Covalent Bonds

Ch 8

Covalent Bonding

• In these bonds electrons are shared between the nuclei of two atoms to form a molecule or polyatomic ion

• Usually formed between two elements on right side of periodic table

• Usually formed between 2 nonmetals or nonmetals and metalloids

Covalent Bonding

• There are several electrostatic interactions in these bonds:– Attractions between electrons

and nuclei– Repulsions between electrons– Repulsions between nuclei

Covalent Compound Properties

• Lower melting points– Lower molecular weights usually have lower

melting and boiling points– Weaker forces between molecules compared

to metallic and ionic compounds– Can be very large (polymers)

Polar Covalent Bonds

• Although atoms often form compounds by sharing electrons, the electrons are not always shared equally.

• Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does.

• Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.

Electronegativity:

• The ability of atoms in a molecule to attract shared electrons to itself.

• On the periodic chart, electronegativity increases as you go…– …from left to right

across a row.– …from the bottom to

the top of a column.

Electronegativity

• Increases as you move across a period because effective nuclear force is greatest causing the Coulomb force of the nucleus to be great enough to attract electrons from other atom

• Decreases as you move down a group because valence electrons are in outer shells far from nucleus causing the charge of the nucleus to be less than atoms whose electrons are close

Distance versus Potential Energy graph

• Graphically shows what happens when two atoms try to bond– The relative strengths of

attractive and repulsive forces as a function of distance determine the shape of the graph

• The bond length is the distance between bonded atoms’ nuclei, and is the distance of minimum potential energy

Polar Covalent Bonds• When two atoms share

electrons unequally, a bond dipole results.

• The more electronegative atom will be partially negative because the electrons are closer to it

• While the least electronegative will have a partial positive charge

• The sum of the partial charges must equal the overall charge of the atom

Polar Covalent Bonds

The greater the difference in electronegativity, the more polar is the bond.

• Polarity of the molecular compound can be determined by the location of the elements in the periodic table and their electronegativity– All bonds have some ionic character, the different

between ionic and covalent bonding is not designated by rather a continuum.

– Elements with the greatest difference in electronegativity are ionic (distinct positive and negative ions)

• Ions have distinct positive and negative charges (difference in electronegativity >1.7)

– Elements with large difference in electronegativity would be polar (between 1.7 > x > 0.5)

– Elements with no difference or a slight difference in electronegativity are nonpolar (<0.5)

• Example: There is a slight difference in electronegativity between C and H, but the difference is not great enough to be classified as polar.

– C and H form nonpolar covalent bonds