This set of Multiple Choice Questions & Answers (MCQs) focuses on “p-Block Elements – Important Trends and Anomalous Properties of Boron”.
- Which of the following is true regarding the acidic character?
a) Aluminium halides acidic character is greater than that of indium halide
b) Boron halide acidic character is less than that of gallium halide
c) Gallium halides acidic character is less than that of indium halide
d) Aluminium halides acidic character is greater than that of boron halide
Answer: a
Explanation: The halides of group 13 elements exhibit Lewis acidic behavior. Their acidic strength decreases in the order: boron > aluminium > gallium > indium. In this context, halides refer to compounds with chlorine, bromine, or iodine. Thallium trichloride, however, does not follow this trend and primarily acts as an oxidizing agent due to its tendency to decompose readily.
- Which of the following is a colourless gas?
a) Boron chloride
b) Boron fluoride
c) Boron Bromide
d) Boron iodide
Answer: b
Explanation: All boron halides exist as trihalides. Boron chloride and boron bromide are colorless, fuming liquids, while boron fluoride is a colorless gas. Boron iodide, on the other hand, appears as a white solid at room temperature. All these boron trihalides act as Lewis acids.
- Which of the following element has the highest melting point?
a) thallium
b) gallium
c) aluminium
d) boron
Answer: d
Explanation: Boron has a significantly higher melting point than the other elements in group 13 due to its three-dimensional structure, where boron atoms are tightly bonded through strong covalent bonds. This strong bonding also contributes to its relatively higher boiling point.
- Boron shows a diagonal relationship with ____________
a) thallium
b) lithium
c) magnesium
d) aluminium
Answer: c
Explanation: Boron (B) exhibits a diagonal relationship with magnesium (Mg) in the periodic table. This is because elements positioned diagonally to each other often display similar properties, owing to their comparable charge densities, electro negativities, and atomic or ionic radii.
- ____________ and Boron are same in case when reacted with concentrated nitric acid.
a) Oxygen
b) Aluminium
c) Hydrogen
d) No other metal
Answer: d
Explanation: Boron reacts with concentrated nitric acid, undergoing oxidation to form boric acid and nitrous oxide. In contrast, aluminium becomes passive in the presence of concentrated nitric acid because a protective oxide layer forms on its surface, preventing further reaction.
- Which of the following is the correct reason for the anomalous behaviour of Boron?
a) low ionization energy
b) smallest size in the group
c) low electronegativity
d) the presence of the orbital and can show allotropy
Answer: b
Explanation: Boron exhibits anomalous behavior compared to other group members due to its smallest atomic size, highest ionization energy, highest electronegativity, absence of vacant d-orbitals, and presence of allotropy — characteristics not shared by the other elements in the group.
- Which of the following compound is formed when aluminium reacts with an alkali?
a) sodium Tetra hydroxyl aluminate V
b) sodium Tetra hydroxyl aluminate III
c) sodium Penta hydroxyl aluminate IV
d) sodium Septa hydroxyl aluminate III
Answer: b
Explanation: When two moles of aluminum react with two moles of sodium hydroxide in the presence of six moles of water, the reaction yields two moles of sodium tetrahydroxoaluminate(III) and three moles of hydrogen gas.
- Group 13 hydrides are formed directly.
a) true
b) false
Answer: b
Explanation: Group 13 elements do not react directly with hydrogen to form hydrides. As a result, their hydrides are synthesized using indirect methods. For instance, diborane can be produced by reacting boron trifluoride with lithium aluminium hydride in the presence of dry ether.
- Do Boron and silicon react with electropositive metals?
a) Yes
b) No
c) Maybe
d) May not be
Answer: a
Explanation: Boron and silicon both form covalent hydrides—boranes and silanes, respectively. They also react with electropositive metals to produce binary compounds, which, upon hydrolysis, generate a mixture of boranes and silanes.
- Which of the following elements is more reactive with air?
a) indium
b) gallium
c) thallium
d) all three are equally reactive
Answer: c
Explanation: Thallium is more reactive than gallium and indium, primarily because it readily forms a unipositive ion, Tl⁺. When exposed to air, thallium reacts with oxygen to form thallium oxide. Among its oxides, Tl₂O is more stable than Tl₂O₃, owing to the inert pair effect.
- ____________ Boron is reactive with air.
a) Neither crystalline nor amorphous
b) Crystalline
c) Both crystalline and amorphous
d) Amorphous
Answer: d
Explanation: Crystalline boron is chemically inert, while amorphous boron is more reactive. At around 700°C, amorphous boron reacts with air, where it separately combines with oxygen and nitrogen to form boron oxide and boron nitride.
- Boron and silicon form covalent and volatile halides.
a) true
b) false
Answer: a
Explanation: Boron from group 13 and silicon from group 14 both form covalent, volatile halides that fume when exposed to moist air due to the release of hydrogen chloride (HCl) gas. Upon reacting with water, these halides produce boric acid and silicon hydroxide, respectively, along with the release of volatile HCl gas.
- All the oxides and hydroxides of Boron family are ____________ in nature.
a) acidic
b) basic
c) acidic and basic
d) acidic, basic and amphoteric
Answer: d
Explanation: As we move down Group 13 in the periodic table, the nature of the oxides and hydroxides shifts from acidic to amphoteric, and eventually to basic. Among them, only boric acid is water-soluble, whereas the hydroxides of the other elements in the group are generally insoluble in water.
- Which of the following property is not a similarity between Boron and silicon?
a) metals
b) non-metals metals
c) semiconductors
d) formation of covalent hydrides
Answer: a
Explanation: Boron from group 13 and silicon from group 14 exhibit a diagonal relationship and share several similarities. Both elements are non-metals and semiconductors. They also form covalent hydrides, which further confirms that they are not metals.
- Which of the following elements do you think can react with water?
a) Thallium
b) Boron
c) Aluminium
d) Gallium
Answer: a
Explanation: Boron and aluminium are unreactive towards water; however, amalgamated aluminium reacts with water, releasing hydrogen gas. Gallium and indium also do not react with water, whether cold or hot, whereas thallium forms a surface oxide layer upon exposure.