General Science MCQs – Page 25 (Q241-250)

Oxygen has the atomic number 8, meaning eight protons in its nucleus, and it is essential for aerobic respiration in most living organisms, serving as the final electron acceptor in cellular processes that generate usable energy from food. Oxygen typically exists as a diatomic molecule, O2, in the atmosphere, but also forms ozone, O3, a triatomic allotrope concentrated in the stratosphere that absorbs harmful ultraviolet radiation from the Sun. Oxygen is highly reactive and supports combustion, which is why fire requires oxygen to burn, and it readily combines with countless other elements to form oxide compounds, including water and rust.

Iron has the atomic number 26, meaning 26 protons in its nucleus, and it is the most abundant element by mass in Earth as a whole, concentrated heavily in the planet's molten outer and solid inner core, where its convective and rotational movement generates Earth's protective magnetic field. Biologically, iron is central to hemoglobin, enabling oxygen transport in blood, and industrially, iron combined with carbon and other elements produces steel, one of the most widely used structural materials in construction, transportation, and manufacturing worldwide, underscoring iron's dual importance across biology, geology, and engineering.

Gold has the atomic number 79, meaning 79 protons in its nucleus, and it is renowned for its exceptional resistance to corrosion and tarnishing, since it is one of the least reactive metals, allowing gold artifacts and jewelry to retain their luster for thousands of years without significant chemical degradation. Gold is also highly malleable and ductile, meaning it can be hammered into extremely thin sheets or drawn into fine wires without breaking, and it conducts electricity well, which is why it is used in reliable electronic connectors despite its high cost. Gold's rarity and enduring properties have made it a valued store of wealth and currency throughout human history.

Uranium, atomic number 92, is a naturally occurring radioactive element used as nuclear fuel in power plants and, historically, in nuclear weapons, due to its ability to undergo nuclear fission, in which a uranium-235 nucleus splits after absorbing a neutron, releasing an enormous amount of energy along with additional neutrons that can sustain a chain reaction. Natural uranium consists mostly of the more stable uranium-238 isotope, with only about 0.7 percent being the fissile uranium-235, which is why uranium must be enriched to increase the U-235 concentration before it becomes usable as reactor fuel or weapons material.

Dmitri Mendeleev, a Russian chemist, is widely credited as the main developer of an early, highly influential version of the periodic table in 1869, arranging the then-known elements by increasing atomic mass while grouping those with similar chemical properties together. Remarkably, Mendeleev left deliberate gaps in his table for elements not yet discovered, successfully predicting the existence and properties of elements like gallium and germanium before they were actually found, a triumph that strongly validated his periodic arrangement. His table, though later refined by organizing elements by atomic number instead of mass, remains the direct ancestor of the modern periodic table used today.

Henry Moseley, an English physicist, is credited with establishing the modern periodic law in 1913, demonstrating through X-ray spectroscopy experiments that elements should be arranged by increasing atomic number, the number of protons in the nucleus, rather than by atomic mass as Mendeleev had originally done. This refinement resolved several inconsistencies present in the atomic-mass-based ordering, such as certain elements appearing out of place when grouped by chemical properties. Moseley's promising scientific career was tragically cut short when he was killed in action during World War I at the Battle of Gallipoli in 1915, at just 27 years old.

Elements placed in the same group, or vertical column, of the periodic table generally share the same number of valence electrons, the electrons in the outermost shell that primarily determine an element's chemical bonding behavior and reactivity. This shared valence electron count is why elements within a group exhibit remarkably similar chemical properties, such as all group 1 alkali metals readily losing a single electron to form a plus-one ion, or all group 17 halogens readily gaining a single electron to form a minus-one ion, making group position a powerful predictor of an element's typical chemical behavior in reactions.

Valence electrons are the electrons found in the outermost occupied shell of an atom, and they are the primary electrons involved in forming chemical bonds with other atoms, since they are the least tightly bound to the nucleus and most accessible for sharing, gaining, or losing during chemical reactions. The number of valence electrons directly determines an element's group placement in the periodic table and largely predicts its chemical reactivity and the types of bonds it tends to form, whether ionic, covalent, or metallic. Inner-shell electrons, by contrast, are generally not involved in ordinary chemical bonding, remaining shielded closer to the nucleus.

On the standard periodic table, metals are generally located on the left side and middle of the table, encompassing the majority of known elements, including the alkali metals, alkaline earth metals, transition metals, and lanthanides and actinides, all typically characterized by properties like luster, malleability, ductility, and good electrical and thermal conductivity. This large metallic region reflects the tendency of elements with fewer valence electrons to lose them relatively easily, forming positive ions and metallic bonds. A zigzag line of metalloid elements, such as silicon and germanium, separates the metals from the nonmetals clustered in the upper right.

On the standard periodic table, nonmetals are generally located on the right side of the table, including elements like nitrogen, oxygen, sulfur, and the halogens, typically characterized by properties opposite to metals, such as poor electrical and thermal conductivity, brittleness in solid form, and a tendency to gain or share electrons rather than lose them during chemical bonding. The noble gases occupy the far-right column as an especially unreactive subset of nonmetals. A zigzag staircase line running through the right portion of the table marks the boundary between nonmetals and the metalloid elements that share intermediate properties of both metals and nonmetals.