Bonding, Formulas & Molecular Structure — Problem set, Grade 11

Build names, formulas, Lewis structures, and structure-property explanations.

Review the key ideas

Ionic Bonding

Ionic bonding is electrostatic attraction in an ionic arrangement. Ion formation, lattice stabilization, formula neutrality and ion mobility explain different parts of a compound’s chemistry; electron transfer alone is not the bond or the full energy balance.

Covalent Bonding

Covalent bonding involves shared electron density. Discrete molecules and extended networks have different structures and properties. Bond order, polarity and energy comparisons need the actual species and bonding context.

Metallic Bonding

Metallic bonding can be modeled using positive ion cores and delocalized valence electrons. Conductivity, deformation and alloy properties also depend on structure, defects, composition and conditions.

Molecular Geometry & Polarity

VSEPR theory says electron groups spread out to set a molecule's shape, and a molecule's overall polarity depends on both its bond polarities and whether that shape lets the dipoles cancel — which in turn sets the intermolecular forces that shape a substance's properties.

Chemical Names, Formulas & Polyatomic Ions

Use charge neutrality for ionic formulas and distinguish ion charge from atom count.

Lewis Structures, Resonance & Bond Models

Count electrons and charge before predicting bonding, shape, and polarity.

Problem 1

What ion does a potassium atom (Group 1) form, and why?
Answer and explanation

K⁺, a 1+ cation.

Potassium has 1 valence electron. It loses that electron to reach a stable full outer shell, leaving one more proton than electron, so it becomes K⁺.

Problem 2

Write the formula for the ionic compound formed from calcium (Ca²⁺) and fluoride (F⁻).
Answer and explanation

CaF₂

Ca²⁺ is +2 and each F⁻ is −1, so two fluoride ions are needed to balance one calcium ion: CaF₂.

Problem 3

Explain why solid table salt does not conduct electricity, but salt dissolved in water does.
Answer and explanation

Dissolving frees the ions to move and carry charge; in the solid they are locked in place.

Conduction needs free-moving charges. In solid NaCl the ions are fixed in the lattice. Dissolving in water separates the Na⁺ and Cl⁻ ions and lets them move, so the solution conducts electricity.

Problem 4

Aluminum forms Al³⁺ and sulfur forms S²⁻. Write the formula of aluminum sulfide and explain the criss-cross reasoning.
Answer and explanation

Al₂S₃

The total charge must be zero. The charges +3 and −2 have a least common multiple of 6: two Al³⁺ give +6 and three S²⁻ give −6. Criss-crossing the charges gives the subscripts Al₂S₃.

Problem 5

Is the bond between two chlorine atoms in Cl₂ ionic or covalent? Explain.
Answer and explanation

Covalent.

Both atoms are the same nonmetal with identical electronegativity, so neither can pull the electrons away. They share a pair of electrons instead of transferring them — a covalent bond.

Problem 6

In a molecule of oxygen gas (O₂), the two atoms share two pairs of electrons. What type of bond is this, and how many electrons are shared?
Answer and explanation

A double bond; 4 electrons shared.

Two shared pairs make a double bond. Two pairs × 2 electrons per pair = 4 shared electrons.

Problem 7

Name two properties of metals that the electron-sea model explains, and briefly say why.
Answer and explanation

Any two of: electrical/thermal conductivity, malleability, ductility, luster.

Delocalized electrons move freely, so they carry electric charge and heat (conductivity), let ion layers slide without breaking (malleability and ductility), and absorb and re-emit light (luster).

Problem 8

Sodium (Group 1) and magnesium (Group 2) are neighboring metals, yet magnesium is harder and melts at a much higher temperature. Use the electron-sea model to explain.
Answer and explanation

Magnesium contributes more electrons to the sea and forms higher-charged ions, so its metallic bonding is stronger.

Each magnesium atom donates 2 valence electrons (versus 1 for sodium) and becomes Mg²⁺ (versus Na⁺). The denser electron sea and higher ion charge create a stronger attraction — stronger metallic bonds — so magnesium is harder and higher-melting.

Problem 9

Compare the melting point and electrical conductivity you would expect from (a) an ionic solid, (b) a molecular (covalent) solid, and (c) a metal, and briefly say why.
Answer and explanation

Ionic: high melting point, conducts only when molten or dissolved. Molecular: low melting point, does not conduct. Metal: usually high melting point, conducts even as a solid.

Ionic solids have strong lattice-wide electrostatic bonds (high melting) with ions free to move only when molten or dissolved (conducts then). Molecular solids are held together by weak intermolecular forces (low melting) with no free charges (insulator). Metals have strong metallic bonding (often high melting) plus a mobile electron sea (conducts even as a solid).

Problem 10

Predict the shape of a water molecule (oxygen with 2 bonding pairs and 2 lone pairs) and give its approximate bond angle.
Answer and explanation

Bent, about 104.5°.

The four electron groups arrange tetrahedrally, but only two atoms are attached, so the visible shape is bent. The two lone pairs repel strongly and squeeze the H–O–H angle to about 104.5°.

Problem 11

Explain why oil (nonpolar) does not dissolve in water (polar).
Answer and explanation

'Like dissolves like' — nonpolar oil and polar water don't attract each other enough to mix.

Water molecules attract each other strongly through hydrogen bonds. Nonpolar oil molecules can't form those attractions, so the water molecules stay bonded to each other and squeeze the oil out. Polar dissolves polar, nonpolar dissolves nonpolar.

