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Unit 5 Part 2 Review – Chemical Bonding, Polarity, and Intermolecular Forces

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Q1a. What is electronegativity?

Background

Topic: Electronegativity

This question is testing your understanding of the concept of electronegativity, which is fundamental to chemical bonding and molecular polarity.

Key Terms:

  • Electronegativity: The tendency of an atom to attract electrons in a chemical bond.

Step-by-Step Guidance

  1. Recall that electronegativity is a property of atoms in molecules, not isolated atoms.

  2. Think about how electronegativity affects the sharing of electrons in covalent bonds.

  3. Consider how differences in electronegativity between atoms lead to polar bonds.

Try solving on your own before revealing the answer!

Q1b. Explain why electronegativity decreases down a group on the periodic table.

Background

Topic: Periodic Trends

This question is about understanding how atomic structure affects electronegativity as you move down a group.

Key Concepts:

  • Atomic radius increases down a group.

  • Electron shielding increases down a group.

Step-by-Step Guidance

  1. Recall that as you move down a group, atoms have more electron shells.

  2. Consider how increased distance between the nucleus and valence electrons affects attraction for bonding electrons.

  3. Think about how electron shielding reduces the effective nuclear charge felt by valence electrons.

Try solving on your own before revealing the answer!

Q1c. Explain why electronegativity increases across a period on the periodic table.

Background

Topic: Periodic Trends

This question asks you to explain the trend in electronegativity as you move from left to right across a period.

Key Concepts:

  • Atomic radius decreases across a period.

  • Nuclear charge increases across a period.

Step-by-Step Guidance

  1. Recall that as you move across a period, the number of protons in the nucleus increases.

  2. Consider how increased nuclear charge affects the attraction for bonding electrons.

  3. Think about how the atomic radius changes and how this impacts electronegativity.

Try solving on your own before revealing the answer!

Q2. Using the Periodic Table, arrange the following elements in order of increasing electronegativity:

  • a) Rb, Na, Li

  • b) P, S, Al

  • c) Fe, As, Br

Background

Topic: Electronegativity Trends

This question tests your ability to apply periodic trends to compare electronegativity values among elements.

Key Concepts:

  • Electronegativity increases across a period and decreases down a group.

Step-by-Step Guidance

  1. Locate each element on the periodic table.

  2. For each set, identify which element is furthest down the group and which is furthest to the right in the period.

  3. Apply the trend: down a group = lower electronegativity; across a period = higher electronegativity.

  4. Arrange each set from lowest to highest electronegativity based on their positions.

Try solving on your own before revealing the answer!

Q3a. How can you tell if a molecule is polar or nonpolar?

Background

Topic: Molecular Polarity

This question is about determining molecular polarity based on structure and electronegativity differences.

Key Concepts:

  • Polarity depends on bond dipoles and molecular geometry.

  • Symmetrical molecules tend to be nonpolar; asymmetrical molecules tend to be polar.

Step-by-Step Guidance

  1. Identify if the molecule contains polar bonds (difference in electronegativity).

  2. Analyze the molecular geometry to see if dipoles cancel out.

  3. Decide if the molecule has a net dipole moment (polar) or not (nonpolar).

Try solving on your own before revealing the answer!

Q3b. What is a dipole?

Background

Topic: Bond Polarity

This question asks you to define a dipole in the context of chemical bonding.

Key Terms:

  • Dipole: A separation of charge within a molecule due to differences in electronegativity.

Step-by-Step Guidance

  1. Recall that a dipole forms when electrons are shared unequally between atoms.

  2. Think about how this creates a partial positive and partial negative end in a bond or molecule.

Try solving on your own before revealing the answer!

Q3c. Draw PH3 and label the partial charges of each atom in the molecule.

Background

Topic: Lewis Structures and Polarity

This question tests your ability to draw a Lewis structure and identify partial charges based on electronegativity.

Key Concepts:

  • Lewis structure: Shows how atoms are bonded and where lone pairs are.

  • Partial charges: Indicate regions of higher or lower electron density.

Step-by-Step Guidance

  1. Count the valence electrons for P and H.

  2. Draw the central atom (P) and arrange the three H atoms around it.

  3. Add lone pairs to P as needed.

  4. Label the partial charges based on electronegativity (P vs H).

Try solving on your own before revealing the answer!

Q4. Draw a Lewis structure for each of the following. In addition, determine the molecular shape, polarity, and intermolecular forces.

  • H2SO

  • CH3OH

  • CH3Cl

  • BF3

Background

Topic: Lewis Structures, Molecular Geometry, Polarity, and Intermolecular Forces

This question tests your ability to draw Lewis structures, predict molecular shapes, determine polarity, and identify intermolecular forces.

Key Concepts and Terms:

  • Lewis structure: Shows bonding and lone pairs.

  • Molecular shape: Determined by VSEPR theory.

  • Polarity: Based on shape and bond dipoles.

