Chapter 8 Covalent Bonding

Molecular Compounds 8.1

Molecules and Molecular Compounds Atoms that are held together by sharing electrons are joined by a covalent bond A molecule is a neutral group of atoms joined together by covalent bonds A diatomic molecule contains two atoms Rule of Thumb: Hydrogen 7, make a 7 H2, O2, N2, F2, Cl2, Br2, I2 A compound composed of molecules is called a molecular compound A molecular formula is the chemical formula of a molecular compound

What information does a molecular formula provide? Key Question What information does a molecular formula provide? The amount of atoms of each element a substance contains. Molecular compounds are made up of nonmetals Molecular compounds typically have low melting and boiling points, this is due to the intermolecular forces; which will be discussed later in the chapter The arrangement of the atoms within a molecule is called its molecular structure

The Nature Of Covalent Bonding 8.2

The Octet Rule in Covalent Bonding Key Question What is the result of electron sharing in covalent bonds? The atoms attain an electron configuration of a noble gas by sharing a pair of electrons. Atoms may also form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons. REMEMBER… The octet rule states that an atom will gain or lose electrons to become stable (8 valence electrons) like a noble gas!

A bond formed by sharing three pairs of electrons is a triple bond Two atoms held together by sharing one pair of valence electrons are joined by a single covalent bond A structural formula represents the covalent bonds as dashes and shows the arrangement of covalently bonded atoms A pair of valence electrons that is not shared between atoms is called an unshared pair An unshared pair is also referred to as a lone pair or nonbonding pair A double covalent bond is a bond that involves two shared pairs of electrons A bond formed by sharing three pairs of electrons is a triple bond

Exceptions To The Octet Rule Key Question What are some 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 less, or more, than a complete octet of valence electrons.

DRAWING LEWIS DOT STRUCTURES (electron dot structures) In Class ONLY!

Rules For Drawing Lewis Dot Structures Sum valence electrons from all atoms Place least electronegative in the center, except H! WHY? Write the symbols for the atoms to show which atoms are attached to which, and connect them with a single bond to the central atom Complete octets around all the atoms bonded to the central atom, starting with the most electronegative atom Place left over electrons on the central atom If there are not enough electrons to give the central atom an octet, try multiple bonds Begin shifting from the most electronegative OLD RULE OF THUMB O likes double bonds, N likes triple bonds

Model and Practice Drawing Lewis Dot Structures See Study Guide Model and Practice Drawing Lewis Dot Structures In Class ONLY!

Bond Dissociation Energies The energy required to break the bond between two covalently bonded atoms is the bond dissociation energy Key Question How is the strength of a covalent bond related to its bond dissociation energy? A large bond dissociation energy corresponds to a strong covalent bond.

See Study guide You will need Table 8.3 on page 236. Model and Practice Calculating Bond Dissociation Energy In Class ONLY!

Bonding Theories 8.3

VSEPR Theory VSEPR Valence Shell Electron Pair Repulsion Theory Key Question What do scientists use the VSEPR theory for? To explain the three-dimensional shape of molecules. VSEPR theory states that the repulsion between electron pairs causes molecular shapes to adjust so that the valence electron pairs stay as far apart as possible The sum of the bonding electrons and lone pair electrons makes up what is called the electron domain geometry The electron domain geometry can be predicted by using summing the number of bonding electrons and lone pair electrons

Model and Practice Predicting the Electron Domain Geometry See Study Guide Model and Practice Predicting the Electron Domain Geometry In Class ONLY!

The molecular geometry of a substance is predicted by separating the bonding electrons from the lone pair of electrons

Model and Practice Predicting Molecular Geometry See Study Guide Model and Practice Predicting Molecular Geometry In Class ONLY!

Polar Bonds and Molecules When the atoms in the bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and each bond formed is a nonpolar covalent bond H2, O2, N2, F2, CO2 all have nonpolar covalent bonds Polar covalent bond (polar bond) is a covalent bond between atoms in which the electrons are shared unequally Whether a substance is polar or not depends on electronegativity REMEMBER… Electronegativity increases from the bottom to the top and from left to right.

Key Question How do electronegativity values determine the charge distribution in a polar bond? The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less-electronegative atom has a slightly positive charge. See page 248 for the symbols that are used to denote polarity.

Model and Practice Predicting the Polarity of a Substance See Study Guide Model and Practice Predicting the Polarity of a Substance In Class ONLY!

A molecule that has two poles is called a dipolar molecule or a dipole A polar molecule has one end that is slightly negative, and another end that is slightly positive A molecule that has two poles is called a dipolar molecule or a dipole Why do the following substances have dipole moments? SiCl, H2O, HCl Does CO2 have a dipole moment? Explain.

Attractions Between Molecules Key Question How do the strengths of intermolecular attractions compare with ionic and covalent bonds? Intermolecular attractions are weaker than either ionic or covalent bonds.

The two weakest attractions between molecules are collectively called van der Waals forces Van der Waals forces consists of dipole interactions and dispersion forces Dipole interactions occur when polar molecules are attracted to one another The slightly polar negative region of a polar molecule is weakly attracted to the slightly positive region of another polar molecule Dipole interactions are similar to, but much weaker than ionic bonds See page 250 (could go way more in depth ion-dipole, dipole-dipole) Dispersion forces are the weakest of all molecular interactions and are caused by the motion of electrons They occur between nonpolar molecules CH4

Do the following exhibit hydrogen bonding? Explain. Hydrogen bonds are attractive forces in which hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom In order for hydrogen bonding to take place a covalent bond must already exist between hydrogen and either N, O, or F, and a lone pair of electrons must be present on the central atom (N, O, F) Do the following exhibit hydrogen bonding? Explain. NH3, CH4, CH3F, H2S, H2O

Intermolecular Attractions and Molecular Properties Key Question Why are the properties of covalent compounds so diverse? Intermolecular attractions!