1. Chapter 8: Covalent Bonding

2. 8.1 The Covalent Bond

3. What is a covalent bond?
The chemical bond that results from sharing valence electrons is a covalent bond.
A molecule is formed when two or more atoms bond covalently
The shared electrons are considered to be part of the outer energy level of both atoms involved.
Generally occurs between elements that are relatively close to each other on the periodic table
The majority of covalent bonds form between nonmetals

4. Covalent bond formation
Diatomic molecules form when two atoms of each element share electrons
Includes hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine
As the atoms are moved closer to each other, each nucleus attracts the other atom’s electron cloud, but the electron clouds repel each other
As the atoms move closer, the attraction of both nuclei for the other atom’s electrons increases until the maximum attraction is achieved
The attractive forces balance the repulsive forces
Most stable arrangement of the two atoms
At this point the two atoms bond covalently
If the atoms are forced closer together, the repulsion forces increase and exceed the attractive forces

5. Single Covalent Bonds
When only one pair of electrons is shared, it is a single covalent bond
The shared electron pair is often referred to as the bonding pair
The shared electrons belong to each atom simultaneously
Lewis structures
Electron-dot diagrams are used to show how electrons are arranged in molecules
A line or pair of vertical dots between symbols of elements represents a single covalent bond in a Lewis structure

6. Group 17 elements form a single covalent bonds with atoms of other nonmetals
An atom of a group 16 element can share two electrons and can form two covalent bonds.
Group 15 elements form three covalent bonds with atoms of nonmetals
Atoms of group 14 elements will form four covalent bonds.

7. The sigma bond Single covalent bonds are also called sigma bonds
Represented by the Greek letter sigma (σ)
Occurs when the pair of shared electrons is in an area centered between the two atoms
The valence atomic orbitals overlap end-to-end, concentrating the electrons in a bonding orbital between the two atoms
A bonding orbital is a localized region where bonding electrons will most likely be found

8. Multiple Covalent Bonds
In some molecules, atoms have noble-gas configurations when they share more than one pair of electrons with one or more atoms
Sharing multiple pairs of electrons forms multiple covalent bonds
In general, the number of valence electrons needed to form an octet equals the number of covalent bonds that can form
A double covalent bond forms when two pairs of electrons are shared between two atoms
A triple bond forms when three pairs of electrons are shared between two atoms.

9. The pi bond
A multiple bond consists of one sigma bond and at least one pi bond
A pi bond, Greek symbol π, forms when parallel orbitals overlap and share electrons
Molecules having multiple covalent bonds contain both sigma and pi bonds
A triple covalent bond consists of one sigma bond and two pi bonds

10. Strength of Covalent Bonds
Because covalent bonds differ in strength, some bonds break more easily than others
In general, covalent bonds are weaker than ionic bonds
The strength of a covalent bond depends on the distance between the bonded nuclei
The distance between the two bonding nuclei at the position of maximum attraction is called bond length
Determined by the sizes of the two bonding atoms and how many electron pairs they share
As the number of shared electron pairs increases, bond length decreases
The shorter the bond length, the stronger the bond

11. Energy is released when a bond forms
Energy must be added to break a bond
The amount of energy required to break a specific covalent bond is called bond-dissociation energy.
Always a positive value
Indicates the strength of a chemical bond
Endothermic reactions occur when a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in the product molecules.
Exothermic reactions occur when more energy is released forming new bonds than is required to break bonds in the initial reactants.

12. 8.2 Naming Molecules

13. Naming Binary Molecular Compounds
A binary molecular compound is composed only of two nonmetal atoms
Rules
The first element in the formula is always named first, using the entire element name
The second element in the formula is named using its root and adding the suffix –ide
Prefixes are used to indicate the number of atoms of each element that are present in the compound.
Most common prefixes shown in Table 3

14. Exceptions
The first element in the compound name never uses the prefix mono-
If using a prefix results in two consecutive vowels, one of the vowels is usually dropped to avoid awkward pronunciation
Ex: carbon monoxide not monocarbon monooxide

15. Many binary molecular compounds were discovered and given common names long before the present-day naming system was developed
Examples: water, ammonia, nitrous oxide

16. Naming Acids
Water solutions of some molecules are acidic and are named acids
If a compound produces hydrogen ions (H+) in solution, it is an acid
Naming binary acids
A binary acid contains hydrogen and one other element.
Rules
The first word has the prefix hydro- to name the hydrogen part of the compound.
The rest of the first word consists of a form of the root of the second element plus the suffix–ic.
The second word is always acid
Ex: HBr = hydrobromic acid

17. Naming oxyacids
An acid that contains both a hydrogen atom and an oxyanion is referred to as an oxyacid
An oxyanion is a polyatomic ion containing one or more oxygen atoms
Rules
Identify the oxyanion present
The first word of an oxyacid’s name consists of the root of the oxyanion and the prefix per- or hypo- if it is part of the oxyanion’s name
The first word of the oxyacid’s name also has a suffix that depends on the oxyanion’s suffix
If the oxyanion suffix is –ate, replace it with the suffix –ic
If the oxyanion suffix is –ite, replace it with the suffix –ous
The second word of the name is always acid
Ex: HNO3 contains nitrate  nitric acid

18. Writing Formulas from Names
The name of a molecular compound reveals its composition
The name of any binary molecule allows you to write the correct formula
Subscripts are determine by the prefixes used
Figure 12 is a flow chart that can be used to name molecular compounds

19. 8.3 Molecular structures

20. Structural Formulas
One of the most useful molecular models is the structural formula, which uses letter symbols and bonds to show relative positions of atoms
You can predict the structural formula for many molecules by drawing the Lewis structure

