Molecular CompoundsSection 9.3 and Chapter 8

Nomenclature for Molecular CompoundsWhen two NONMETALS are bonded together:1. Write the names of the elements in the order listed in the formula.2. Use prefixes appropriately to indicate the number of each kind of

atom.• If there is one atom of the first element, DO NOT use mono-• If there is a repeating vowel at the end of a prefix, the last letter of the prefix

is sometimes dropped when the name of the element begins with a vowel.

3. End the name of the second element with the suffix –ide.

Molecular Compounds

Molecules and Molecular Compounds•Molecules and molecular compounds are held

together by covalent bonds.• It’s all about sharing electrons

•Molecular compounds are made up of representative units called molecules.•Diatomic molecule: O2 , N2 , F2 ,H2 ,

Cl2 , Br2

Representing Molecules•Molecular formulas show how many atoms of each element

a substance contains- it is not always in the lowest whole number ratio (you do not need to simplify)• A molecular formula does not tell you about a molecule’s

structure.

The Octet Rule in Covalent Bonding• In covalent bonds, electron sharing usually occurs so that atoms attain the electron configuration of noble gases.•Use Lewis Dot Structures to represent bonds formed in molecular compounds. •Draw the dot structure for each element•Bond unshared pair of valence electrons so each

atom has 8 surrounding valence electrons.

VSEPR Theory Valence-Shell Electron-Pair Repulsion theory

•The repulsion between electron pairs causes molecular shapes to adjust so that the valence electron pairs stay as far apart as possible.•Explains the 3D shape of molecules

Bond Polarity

• Covalent bonds are formed by the sharing of electrons, but covalent bonds differ in how bonded atoms share electrons.

Nonpolar covalent bond

• Electrons are shared equally in the bond (all diatomic molecules have nonpolar covalent bonds)

Polar Covalent Bonds• Electrons are NOT shared equally in the bond• The more electronegative atom attracts electrons more strongly

and gains a slightly negative charge.• The less electronegative atom has a slightly positive charge.

Describing Bond Polarity

Describe the bond polarity between the atoms of each pair.

1.C and O2.Cl and F3.Br and Br

Molecular Polarity (looking at the entire molecule)• If there is a polar bond, then the entire molecule will

most likely be polar molecule. •Dipolar molecule- a molecule that has two poles

Molecular Polarity

• You must also consider the shape of the molecule.• If the polarity arrows are point in the opposite

direction, then it is a nonpolar molecule

Determine if the following molecules are polar or nonpolar.

1.N2

2.NH3

3.CH3OH4.CH4

Attractions between Molecules- Hydrogen bonds• The strongest intermolecular force•Hydrogen covalently bonded to a very electronegative

atom that is also weakly bonded to an unshared pair of another electronegative atom.

Attractions between Molecules- Dipole interactions•Occur when polar molecules are attracted to one

another

Attractions between Molecules- Dispersion forces•Weakest of all molecular interactions• Caused by the motion of electrons and occur between

nonpolar molecules

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