Comprehensive Step-by-Step Guidance for Introductory Chemistry Exam 3 Review

Q1. Write the Lewis structure for lithium nitride. What kind of chemical bond is formed?

Background

Topic: Lewis Structures and Chemical Bonding

This question tests your ability to draw Lewis structures for ionic compounds and to identify the type of chemical bond present.

Key Terms and Concepts:

• Lewis Structure: A diagram that shows the arrangement of valence electrons among atoms in a molecule or ion.

• Ionic Bond: A chemical bond formed through the transfer of electrons from a metal to a nonmetal.

• Lithium Nitride Formula: $\mathrm{Li_3N}$

Step-by-Step Guidance

1. Determine the chemical formula for lithium nitride. Lithium forms $+1$ ions and nitrogen forms $-3$ ions, so the formula is $\mathrm{Li_3N}$.

2. Draw the Lewis structure for each atom: Lithium ($\mathrm{Li}$) has one valence electron, and nitrogen ($\mathrm{N}$) has five valence electrons.

3. Show the transfer of electrons from each lithium atom to the nitrogen atom, resulting in three $\mathrm{Li^+}$ ions and one $\mathrm{N^{3-}}$ ion.

4. Represent the ions in the Lewis structure, indicating the charges and the resulting ionic bond.

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Q2. How many resonance structures are possible for the nitrate ion? Show all of them.

Background

Topic: Resonance Structures

This question tests your understanding of resonance in polyatomic ions and your ability to draw all valid resonance structures for a given ion.

Key Terms and Concepts:

• Resonance Structures: Different Lewis structures for the same molecule or ion that show the same arrangement of atoms but different arrangements of electrons.

• Nitrate Ion: $\mathrm{NO_3^-}$

Step-by-Step Guidance

1. Count the total number of valence electrons for $\mathrm{NO_3^-}$: Nitrogen has 5, each oxygen has 6, and there is one extra electron for the negative charge.

2. Draw one valid Lewis structure for $\mathrm{NO_3^-}$, ensuring all atoms have full octets where possible.

3. Move the double bond between nitrogen and each oxygen atom in turn to generate all possible resonance structures.

4. Make sure to use arrows to indicate resonance between the structures.

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Q3. Show the Lewis structure of NH3. How many bonding electrons are in NH3?

Background

Topic: Lewis Structures and Bonding Electrons

This question tests your ability to draw the Lewis structure for a covalent molecule and to count the number of electrons involved in bonding.

Key Terms and Concepts:

• Bonding Electrons: Electrons shared between atoms to form covalent bonds.

• NH3 (Ammonia): Consists of one nitrogen atom and three hydrogen atoms.

Step-by-Step Guidance

1. Count the total number of valence electrons: Nitrogen has 5, each hydrogen has 1, for a total of 8 valence electrons.

2. Draw the Lewis structure with nitrogen in the center and three hydrogens bonded to it.

3. Identify the bonding pairs (shared electrons) between nitrogen and each hydrogen atom.

4. Count the total number of electrons involved in these bonds.

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Q4. Show the Lewis structure of carbon tetrachloride. How many lone pairs of electrons are in the Lewis structure of carbon tetrachloride?

Background

Topic: Lewis Structures and Lone Pairs

This question tests your ability to draw the Lewis structure for a molecule and to count the number of lone pairs present.

Key Terms and Concepts:

• Lone Pair: A pair of valence electrons not involved in bonding.

• Carbon Tetrachloride: $\mathrm{CCl_4}$

Step-by-Step Guidance

1. Count the total number of valence electrons: Carbon has 4, each chlorine has 7, for a total of $4 + (4 \times 7) = 32$ valence electrons.

2. Draw the Lewis structure with carbon in the center and four chlorines bonded to it.

3. Assign the remaining electrons as lone pairs on the chlorine atoms.

4. Count the total number of lone pairs present in the structure.

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Q5. What is the molecular geometry of boron trifluoride?

Background

Topic: Molecular Geometry (VSEPR Theory)

This question tests your ability to predict the shape of a molecule using the Valence Shell Electron Pair Repulsion (VSEPR) theory.

Key Terms and Concepts:

• Boron Trifluoride: $\mathrm{BF_3}$

• VSEPR Theory: Predicts the geometry of molecules based on electron pair repulsion.

Step-by-Step Guidance

1. Draw the Lewis structure for $\mathrm{BF_3}$: Boron in the center, three fluorines bonded to it.

2. Count the number of electron domains (bonding pairs and lone pairs) around the central atom (boron).

3. Use VSEPR theory to determine the molecular geometry based on the number of electron domains.

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