The Nature of Covalent Bonding Prentice-Hall Chapter 8.2 Dr. Yager

Objectives Describe how electrons are shared to form covalent bonds and identify exceptions to the octet rule Demonstrate how electron dot structure represent shared electrons. Describe how atoms form double or triple bonds. Distinguish between a covalent bond and a coordinate covalent bond and describe how the strength of a covalent bond is related to its bond dissociation energy. Describe how oxygen atoms are bonded in ozone

In covalent bonds, electrons are shared to achieve the electron configuration of nobel gases (Octet Rule). Bonds can be: Single 2 e- Double 4 e- Triple 6 e-

Single Covalent Bonds Showing electron dot structure

An electron dot structure such as H:H shows the shared pair of electrons in the covalent bond by two dots. A structural formula shows the covalent bonds by dashes and shows the arrangement of covalently bonded atoms. H – H

The halogens form single covalent bonds in their diatomic molecules The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example.

The hydrogen and oxygen atoms attain noble- gas configurations by sharing electrons.

A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair.

The ammonia molecule has one unshared pair of electrons.

Methane has no unshared pairs of electrons.

Double and Triple Bonds Atoms form double or triple covalent bonds if they can attain a noble gas structure by: Sharing two pairs of e- for a double bond. Sharing three pairs of e- for a triple bond.

Double Bond Each nitrogen atom has one unshared pair of electrons.

Carbon dioxide gas is soluble in water and is used to carbonate many beverages. A carbon dioxide molecule has two carbon-oxygen double bonds.

A carbon dioxide molecule has two carbon-oxygen double bonds and is called a tri-atomic molecule.

Coordinate Covalent Bond A coordinate covalent bond is a covalent bond in which one atom contributes both bonding electrons. In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them.

In carbon monoxide, oxygen has a stable configuration but the carbon does not.

As shown below, the dilemma is solved if the oxygen donates one of its unshared pairs of electrons for bonding. In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms.

A polyatomic ion, such as NH4+, is a tightly bound group of atoms that has a positive or negative charge and behaves as a unit. Most plants need nitrogen that is already combined in a compound to grow.

Polyatomic Ions We’ve seen these in Chapter 7: NO3- SO42- CO32- PO43- NH4+ There are others! All of the atoms are covalently bonded.

Draw the electron dot structure of BF4-. Practice Problem Draw the electron dot structure of BF4-. Count the total number of valence electrons. Usually (not always) place the most electronegative atom in the center; otherwise, (as in this case, B) place the single different atom in the center. Place other atoms around central atom and distribute electron dots in ways that allow each atom to achieve an octet. (32)

BF4- ׃ ׃ ׃ F ׃ ׃ ׃ ׃ ׃ ׃ ׃ F B F ׃ ׃ ׃ ׃ ׃ F ׃

Bond Dissociation Energy The energy required to break the bond between two covalently bonded atoms is known as the bond dissociation energy. A large bond dissociation energy corresponds to a strong covalent bond.

Bond Dissociation Energies Expressed for a mole of bonds A mole is 6.02 x 1023 of bonds Typical carbon - carbon bond Single bond: 347 kJ/mol Double bond: 657 kJ/mol Triple bond: 908 kJ/mol

Resonance Ozone or O3 in the upper atmosphere blocks harmful ultraviolet radiation from the sun. At lower elevations, it contributes to smog.

Resonance A resonance structure is a structure that occurs when it is possible to draw two or more valid electron dot structures that have the same number of electron pairs for a molecule or ion. The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms.

Exceptions to the Octet Rule Sometimes there are exceptions to the Octet Rule. The octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number. There are also molecules in which an atom has fewer, or more, than a complete octet of valence electrons.

Two electron dot structures can be drawn for the NO2 molecule.

NO2 is produced naturally by lightening strikes.

Paramagnetic Substances Each electron can be considered as a small, spinning electric charge that creates a magnetic field. (opposite spins of paired electrons cancel, remember?) Substances that have all paired electrons are diamagnetic. Diamagnetic substances are weakly repelled by an external magnetic field. In contrast, paramagnetic substances show a relatively strong attraction to an external magnetic field. They contain one or more unpaired electrons.

Paramagnetism should not be confused with ferro-magnetism, which is the much stronger attraction of iron, cobalt and nickel for magnetic fields. A structure can be written for the O2 molecule in which both atoms are surrounded by 8 electrons: .. .. :O::O: all electrons are paired But this structure for O2 is incorrect because O2 is paramagnetic and must therefore have one unpaired electron – must be a resonance structure! .. .. .. .. :O - O: ↔ :O::O: . .

The electron dot structure for PCl5 can be written so that phosphorus has ten valence electrons.

1. In covalent bonding, atoms attain the configuration of noble gases by losing electrons. gaining electrons. transferring electrons. sharing electrons.

1. In covalent bonding, atoms attain the configuration of noble gases by losing electrons. gaining electrons. transferring electrons. sharing electrons.

2. Electron dot diagrams are superior to molecular formulas in that they show which electrons are shared. indicate the number of each kind of atom in the molecule. show the arrangement of atoms in the molecule. are easier to write or draw.

2. Electron dot diagrams are superior to molecular formulas in that they show which electrons are shared. indicate the number of each kind of atom in the molecule. show the arrangement of atoms in the molecule. are easier to write or draw.

3. Which of the following molecules would contain a bond formed when atoms share three pairs of electrons? Se2 As2 Br2 Te2

3. Which of the following molecules would contain a bond formed when atoms share three pairs of electrons? Se2 As2 Br2 Te2