APCE Chemical Bonding & Molecular Structure 1 — Questions and Answers
Question 1: What type of bond is formed when two atoms share electrons?
- Ionic bond.
- Covalent bond. (Correct answer)
- Metallic bond.
- Van der Waals bond.
Correct answer: Covalent bond.
A covalent bond is formed when two atoms share one or more pairs of electrons to achieve a more stable electron configuration, typically an octet. This sharing occurs between nonmetal atoms and results in the formation of molecules. The shared electrons are attracted to the nuclei of both atoms, holding them together.
Question 2: What is the octet rule?
- Atoms tend to form bonds to increase their atomic number.
- Atoms tend to have 8 electrons in their valence shell. (Correct answer)
- Atoms tend to gain only electrons.
- Atoms can have more than 8 electrons in their valence shell.
Correct answer: Atoms tend to have 8 electrons in their valence shell.
The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with eight electrons in their outermost (valence) shell. This configuration, similar to that of noble gases, is energetically favorable and drives much of chemical bonding. While there are exceptions, it's a fundamental principle for many main group elements.
Question 3: What is the main characteristic of ionic bonding?
- Electrons are shared equally between atoms.
- One atom donates electrons to another atom, forming oppositely charged ions. (Correct answer)
- Atoms do not gain or lose electrons.
- Electrons are transferred but not shared.
Correct answer: One atom donates electrons to another atom, forming oppositely charged ions.
Ionic bonding involves the complete transfer of one or more valence electrons from a metal atom to a nonmetal atom. This transfer results in the formation of positively charged cations and negatively charged anions, which are then held together by strong electrostatic forces of attraction. The large electronegativity difference between the atoms facilitates this electron transfer.
Question 4: What is the bond angle in a molecule with a tetrahedral geometry?
- 90°.
- 120°.
- 109.5°. (Correct answer)
- 180°.
Correct answer: 109.5°.
In a molecule with tetrahedral geometry, such as methane (CH₄), a central atom is bonded to four other atoms with no lone pairs. The electron domains repel each other equally, arranging themselves in three dimensions to maximize separation. This specific arrangement results in bond angles of approximately 109.5 degrees between any two bonds.
Question 5: Which of the following molecules has a polar covalent bond?
- O₂.
- H₂O. (Correct answer)
- Cl₂.
- N₂.
Correct answer: H₂O.
A polar covalent bond forms when there is an unequal sharing of electrons between two atoms due to a difference in their electronegativities. In H₂O, oxygen is significantly more electronegative than hydrogen, pulling the shared electrons closer to itself. This creates partial negative charges on oxygen and partial positive charges on hydrogen, making the O-H bonds polar.
Question 6: What is the molecular geometry of a molecule with two bonding pairs and no lone pairs of electrons on the central atom?
- Tetrahedral.
- Linear. (Correct answer)
- Bent.
- Trigonal planar.
Correct answer: Linear.
According to VSEPR theory, if a central atom has two bonding pairs and no lone pairs, the electron domains will arrange themselves as far apart as possible to minimize repulsion. This results in a linear geometry with a bond angle of 180 degrees. An example is carbon dioxide (CO₂), where the central carbon atom is double-bonded to two oxygen atoms.
Question 7: Which of the following elements has the highest electronegativity?
- Oxygen.
- Fluorine. (Correct answer)
- Chlorine.
- Nitrogen.
Correct answer: Fluorine.
Electronegativity generally increases across a period and decreases down a group. Fluorine is located in the top right corner of the periodic table (excluding noble gases) and is the most electronegative element. Its small atomic radius and high effective nuclear charge allow it to strongly attract electrons in a chemical bond.
Question 8: What is the difference between sigma and pi bonds?
- Sigma bonds are weaker than pi bonds.
- Sigma bonds are formed by head-on overlapping, pi bonds by side-on overlapping. (Correct answer)
- Pi bonds are stronger than sigma bonds.
- Pi bonds are formed by head-on overlapping.
Correct answer: Sigma bonds are formed by head-on overlapping, pi bonds by side-on overlapping.
Sigma (σ) bonds are the strongest type of covalent bond, formed by the direct, head-on (end-to-end) overlap of atomic orbitals along the internuclear axis. Pi (π) bonds, on the other hand, are weaker and are formed by the side-on (lateral) overlap of unhybridized p orbitals above and below the internuclear axis. A single bond is always a sigma bond, while double and triple bonds contain one sigma and one or two pi bonds, respectively.
Question 9: Which of the following compounds is ionic?
- NaCl. (Correct answer)
- H₂O.
- CO₂.
- O₂.
Correct answer: NaCl.
Sodium chloride (NaCl) is an ionic compound because it is formed between a metal (sodium, Na) and a nonmetal (chlorine, Cl) with a large electronegativity difference. Sodium readily loses an electron to become Na⁺, and chlorine readily gains an electron to become Cl⁻. These oppositely charged ions are held together by strong electrostatic forces.
APCE Chemical Bonding & Molecular Structure 1
What type of bond is formed when two atoms share electrons?