General Science MCQs – Page 27 (Q261-270)

Graphite, a common allotrope of carbon, is unusual among nonmetals because it can conduct electricity, a property arising from its distinctive layered structure, in which each carbon atom bonds to only three neighboring atoms within flat hexagonal sheets, leaving one delocalized electron per atom free to move across the layer, similar in effect to the electron sea found in metals. This same layered structure, with weak forces holding the sheets together, also makes graphite soft and slippery, useful as a lubricant and as the writing material in pencils. Diamond, another carbon allotrope with all four electrons locked into rigid bonds, does not share this conductivity.

Unlike graphite, diamond does not conduct electricity because it lacks free-moving electrons, since every one of its carbon atoms forms four strong, rigid covalent bonds to neighboring carbon atoms in a three-dimensional tetrahedral lattice, leaving no delocalized electrons available to carry electric current. This same fully bonded, rigid structure is precisely what gives diamond its extreme hardness and high melting point, in stark contrast to graphite's layered structure, where each carbon atom bonds to only three neighbors, leaving one free electron per atom that can move and conduct electricity. Both substances are pure carbon, demonstrating how structural arrangement alone can produce dramatically different physical and electrical properties.

Oxygen has an atomic number of 8, meaning each oxygen atom contains eight protons in its nucleus, placing it in period 2 and group 16 of the periodic table, alongside sulfur and other chalcogen elements. With six valence electrons, oxygen readily gains two additional electrons to complete its outer shell, forming stable minus-two oxide ions or sharing electrons through covalent bonds, as seen in water and carbon dioxide. Oxygen is the most abundant element in Earth's crust by mass and essential for aerobic respiration in most living organisms, making it one of the most exam-relevant elements across both chemistry and biology topics.

Chlorine has an atomic number of 17, meaning each chlorine atom contains seventeen protons in its nucleus, placing it in period 3 and group 17, the halogens, alongside fluorine, bromine, iodine, and astatine. With seven valence electrons, chlorine readily gains one additional electron to achieve a stable, noble-gas-like configuration, making it highly reactive, as seen in its vigorous combination with sodium to form common table salt. Chlorine gas is toxic and was notoriously used as a chemical weapon in World War I, but in controlled, diluted form it is widely used today to disinfect drinking water and swimming pools by killing harmful microorganisms.

Helium has an atomic number of 2, meaning each helium atom contains two protons in its nucleus, making it the second element on the periodic table and the second lightest element overall after hydrogen. Helium's two electrons completely fill its single electron shell, giving it exceptional chemical stability as the first member of the noble gas family, meaning it essentially never reacts with other elements under normal conditions. Formed primarily through nuclear fusion in stars, including the Sun, helium is used to fill balloons and airships due to its low density and non-flammability, a much safer alternative to the highly flammable hydrogen once used for the same purpose.

Helium is generally considered the second lightest element after hydrogen, with an atomic mass of about 4 atomic mass units compared to hydrogen's roughly 1 atomic mass unit, reflecting hydrogen's single proton nucleus versus helium's two protons and typically two neutrons. Despite being heavier than hydrogen, helium is still far less dense than air, which is why helium-filled balloons still rise, though not as vigorously as the now largely abandoned hydrogen-filled versions. Helium is chemically inert as a noble gas, unlike highly flammable hydrogen, making it the much safer choice for filling balloons, blimps, and airships despite its higher relative cost and increasing global scarcity.

Neon, argon, krypton, xenon, and radon, together with helium, belong to the category of elements known as noble gases, occupying group 18 of the periodic table and sharing the defining characteristic of having a complete, stable outer electron shell, which makes them exceptionally unreactive under normal conditions. Argon is the most abundant noble gas in Earth's atmosphere, making up nearly 1 percent of the air, and is used to provide an inert atmosphere for welding and inside incandescent bulbs, while radon is radioactive and considered a health hazard when it accumulates in poorly ventilated basements from underlying uranium-containing rock and soil.

Neon is commonly used to fill glowing signs and advertising lights, since passing an electric current through neon gas at low pressure excites its atoms, causing them to emit a distinctive bright reddish-orange glow as electrons fall back to lower energy states, a phenomenon first commercially demonstrated by French engineer Georges Claude in the early twentieth century. Other noble gases can be mixed in or used alone to produce different colors in similar discharge tubes, such as argon producing a pale lavender-blue glow, though the term neon sign has become a generic name for this entire category of gas-discharge lighting regardless of which specific gas is actually used.

Mercury is the metal well known for being liquid at normal room temperature, a striking exception among metals caused by unusually weak metallic bonding between its atoms, an effect linked to relativistic behavior of its inner electrons that reduces their availability for strong bonding. Historically used in thermometers and barometers due to its uniform, predictable thermal expansion and high density, mercury's significant toxicity to the nervous system has led most modern applications to shift toward safer alternatives like digital sensors, galinstan alloy thermometers, and aneroid barometers, though mercury remains present in some fluorescent lighting and certain industrial processes.

Bromine is the nonmetal element well known for being liquid at normal room temperature, a reddish-brown, dense, and highly corrosive liquid that readily evaporates into a similarly colored, pungent, toxic vapor, making it one of only two elements, along with mercury, that exist as liquids under standard conditions. As a member of the halogen group, bromine is highly reactive, and its compounds are used in flame retardants, certain pharmaceuticals, and historically in photographic film processing. Bromine's toxicity and corrosive nature require careful handling, and it must be stored in sealed, chemically resistant containers to prevent dangerous exposure or reaction.