1. Chapter 8 – Basic Concepts of Chemical Bonding
Abegim Undieh

2. 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

3. 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

4. Picture of a Lewis Symbol
Lewis Dot Symbol for Se:

5. 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

6. 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

7. Formula showing a sample Ionic Bond
Na + Cl  NaCl Na + Cl  Na+ + [Cl]-

8. 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

9. Ionic Bonding (cont.) Formula for lattice energy: E = kQ1Q2 D
Lattice 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

10. 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

11. 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

12. 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

13. Ionic Radii

14. 8.4 – Covalent Bonding
In covalent bonding, two nonmetals share electrons
The atoms are held together by the electrostatic attraction between the two nuclei

15. 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

16. 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

17. 8.5 – Bond Polarity and Electronegativity

18. 8.6 – Drawing Lewis Structures

19. 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

20. 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

21. Resonance in Benzene
Benzene, C6H6, has two equivalent Lewis structures, each showing three C-C single bonds and three C=C double bonds

22. Resonance in Benzene Benzene is often represented as such: OR
Millions of organic compounds have structures based on Benzene

23. 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

24. 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

25. 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.

26. 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.

27. 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

28. 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

29. 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