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.
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
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