Molecular Molecular compoundscompounds

SO FAR . . . We’ve seen how ions combine to

form ionic compounds Electrons are gained or lost so that

the atom can form an ion Opposite charges attract Tend to form crystal structures as

salts Yet, this is not the only way that

compounds are formed

COVALENT BONDSSometimes there is not enough

energy to fully remove an e- In this case, the atoms still want to be

like the noble gases.The only way to achieve this is by

SHARING electrons.Molecular compound: a compound

formed when two or more atoms combine by sharing electrons.

Covalent bondCovalent bond: a bond in which e- are shared between two atoms.

Single bond: a bond in which two atoms share one pair of electrons between them.

Single BondAbbreviated as: H-H

H H

Distance of BondThe hydrogen atoms have a definite distance

If the two hydrogen’s get too close together then the electrons repel one another

Electronegativity

Electronegativity: the tendency of an atom to attract bonding electrons to itself when it bonds with another atom.

Electronegativity (Handout)Electronegativity refers to how likely an element will form an ionic bond.

F is the most electronegative element, thus it is assigned a 4.0.

Electronegativity decreases as it goes down a group and increases as it goes across a period.

Nonpolar Covalent BondNonpolar covalent bond: a bond in which the electronegativities of two atoms are equal.

For instance, H-H bond.In a nonpolar covalent compound, the e- are shared evenly

Polar Covalent BondPolar covalent bond: a bond in which two atoms form a covalent bond, but one atoms attracts electrons more strongly than the other atom.

Electronegativity Difference

How do we determine if a compound is covalent or ionic?

Electronegativity difference: the difference between two elements electronegativities.

Electronegativity Difference

What is the electronegativity difference between HBr and classify it as nonpolar covalent, polar covalent, or ionic?

Electronegativity DifferenceWhat is the electronegativity difference between AlF3 and classify it as nonpolar covalent, polar covalent, or ionic?

When doing compounds with multiple bonds, you look at all bonds

DipoleDipole: a molecule in which one end has a partial positive charge and the other end has a partial negative charge.

Polarity: when one end of the molecule is more electronegative than the other

DipoleFor instance, HF Which element is more

electronegative?

F

So, we say that H is more positive and F is more negative.

DipoleDipole moment: the

electronegativity difference.Dipole moments only occur in polar

covalent compounds.Remember ionic compounds don’t

share e-, so there is no partial charge.

So, HF the dipole moment is 1.8.We write a dipole molecule as HΔ+-FΔ-

Δ = a small change in charge.

SPECIAL CASE: HYDROGEN BONDS

Water has some special properties because of polarity:Surface tensionHigh specific heat (we will

discuss this later in the quarter)

Density

PRACTICE PROBLEMS For the following, find the

electronegativity difference. Is the compound, ionic, polar

covalent or nonpolar covalent?

If the compound is covalent, what is the dipole moment?

PRACTICE PROBLEMS

1. CO2

2. RbCl3. N2O3

4. FeN5. AgCl6. ZrO

1. PO4

2. NaCl3. H2

4. CdCl25. SnO6. LiF

ANSWERS1. Polar covalent,

1.02. Ionic, 2.23. Polar covalent,

0.54. Polar covalent,

1.25. Polar covalent,

1.16. Ionic, 2.1

1. Polar covalent, 1.4

2. Ionic, 2.13. Nonpolar

covalent, 0.04. Polar covalent,

1.35. Polar covalent,

1.76. Ionic, 3.0