Chapter 8 Notes Covalent Bonding

Molecules and Molecular Compounds

Helium and Neon are monoatomic, meaning they exist as single atoms

Some compounds exist as crystalline solids, such as NaCl

Others exist as liquids (H2O) or gases (HCl) at room temperature

HCl and H2O bond very differently than NaCl

Instead of giving up or taking electrons, they are shared between the atoms

Known as a Covalent Bond

Molecules and Molecular Compounds

H2O and CO are examples of molecules, meaning that in nature they exist as a neutral group of atoms held together by covalent bonds

How does this occur?

A diatomic molecule consist of 2 atoms

Ex. O2, H2, N2

A compound composed of molecules is called a molecular compound

Remember – ionic bonds are collections of + and – ions held together

There is no such thing as a molecule of NaCl

See fig. 8.3 p. 214

Molecules and Molecular Compounds

Properties of Molecular Compounds

1. Tend to have relatively lower melting and boiling points

Boiling point of water is 100C

2. Most molecular compounds are composed of atoms of two or more nonmetals

Molecular Formulas

A molecular formula shows how many atoms of each element a molecule contains.

Ex. H2O indicates that every molecule of water consists of 2 Hydrogen and 1 Oxygen

Ex. C6H12O6 – indicates that every molecule of glucose has 6 Carbon, 12 Hydrogen, and 6 Oxygen

Unlike formulas for ionic bonds, a molecular formula reflects the actual number of atoms in each molecule

Molecular Formulas

Molecular formulas do not tell you how the molecules are arranged or which atoms are sharing electrons

CO2 forms a linear structure while H2O forms a triangular structure

The structure depends on which atoms are linked and how many electrons they are sharing

Fig. 8.6 p. 216

Section 8-2 Notes The Nature of Covalent Bonding

Octet Rule in Covalent Bonding

In forming covalent bonds, electron sharing will occur so that atoms attain the electron configuration of a noble gas.

Ex. Hydrogen only has 1 e-, so to reach a noble gas configuration Hydrogen forms a diatomic molecule with another Hydrogen atom (H2)

Nonmetallic elements in groups 4A, 5A, 6A, and 7A tend to form covalent bonds by sharing electrons

Single Covalent Bonds

Two atoms held together by sharing a pair of electrons are joined by a single covalent bond.

Ex. H2

Electron dot structures can illustrate which electrons are shared in the covalent bond

Ex. H2 = H:H

A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms

Ex. H-H

A molecular formula only shows the number and kinds of atoms in the bond

Ex. H2

Single Covalent Bonds

Halogens also form single bonds in their diatomic molecules

Show how Cl forms a diatomic bond

A pair of valence electrons that is not shared between atoms is called an unshared pair

How many unshared pairs do the following molecules have?

H2O, CO, CO2, CH4, NH3

Double & Triple Covalent Bonds

Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing 2 or 3 pairs of electrons

Double Bond – when 2 pairs of electrons are shared

CO2

Triple Bond – when 3 pairs of electrons are shared

Ex. N2

Section 8-4 Notes Polar Bonds and Molecules

Bond Polarity

The character of a bond in a given molecule depends on the kind and # of atoms joined together

When bonding pairs of e- pull equally (and therefore are shared equally) a nonpolar covalent bond occurs

Ex. H2, O2, N2, Cl2

A polar covalent bond occurs when there is an unequal sharing of electrons

The electronegativity of each atom will determine the pull of the e-

The higher the electronegativity, the stronger the pull for the electrons being shared

Bond Polarity

Polar Bond cont.

HCl – hydrogen has an electronegativty of 2.1 while chlorine has an electronegativity of 3.0

Cl will have a stronger attraction to the e- and therefore will have a slightly negative charge

H has a weaker attraction to the e- and will have a slightly positive charge

The polar nature of a bond will often be represented by an arrow pointing in the direction of the more electronegative atom

The electronegativity difference between 2 atoms will determine the type of bond that will form

See Table 8.3 p. 238

Polar Molecules

The presence of a polar bond makes the entire molecule polar

Polar molecules have a slightly negative end and slightly positive end

A molecule having 2 poles is called a dipole

The effect of polar bonds on the polarity of the entire molecule depends on the shape of the molecule and the orientation of the polar bonds

Ex. CO2 is nonpolar, while H2O is polar

Attraction Between Molecules

Intermolecular attractions are weaker than either ionic or covalent bonds

Two kinds of attractions collectively called Van der Waals Forces

1. Dipole interactions – occur when polar molecules are attracted to each other

The slightly negative end is attracted to the slightly positive end

2. Dispersion Forces – weakest attraction caused by the motion of electrons

Occurs when e- momentarily move more to one side of a molecule closest to a neighboring molecule

Attraction Between Molecules

Hydrogen Bonds are a form of dipole interactions

Occurs when a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom

Hydrogen bonds are the strongest of the intermolecular forces

Molecular Properties

The physical properties of a compound depend on the type of bonding it displays

In most solids formed from molecules, only weak attractions need to be broken resulting in lower melting points

A few solids consists of molecules that do not melt until temperatures reach 1000 C or higher

Known as network solids which are very stable substances

Ex. Diamond