21. Lewis structures: When drawing a Lewis structure, follow these steps
Predict the location of certain atoms
The atom with the least attraction for shared electrons is the central atom (usually the element further left on the periodic table)
Hydrogen is always a terminal, or end, atom
Determine the number of electrons available for bonding, the valence electrons
Determine the number of bonding pairs
Divide the number of electrons available by two

22. Place the bonding pairs
Place one bonding pair (single bond) between the central atom and each of the terminal atoms
Determine the number of electron pairs remaining
Subtract the number of pairs used in step 4 from the total number determined in step 3.
The remaining electron pairs are lone pairs as well as double, or triple bonds. Place lone pairs around each terminal atom (except H atoms) to satisfy the octet rule.
Any remaining pairs are assigned to the central atom
Determine whether the central atom satisfies the octet rule.
If the central atom does not have an octet, convert lone pairs on the terminal atoms to form a double or triple bond between the terminal and central atoms.

23. Lewis structures for polyatomic ions
Although the unit acts as an anion, the atoms within a polyatomic ion are covalently bonded
Compared to the number of valence electrons present in the atoms that make up the ion, more electrons are present if the ion is negatively charged and fewer are present if the ion is positively charged.
To find the total number of electrons available for bonding:
Find the number of electrons available in the atoms present in the ion
Subtract the ion charge if the ion is positive, and add the charge if the ion is negative

24. Resonance Structures
Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion
Referred to as resonance structures
Differ only in the position of the electron pairs, never the atom positions
Each molecule or ion that undergoes resonance behaves as if it has only one structure
The actual bond length is an average of the bonds in the resonance structures

25. Exceptions to the Octet Rule
Some molecules and ions do not obey the octet rule
A small group of molecules might have an odd number of valence electrons and be unable to form an octet around each atom.
Some compounds form suboctects – stable configurations with fewer than eight electrons present around an atom
A coordinate covalent bond forms when one atom donates both of the electrons to be shared with an atom or ion that needs two electrons to become stable

26. Some compounds have a central atom that contain more than eight valence electrons
Expanded octet
When you draw the Lewis structure, either extra lone pairs are added to the central atom, or more than four bonding atoms are present in the molecule

27. 8.4 Molecular Shape

28. VSEPR Model
The molecular geometry, or shape, of a molecule determines many of its physical and chemical properties.
The model used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion model, or VESPR model.
Based on an arrangement that minimizes the repulsion of shared and unshared pairs (of electrons) around the central atom

29. Table 6 illustrates some common shapes of molecules including:
Repulsion forces cause the atoms in a molecule to be positioned at fixed angles relative to one another
The angle formed by two terminal atoms and the central atom is a bond angle
Unshared pairs of electrons occupy a slightly larger orbital than shared electrons
Table 6 illustrates some common shapes of molecules including:
Linear
Trigonal planar
Tetrahedral
Bent

30. Hybridization
A hybrid occurs when two things are combined and the result has characteristics of both
Hybridization is a process in which atomic orbitals are mixed and form new, identical hybrid orbitals
Each orbital contains one electron that it can share with another atom
The hybrid orbital sp3 has four hybrid orbitals that form from one s orbital and three p orbitals.
Lone pairs also occupy hybrid orbitals

31. 8.5 Electronegativity and Polarity

32. Electronegativity and Bond Character
The scale of electronegativity evaluates the electron affinity of specific atoms in a compound
Electronegativity indicates the relative ability of an atom to attract electrons in a chemical bond
Electrons in bonds between identical atoms have an electronegativity difference of zero – meaning that the electrons are equally shared
Nonpolar covalent bond

33. Unequal sharing results in a polar covalent bond
Occurs when elements of different electronegativity form a compound
Large differences in electronegativity indicate an electron transfer, resulting in a bond that is primarily ionic

34. Polar Covalent Bonds
When a polar bond forms, the shared electron pair or pairs are pulled toward one of the atoms
The electrons spend more time around one atom, resulting in partial charges at the ends of the bond
The Greek letter delta (δ) is used to represent a partial charge
δ- indicates slightly negative charge
δ+ indicates slightly positive charge

35. Molecular polarity
Molecules are either nonpolar or polar, depending on the location and nature of the covalent bonds they contain
Nonpolar molecules are not attracted by an electric field while polar molecules are

36. Polarity and molecular shape
Symmetric molecules are usually nonpolar
Even if the molecule contains polar bonds, the polar bonds cancel each other out
Asymmetric molecules are polar as long as the bond type is polar
If the charge distribution is unequal because the molecule is asymmetric the molecule is polar

37. Solubility of polar molecules
Solubility = ability of a substance to dissolve in another substance
Determined by bond type and shape of the molecules present
Polar molecules and ionic compounds are usually soluble in polar substances, but nonpolar molecules dissolve only in nonpolar substances

38. Properties of Covalent Compounds
Differences in properties are a result of differences in attractive forces.
In a covalent compound, the covalent bonds between atoms in molecules are strong, but the attraction forces between molecules are relatively weak.
Forces between molecules are called intermolecular forces, or van der Waals forces

39. There are different types of intermolecular forces
Between nonpolar molecules, the force is weak and called a dispersion force, or induced dipole
The force between oppositely charged ends of two polar molecules is called a dipole-dipole force
A hydrogen bond forms between the hydrogen end of one dipole and a fluorine, oxygen, or nitrogen atom on another dipole
Especially strong

40. The properties of covalent molecular compounds are related to their weak intermolecular forces
Low melting and boiling points (compared to ionic compounds)
Covalent molecules are relatively soft solids

41. Covalent Network Solids
Solid composed only of atoms interconnected by a network of covalent bonds
Ex: quartz, diamond
Typically brittle, nonconductive, and extremely hard