3B3 Polar and Nonpolar Molecules Flashcards
Differentiate polar and nonpolar molecules based on bond polarity and molecular symmetry.
Define:
electronegativity
A measure of the tendency of an atom to attract a bonding pair of electrons.
It helps in determining the ionic/covalent character of bonds.
Who developed the electronegativity scale?
Linus Pauling
The scale ranges from 0.0 to 4.0, with fluorine being the most electronegative element at 4.0.
What is the most straightforward way to determine if a bond is ionic or covalent?
Compare the electronegativities of the two elements involved.
A large difference suggests ionic, while a small difference suggests covalent.
Where are elements with more ionic tendencies located on the periodic table?
On the left-hand side.
These elements have lower electronegativity values.
What indicates an ionic bond based on electronegativity difference?
A difference greater than 1.7.
This categorization helps in identifying the nature of the bond.
Define:
bond polarity
The unequal sharing of electrons between two atoms in a bond due to differences in electronegativity.
This creates a dipole moment where one atom has a partial negative charge, and the other has a partial positive charge.
What is a nonpolar bond?
A bond that occurs when electrons are shared equally between two atoms with similar or equal electronegativities.
An example is the H-H bond in hydrogen gas.
Define:
polar bond
A covalent bond with unequal sharing of electrons between two atoms with different electronegativities.
This unequal sharing results in one slightly negatively charged end, and one slightly positively charged end.
How does electronegativity affect bond polarity?
A greater electronegativity difference increases bond polarity.
For example, hydrogen fluoride (H-F) is highly polar due to fluorine’s high electronegativity.
How does electronegativity difference create a polar bond?
A higher electronegativity atom pulls electrons closer, creating partial charges.
This results in a dipole moment.
How can bond polarity be predicted using the periodic table?
By identifying the atoms as metals or non-metals.
Metals are on the left side and non-metals are on the top right.
What difference in electronegativity indicates a polar covalent bond?
A difference between 0.4-1.7.
This range signifies an unequal sharing of electrons.
What difference in electronegativity indicates a non-polar covalent bond?
A difference less than 0.4.
This signifies an equal sharing of electrons between atoms.
Example: C-H bond
What is a dipole moment?
A measure of the separation of positive and negative charges in a molecule, indicating its polarity.
Depends on electronegativity and molecular geometry.
List two factors used to determine molecular polarity.
- Bond polarity
- Molecular symmetry
Symmetrical molecules with polar bonds can be nonpolar if the dipoles cancel out.
What is the relationship between bond polarity and molecular polarity?
- Bond polarity: It contributes to molecular polarity.
- Molecular symmetry: It determines the overall dipole moment.
A molecule can have polar bonds but still be nonpolar if symmetric.
List three examples of polar molecules.
- Water (H₂O)
- Ammonia (NH₃)
- Hydrogen fluoride (HF)
These molecules have polar bonds and asymmetrical shapes.
List three examples of nonpolar molecules.
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Diatomic oxygen (O₂)
These molecules either have no polar bonds or are symmetrical.
How does symmetry affect molecular polarity?
It determines whether the dipole moments of individual bonds cancel out.
Dipole moments in symmetric molecules cancel out resulting in a nonpolar molecule even with polar bonds present.
Non-symmetric molecules with polar bonds are usually polar.
What is the symmetry of a linear molecule with identical terminal atoms?
Symmetrical
Examples include carbon dioxide (CO₂) and hydrogen (H₂).
How does the shape of a molecule affect its polarity?
It determines whether the polar bonds dipoles cancel out or reinforce each other.
For example, tetrahedral methane (CH₄) is nonpolar, while bent water (H₂O) is polar.
Fill in the blank:
Molecules with symmetrical shapes and _______ bonds are typically nonpolar.
nonpolar
Why is ammonia (NH₃) polar?
- Asymmetrical trigonal pyramidal shape
- Polar N-H bonds
The lone pair on nitrogen enhances the asymmetry.
What makes hydrogen sulfide (H₂S) polar?
- Asymmetrical bent shape
- Polar H-S bonds
Similar to water, but with less polarity due to smaller electronegativity differences.
What property makes a molecule like methane (CH₄) nonpolar?
Symmetrical tetrahedral geometry.
The dipoles of the C-H bonds cancel out due to symmetry.
Fill in the blank:
A molecule with polar bonds and a/an _________ shape is polar.
asymmetrical
The lack of symmetry prevents dipole cancellation.
Describe the polarity of sulfur hexafluoride (SF₆).
SF₆ is nonpolar due to its symmetrical octahedral shape.
The polar S-F bonds cancel each other out.
Describe the polarity of boron trifluoride (BF₃).
BF₃ is nonpolar due to its symmetrical trigonal planar shape.
The dipoles of the B-F bonds cancel out.
Why is benzene (C₆H₆) nonpolar?
It has a symmetrical planar shape with equal electron distribution.
Despite having polar bonds, symmetry cancels dipoles.
What is the significance of lone pairs in molecular polarity?
Lone pairs create asymmetry, making a molecule polar.
Examples include water (H₂O) and ammonia (NH₃).
True or False:
A molecule with no lone pairs is always nonpolar.
False
A molecule’s polarity also depends on bond polarity and shape.
True or False:
All molecules with lone pairs are polar.
False
Lone pairs increase polarity, but overall symmetry can make the molecule nonpolar.
How does bond polarity affect solubility?
- Polar molecules dissolve in polar solvents.
- Nonpolar molecules dissolve in nonpolar solvents.
‘Like dissolves like’ is a key principle in chemistry.
How can polarity be experimentally measured?
By measuring the dipole moment using spectroscopy or dielectric constant.
Polar molecules exhibit higher dielectric constants.
What is the effect of molecular polarity on boiling points.
Polar molecules have higher boiling points.
For example, water (H₂O) has a higher boiling point than methane (CH₄).
How does polarity affect the behavior of molecules in an electric field?
Polar molecules align with the electric field due to their dipole moment.
Nonpolar molecules show no alignment.
What is the relationship between polarity and intermolecular forces?
Polar molecules experience stronger intermolecular forces like dipole-dipole interactions.
Nonpolar molecules experience weaker forces like London dispersion forces.
