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Indicates the kind, number, arrangement, and bonds but not the unshared pairs of the atoms in a molecule.

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STRUCTURAL FORMULA

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Draw the Lewis structure of iodomethane, CH3I

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Draw the Lewis structure of ammonia, NH3

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Draw the Lewis structure for hydrogen sulfide, H2S

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Draw the Lewis structure for methanal (aka formaldehyde), CH2O

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PCl5

C6H10Cl2OH

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Bond Bond Length (pm)

Bond Energy (kJ/mol)

Bond Bond Length (pm)

Bond Energy (kJ/mol)

C-C 154 346 C-O 143 358

C=C 134 612 C=O 120 732

C C 120 835 C O 113 1072

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BOND LENGTHS AND ENERGIES FOR SINGLE AND MULTIPLE BONDS

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Refers to bonding in molecules that cannot be correctly represented by a single Lewis structure…..the true structure lies somewhere between the two resonance structures. A double arrow is used indicate a molecule’s resonance structure.

O=O-O O-O=O

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RESONANCE

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Chapter 6 Section 3

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IONIC BONDS AND COMPOUNDS

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Most of the rocks & minerals in the Earth’s crust are composed of positive & negative ions held together by bonds.

Ionic compound – composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal…generally crystals.

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IONIC BONDING & IONIC COMPOUNDS

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Chem. Formula for an ion represents the simplest ratio of the combined ions that gives electrical neutrality.

Formula unit shows the simplest collection of atoms from which an ionic compound’s formula can be established.

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Example: Rock Salt, sodium chloride, NaCl is composed of Na+ & Cl-

Example: calcium fluoride(the liquid form is used to melt ice on highways or in the use of oxygen sensitive applications such as the making of metal alloys), CaF2 is composed of Ca2+ & F-

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Can be used to demonstrate the changes that occur in ionic bonding.

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ELECTRON-DOT NOTATION….

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Since nature favors arrangements where PE is @ a min. ionic crystals are arranged in a crystal lattice.

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CHARACTERISTICS OF IONIC BONDING

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Forces of attraction = oppositely charged ions & those between the nuclei & electrons of adjacent ions.

Forces of repulsion = between like charged ions & those between electrons of adjacent ions.

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FORCES AT ACT IN THE CRYSTAL LATTICE

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Ionic: the forces that hold it together are very strong.

Higher melting & boiling pts. Also, do not vaporize @ room temp.

Hard but brittle due to a large build-up of repulsion when a shift occurs.

Molten state can conduct since ions are free to move.

Some ionic compounds do not dissolve in water because the attraction of water cannot overcome the attraction between ions.

Solid state ions cannot move so they do not conduct.

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IONIC VS. MOLECULAR

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Molecular – covalent bonds are also strong but not in comparison to ionic.

Melt at lower temperatures or are gases @ room temp.

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Is a charged group of covalently bonded atoms…can be represented by Lewis structures

Examples: Ammonium NH4

+, Nitrate NO3

-, Sulfate SO4

2- & Phosphate PO4

2-

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POLYATOMIC IONS

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Chapter 6 Section 4

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METALLIC BONDING

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Explains why they are such excellent conductors of heat & electricity in the solid state compared to molten ionic compounds.

This is due to the highly mobile valence electrons of the atoms that make up a metal.

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METALLIC BONDING

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This mobility is not possible in molecular or ionic compounds since they are localized or bound to individual ions that are held in place.

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The vacancy that is observed in s-block and d-block metals allows for an overlap of the vacant orbitals that in turn allows the outer electrons to roam.

These electrons are considered to be delocalized, meaning they do not belong to an individual atom.

These mobile electrons are referred to as a sea of electrons.

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This freedom accounts for electrical & thermal conductivity.

Metals can absorb wide range of light frequencies (Flame Test) responsible for shiny appearance.

Also, since metallic bonding is the same in all directions this accounts for why metals are both malleable (hammered into thin sheets) or ductile (drawn into a wire).

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Varies with nuclear charge and the # of electrons in the metal’s electron sea.

This is reflected in the heat of vaporization of a metal.

The amt of (heat) required to vaporize the metal is a measure of the strength of the bonds that hold the metal together.

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METALLIC BOND STRENGTH

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Period Element Element Element

Second Li – 147 Be – 297

Third Na – 97 Mg – 128 Al – 294

Fourth K – 77 Ca – 155 Sc – 333

Fifth Rb – 76 Sr – 137 Y – 365

Sixth Cs – 64 Ba – 140 La - 402

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TABLE 6-4 HEATS OF VAPORIZATION OF SOME METALS (KJ/MOL)

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VSEPR Theory (valence-shell, electron-pair repulsion) states that the repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.

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MOLECULAR GEOMETRY

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VESPR MODEL

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VESPR MODEL

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Hybridization is the mixing of two or more atomic orbitals of similar energies on the same atom to produce new orbitals of equal energies.

Example: Methane CH4 1s2 2s2 2p2 (see board)

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GEOMETRY OF HYBRID ORBITALS

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VESPR AND MOLECULAR GEOMETRY

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VESPR AND MOLECULAR GEOMETRY

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HICBr4

AlBr3

CH2Cl2

NH3

H2O

PCl5

SF6

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PREDICT THE MOLECULAR GEOMETRY FOR THE FOLLOWING:

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Intermolecular Forces – are the forces of attraction between molecules…

The stronger the forces are the higher the boiling point will be..

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PREDICTING POLARITY

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PREDICTING POLARITY

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MOLECULAR POLARITY