Chapter 8 – Basic Concepts of Chemical Bonding

Abegim Undieh

8.1 – Chemical Bonds, Lewis Symbols, and the Octet Rule

• When atoms or ions are strongly attached to one another, there is a chemical bond between them

• There are THREE types of chemical bonds: ionic, covalent, and metallic

Lewis Symbols

• The electrons involved in chemical bonding are called valence electrons

• Valence electrons are in the outer shell of an atom

• Valence electrons are symbolized using Lewis Symbols

• The Lewis Symbol of an element shows the chemical symbol for the element and a dot for each valence electron

Picture of a Lewis Symbol

• Lewis Dot Symbol for Se:

http://www.cbu.edu/~mcondren/Se-Lewis-dot-structure.jpg

The Octet Rule

• All noble gases (besides He) have 8 valence electrons

• Noble gases have the most stable electron arrangements

• The Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons

8.2 – Ionic Bonding

• The bonding of a metal and a non-metal through the electrostatic attraction of opposite charges is called ionic bonding

• In an ionic bond, the metal transfers electrons to the nonmetal

• This occurs when there is a metal of low ionization energy and a nonmetal with a high electron affinity

Formula showing a sample Ionic Bond

Na + Cl NaClNa + Cl Na+ + [Cl]-

http://upload.wikimedia.org/wikipedia/commons/7/75/Ionic_bonding_animation.gif

Ionic Bonding (cont.)

• Ionic bonding is a very exothermic process• The measure of the stability of an ionic bond

is called the lattice energy• Lattice energy – the energy required to

completely separate a mole of a solid ionic compound into gaseous ions

Ionic Bonding (cont.)• Formula for lattice energy:

E = kQ1Q2

DLattice energy increases as the charges on the ions

increase and as their radii decrease

Ionic radii do not vary much over a wide range, so the magnitude of lattice energy depends mainly on the ionic charges

8.3 – Sizes of Ions

• Ionic size determines many properties of ionic solids, such as lattice energy, the way it packs in a solid, and the properties of the ions in solution

• The size of an ion depends on – its nuclear charge– the number of electrons it possesses– the orbitals in which the valence electrons reside

Sizes of Ions (cont)• In positive ions, electrons from the outermost

orbital have been removed• This decreases the total electron-electron

repulsion, allowing the atom to pull closer together

• Therefore, cations are smaller than their parent atoms, and conversely..

• Anions are larger than their parent atoms• For ions of the same charge, size increases as you

go down a group in the periodic table

Sizes of Ions (cont)

• an isoelectronic series is a series of ions that have the same number of electrons

• Example: O2-, F-, Na+, Mg2+, Al3+ all have 10 electrons

arranged in a 1s22s22p6 configuration like [Ne]

• In an isoelectronic series, the radius of the ion decreases with increasing nuclear charge

Ionic Radii

http://www.chem.umass.edu/~botch/Chem111F04/Chapters/Ch8/IonicRadii.jpg

8.4 – Covalent Bonding

• In covalent bonding, two nonmetals share electrons

• The atoms are held together by the electrostatic attraction between the two nuclei

Multiple Bonds

• The sharing of a pair of electrons produces one covalent bond, called a single bond

• In some cases, molecules form multiple bonds in order to achieve an octet

Multiple Bonds

• Two shared electrons produce a double bond• Three shared electrons produce a triple bond

the triple bond in N2 as shown by a Lewis structure

• As a rule, the distance between bonded atoms decreases as the number of shared electron pairs increases

8.5 – Bond Polarity and Electronegativity

8.6 – Drawing Lewis Structures

8.7 – Resonance Structures

• In some molecules, the arrangement of atoms cannot fully be described by a single Lewis structure

• This requires showing multiple equivalent Lewis structures, called resonance structures

• The actual structure of the molecule is a blend of its resonance structures

Resonance Structures (cont.)• Example: – In ozone, both of these resonance structures are

equally as correct, and the molecule does not oscillate between the two:

Remember to put brackets ‘[ ]’ around each resonance structure that you draw, with the double arrow in-between

http://www.mikeblaber.org/oldwine/chm1045/notes/Bonding/Resonan/IMG00007.GIF

Resonance in Benzene

• Benzene, C6H6, has two equivalent Lewis structures, each showing three C-C single bonds and three C=C double bonds

http://wpcontent.answers.com/wikipedia/commons/thumb/7/79/Benz3.svg/300px-Benz3.svg.png

Resonance in Benzene

• Benzene is often represented as such:

• OR

• Millions of organic compounds have structures based on Benzene

8.8 – Exceptions to the Octet Rule

• Molecules with an odd number of electrons

• Molecules in which an atom has less than an octet

• Molecules in which an atom has more than an octet

Odd Number of Electrons

• In some cases, complete pairing of electrons is impossible and an octet around each atom cannot be achieved

• Examples: ClO2, NO, NO2

ClO2 Lewis Diagram

Less than an Octet

• There are also cases in which an atom has less than 8 electrons

• This most frequently occurs in compounds containing boron or beryllium

• Example:

BF3 Lewis structure, showing that B only has 6 electrons surrounding it.

http://www.up.ac.za/academic/chem/mol_geom/bf32.gif

More than an Octet

• Many molecules/ions have more than an octet of electrons

• This only occurs for elements in period 3 and beyond

Diagram for PCl5, showing an expanded octet around P.

http://www.wwnorton.com/college/chemistry/gilbert/concepts/chapter7/pcl5.gif

More than an Octet

• The larger the central atom, the larger the number of surrounding atoms it can hold

• Expanded shells occur most often when the central atom is bonded to the smallest and most electronegative atoms, such as F, Cl, O

8.9 – Strengths of Covalent Bonds

• Bond enthalpy – the enthalpy change for the breaking of a particular bond into a mole of gaseous substance

• Bond enthalpy is always a positive quantity• The strengths of covalent bonds increase with

the number of shared electron pairs

Strengths of Covalent Bonds

• The bond length between two bonded atoms is the distance between the two nuclei

• The average bond length between two atoms decreases as the number of bonds between them increases