Molecular compounds

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 BONDS  Sometimes 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 bond  Covalent 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 Bond  Abbreviated as: H-H HH

Distance of Bond  The 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 Bond  Nonpolar 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 Bond  Polar 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 Difference  What is the electronegativity difference between AlF 3 and classify it as nonpolar covalent, polar covalent, or ionic?  When doing compounds with multiple bonds, you look at all bonds

Dipole  Dipole: 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

Dipole  For instance, HF  Which element is more electronegative? FF  So, we say that H is more positive and F is more negative.

Dipole  Dipole 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  H Δ+ -F Δ-  Δ = a small change in charge.

SPECIAL CASE: HYDROGEN BONDS  Water has some special properties because of polarity:  Surface tension  High 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. CO 2 2. RbCl 3. N 2 O 3 4. FeN 5. AgCl 6. ZrO 1. PO 4 2. NaCl 3. H 2 4. CdCl 2 5. SnO 6. LiF

ANSWERS 1. Polar covalent, Ionic, Polar covalent, Polar covalent, Polar covalent, Ionic, Polar covalent, Ionic, Nonpolar covalent, Polar covalent, Polar covalent, Ionic, 3.0