Question
In which molecule does the central atom have an incomplete octet of electrons?
A. \(\rm H_2Se\)
B. \(\rm PH_3\)
C. \(\rm OF_2\)
D. \(\rm BF_3\)
▶️Answer/Explanation
Markscheme: D
An incomplete octet occurs when a central atom does not have eight electrons in its valence shell. Let’s examine the options:
A. \(\rm H_2Se\) – Selenium (Se) is in Group 16 and can accommodate more than 8 electrons. This molecule has a complete octet around selenium.
B. \(\rm PH_3\) – Phosphorus (P) is in Group 15, so it can have fewer than 8 electrons in its valence shell. This molecule has an incomplete octet around phosphorus.
C. \(\rm OF_2\) – Oxygen (O) is in Group 16 and generally follows the octet rule. This molecule has a complete octet around oxygen.
D. \(\rm BF_3\) – Boron (B) is in Group 13 and often has fewer than 8 electrons in its valence shell. This molecule has an incomplete octet around boron.
Therefore, the molecule in which the central atom has an incomplete octet of electrons is D. \(\rm BF_3\).
Question
What is the electron domain geometry of sulfur dioxide, SO2?
A. bent
B. linear
C. tetrahedral
D. trigonal planar
▶️Answer/Explanation
Markscheme: D
The Lewis structure of \(SO_2\) shows that sulfur has one lone pair and forms two sigma bonds with oxygen atoms. This results in three electron domains around the sulfur atom.
Given that there are three electron domains, the electron domain geometry is trigonal planar. Each of these domains can be either a lone pair or a bonding pair.
So, the correct answer for the electron domain geometry of sulfur dioxide (\(SO_2\)) is indeed:
D. trigonal planar
Question
Which species has the longest carbon to oxygen bond length?
A. CO
B. CH3OH
C. CH3CO2−
D. H2CO
Answer/Explanation
Markscheme
B
Examiners report
Question
Which substance has the following properties?
• Low melting point
• Very soluble in water
• Does not conduct electricity when molten
A. Glucose, \({{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}\)
B. Silicon dioxide, \({\text{Si}}{{\text{O}}_{\text{2}}}\)
C. Sodium chloride, NaCl
D. Tetrachloromethane, \({\text{CC}}{{\text{l}}_{\text{4}}}\)
Answer/Explanation
Markscheme
A
Question
Which molecule contains a bond angle of approximately 120°?
A. \({\text{C}}{{\text{H}}_{\text{4}}}\)
B. \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}\)
C. \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}\)
D. \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}\)
Answer/Explanation
Markscheme
C