Common Mistakes
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Understanding common errors helps you avoid them on tests. This lesson identifies frequent mistakes students make with chemical bonding.
Top 5 Common Mistakes
Mistake #1: Confusing Polar Bonds with Polar Molecules
The Error: Assuming that molecules with polar bonds are always polar molecules.
Correct Understanding: A molecule can have polar bonds but be nonpolar overall if the geometry is symmetric and bond dipoles cancel (e.g., CO2, CCl4).
Mistake #2: Wrong Electron Count in Lewis Structures
The Error: Miscounting valence electrons or using wrong numbers.
Correct Understanding: Count valence electrons carefully. Use group numbers: Group 1=1, Group 2=2, Group 13=3, Group 14=4, Group 15=5, Group 16=6, Group 17=7.
Mistake #3: Ignoring Geometry When Determining Polarity
The Error: Only considering electronegativity differences without thinking about molecular shape.
Correct Understanding: Both bond polarity AND molecular geometry determine overall polarity. Symmetric shapes can cancel polar bonds.
Mistake #4: Assuming All Ionic Compounds Dissolve in Water
The Error: Thinking all ionic compounds are soluble.
Correct Understanding: Many ionic compounds are soluble, but some are not (e.g., AgCl, BaSO4). Solubility rules determine which dissolve.
Mistake #5: Confusing Intermolecular Forces with Chemical Bonds
The Error: Treating hydrogen bonds and London forces as the same as covalent bonds.
Correct Understanding: Covalent bonds hold atoms together within molecules. Intermolecular forces hold molecules to each other and are much weaker.
Practice: Find and Fix the Errors
Problem 1: Student says: "BeCl2 is polar because chlorine is more electronegative than beryllium." Is this correct?
Error: Student ignored molecular geometry.
Correct: BeCl2 is linear and symmetric. The polar Be-Cl bonds point in opposite directions and cancel, making the molecule nonpolar.
Problem 2: Student draws the Lewis structure of CO with 8 total valence electrons. Is this correct?
Error: Wrong electron count.
Correct: C has 4 valence electrons, O has 6. Total = 10 electrons, not 8.
Problem 3: Student says: "NaCl conducts electricity because it has ions." Is this correct?
Error: Incomplete answer.
Correct: NaCl only conducts when dissolved in water or molten, not as a solid. The ions must be free to move.
Problem 4: Student says: "Water has a high boiling point because of strong covalent bonds." Is this correct?
Error: Confused covalent bonds with intermolecular forces.
Correct: Water's high boiling point is due to strong hydrogen bonds between molecules, not the covalent bonds within molecules.
Problem 5: Student says: "CH4 is polar because C and H have different electronegativities." Is this correct?
Error: Student didn't consider geometry.
Correct: CH4 is tetrahedral and symmetric. The small C-H dipoles cancel out, making methane nonpolar.
Problem 6: Student says: "Silver chloride (AgCl) dissolves in water because it's ionic." Is this correct?
Error: Not all ionic compounds dissolve.
Correct: AgCl is actually insoluble in water. Solubility rules indicate that most chlorides are soluble EXCEPT AgCl, PbCl2, and Hg2Cl2.
Problem 7: Student draws 12 electrons in a Lewis structure for H2O. Is this correct?
Error: Wrong electron count.
Correct: H2O has 6 (from O) + 2(1) (from H's) = 8 valence electrons, not 12.
Problem 8: Student says: "Oil is polar, which is why it doesn't mix with water." Is this correct?
Error: Oil is nonpolar, not polar.
Correct: Oil is nonpolar. It doesn't mix with polar water because "like dissolves like."
Problem 9: Student says: "SF6 is polar because F is highly electronegative." Is this correct?
Error: Student ignored geometry.
Correct: SF6 has octahedral geometry, which is perfectly symmetric. All S-F dipoles cancel, making SF6 nonpolar.
Problem 10: Student says: "Hydrogen bonds are the strongest type of bond." Is this correct?
Error: Confused intermolecular forces with chemical bonds.
Correct: Hydrogen bonds are the strongest intermolecular force, but they're much weaker than actual chemical bonds (ionic, covalent). Covalent bonds are typically strongest.
Check Your Understanding
- What's the difference between a polar bond and a polar molecule?
- How do you count valence electrons correctly?
- What determines whether an ionic compound dissolves in water?
Next Steps
- Review any concepts that caused errors
- Take the Unit Quiz to assess your mastery