Chapter 8: Covalent Bonding

8.1 The Covalent Bond

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

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

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

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.

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

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.

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

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

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.

8.2 Naming Molecules

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

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

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

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

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

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

8.3 Molecular structures

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

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

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.

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

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

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

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

8.4 Molecular Shape

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

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

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

8.5 Electronegativity and Polarity

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

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

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

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

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

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

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

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

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

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