  • Intermolecular forces: Types include London dispersion, dipole-dipole, hydrogen bonding.

Step-by-Step Guidance

  1. For each molecule, count the total valence electrons.

  2. Draw the skeletal structure and add lone pairs as needed.

  3. Use VSEPR theory to predict the molecular shape.

  4. Determine if the molecule is polar or nonpolar based on shape and bond polarity.

  5. Identify the dominant intermolecular forces present.

Try solving on your own before revealing the answer!

Q5–Q16. Identify the type of force described in each statement.

Background

Topic: Intramolecular and Intermolecular Forces

These questions test your ability to distinguish between forces within molecules (intramolecular) and forces between molecules (intermolecular), and to name specific types (ionic, covalent, hydrogen bonding, dipole-dipole, London dispersion).

Key Terms:

  • Intramolecular forces: Forces holding atoms together within a molecule (e.g., covalent, ionic bonds).

  • Intermolecular forces: Forces between molecules (e.g., hydrogen bonds, dipole-dipole, London dispersion).

Step-by-Step Guidance

  1. Read each statement and identify if it refers to forces within a molecule or between molecules.

  2. Recall the definitions and examples of each type of force.

  3. Match the description to the correct force (e.g., "holds molecules together" = intermolecular; "holds atoms together" = intramolecular).

Try solving on your own before revealing the answer!

Q17. What is harder to break: the H–O bond in water or a hydrogen bond between two water molecules? Explain why.

Background

Topic: Bond Strength and Intermolecular Forces

This question asks you to compare the strength of covalent bonds (intramolecular) and hydrogen bonds (intermolecular).

Key Concepts:

  • Covalent bonds are generally much stronger than intermolecular forces.

  • Hydrogen bonds are a type of intermolecular force.

Step-by-Step Guidance

  1. Recall the difference between covalent bonds and hydrogen bonds.

  2. Consider the energy required to break each type of bond.

  3. Think about what happens during boiling or melting (breaking intermolecular vs intramolecular bonds).

Try solving on your own before revealing the answer!

Q18. Why are London dispersion forces weaker than dipole-dipole forces and hydrogen bonds?

Background

Topic: Intermolecular Forces

This question tests your understanding of the relative strengths of different intermolecular forces.

Key Concepts:

  • London dispersion forces arise from temporary dipoles.

  • Dipole-dipole forces arise from permanent dipoles.

  • Hydrogen bonds are a special, strong type of dipole-dipole interaction.

Step-by-Step Guidance

  1. Recall what causes London dispersion forces (instantaneous dipoles).

  2. Compare the magnitude and duration of these forces to dipole-dipole and hydrogen bonds.

  3. Think about the types of molecules that exhibit each force.

Try solving on your own before revealing the answer!

Q19. Explain what is happening to molecules when they undergo a phase change. Draw a picture to support your explanation.

Background

Topic: Phase Changes

This question asks you to describe molecular behavior during phase changes (e.g., melting, boiling).

Key Concepts:

  • Phase changes involve changes in intermolecular forces, not intramolecular bonds.

  • Energy is absorbed or released during phase changes.

Step-by-Step Guidance

  1. Describe what happens to the arrangement and movement of molecules during a phase change.

  2. Explain how energy input or removal affects intermolecular forces.

  3. Sketch a simple diagram showing molecules in different phases (solid, liquid, gas).

Try solving on your own before revealing the answer!

Q20. Explain how a polar molecule can induce a dipole on a nonpolar molecule.

Background

Topic: Induced Dipoles and Intermolecular Forces

This question is about how polar molecules can affect nearby nonpolar molecules, leading to induced dipoles.

Key Concepts:

  • Induced dipole: A temporary dipole created in a nonpolar molecule by the presence of a nearby polar molecule.

  • This leads to dipole-induced dipole interactions.

Step-by-Step Guidance

  1. Recall that polar molecules have a permanent dipole.

  2. Consider how the electric field from a polar molecule can distort the electron cloud of a nonpolar molecule.

  3. Describe how this creates a temporary dipole in the nonpolar molecule.

Try solving on your own before revealing the answer!

Q21. Draw each of the following molecules and label the intermolecular forces they have: NH3, N2, CH2S. Which of these will have the highest boiling point?

Background

Topic: Molecular Structure and Intermolecular Forces

This question tests your ability to draw Lewis structures, identify intermolecular forces, and predict boiling points based on these forces.

Key Concepts:

  • NH3: Polar molecule, hydrogen bonding.

  • N2: Nonpolar molecule, London dispersion forces.

  • CH2S: Polar molecule, dipole-dipole forces.

Step-by-Step Guidance

  1. Draw the Lewis structure for each molecule.

  2. Determine the molecular geometry and polarity.

  3. Identify the dominant intermolecular forces present in each.

  4. Compare the strength of these forces to predict which molecule has the highest boiling point.

Try solving on your own before revealing the answer!

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