Problem 12

A molecule of BF₃ has a central boron bonded to three fluorines with no lone pairs on boron. Each B–F bond is polar. Predict the molecular shape and state whether the whole molecule is polar or nonpolar.
Answer and explanation

Trigonal planar; nonpolar.

Three bonding groups and no lone pairs spread out at 120° into a flat, symmetric trigonal-planar shape. The three equal bond dipoles point outward at 120° and cancel exactly, so the molecule is nonpolar despite its polar bonds.

Problem 13

Why does Ca form CaCl₂ rather than CaCl?
Answer and explanation

Ca²⁺ needs two Cl⁻ ions for neutrality; a 1:1 formula would carry +1 net charge.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 14

Chapter review: Al³⁺ combines with O²⁻. Find the simplest neutral formula and explain why it is not a five-ion molecule drawing for the solid.
Answer and explanation

The least common charge magnitude is 6: two Al³⁺ and three O²⁻ give +6−6=0, so Al₂O₃. The formula states a stoichiometric ratio in an extended solid, not a claim of isolated five-ion molecules.

The least common charge magnitude is 6: two Al³⁺ and three O²⁻ give +6−6=0, so Al₂O₃. The formula states a stoichiometric ratio in an extended solid, not a claim of isolated five-ion molecules.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

How many electrons are in a triple bond?
Answer and explanation

Three shared pairs give six bonding electrons, not three.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 16

Chapter review: Compare CO₂ and H₂O: both have polar bonds. Explain why bond polarity alone cannot establish that both molecules have a net dipole.
Answer and explanation

CO₂ is linear with equal opposing bond contributions that cancel in the ideal symmetric molecule. H₂O is bent, so its bond dipoles do not cancel. Geometry and the full charge distribution matter.

CO₂ is linear with equal opposing bond contributions that cancel in the ideal symmetric molecule. H₂O is bent, so its bond dipoles do not cancel. Geometry and the full charge distribution matter.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 17

Why can solid copper conduct while solid salt generally cannot?
Answer and explanation

Copper has delocalized mobile electrons; salt’s ions are fixed in a lattice under ordinary conditions.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 18

Chapter review: A part requires strength index ≥60 and conduction index ≥70. Materials P, Q and R score (65,80), (90,50), and (50,95). Select a feasible option and name a missing design consideration.
Answer and explanation

P satisfies both stated constraints. Q fails conduction and R fails strength. Cost, corrosion, fatigue, fabrication or service temperature could still change the decision.

P satisfies both stated constraints. Q fails conduction and R fails strength. Cost, corrosion, fatigue, fabrication or service temperature could still change the decision.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 19

Compare CO₂ and water polarity.
Answer and explanation

CO₂ is linear with equal opposite bond dipoles that cancel; bent water has a nonzero molecular dipole.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 20

Chapter review: A molecule has four electron domains around its center, with two bonding pairs and two lone pairs. Predict electron-domain geometry and molecular shape; explain why bond angles need not be 109.5°.
Answer and explanation

The electron-domain arrangement is approximately tetrahedral and the molecular shape is bent. Lone-pair interactions and the actual chemical species alter angles; 109.5° is an ideal tetrahedral reference rather than an exact universal bond angle.

The electron-domain arrangement is approximately tetrahedral and the molecular shape is bent. Lone-pair interactions and the actual chemical species alter angles; 109.5° is an ideal tetrahedral reference rather than an exact universal bond angle.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 21

Name N₂O₄ using molecular prefixes.
Answer and explanation

Dinitrogen tetroxide: two nitrogen and four oxygen atoms.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 22

Why is calcium carbonate CaCO₃ rather than Ca₂(CO₃)₂?
Answer and explanation

Ca²⁺ and CO₃²⁻ balance in the smallest 1:1 ratio.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 23

Chapter review: Write the formula for iron(III) sulfate and count all atoms per formula unit. Then name N₂O₃ using molecular prefixes.
Answer and explanation

Fe³⁺ and SO₄²⁻ require Fe₂(SO₄)₃: two Fe, three S and twelve O atoms. N₂O₃ is dinitrogen trioxide. The Roman numeral and prefixes serve different bookkeeping roles.

Fe³⁺ and SO₄²⁻ require Fe₂(SO₄)₃: two Fe, three S and twelve O atoms. N₂O₃ is dinitrogen trioxide. The Roman numeral and prefixes serve different bookkeeping roles.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 24

Explain the difference between water’s electron geometry and molecular shape.
Answer and explanation

Four domains are arranged approximately tetrahedrally; considering only atom positions gives a bent molecule.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 25

Does nitrate repeatedly switch between three structures?
Answer and explanation

No. Resonance drawings represent one delocalized bonding structure with equivalent bonds in the idealized ion.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 26

Chapter review: For NH₄⁺, count valence electrons, assign nitrogen’s formal charge for four N–H single bonds and no lone pair, and distinguish that charge from molecular shape.
Answer and explanation

Total valence electrons are 5+4(1)−1=8. Nitrogen formal charge is 5−0−4=+1 and each H is zero. The flat Lewis connectivity does not specify measured angles; the common molecular arrangement is tetrahedral.

Total valence electrons are 5+4(1)−1=8. Nitrogen formal charge is 5−0−4=+1 and each H is zero. The flat Lewis connectivity does not specify measured angles; the common molecular arrangement is tetrahedral.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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