General Science MCQs – Page 26 (Q251-260)

Elements such as silicon and boron, which display properties of both metals and nonmetals, are called metalloids, positioned along the zigzag diagonal line separating true metals from nonmetals on the periodic table, and typically including boron, silicon, germanium, arsenic, antimony, and tellurium. Metalloids often appear shiny like metals but are brittle like nonmetals, and their most important shared characteristic is semiconducting behavior, meaning their electrical conductivity falls between conductors and insulators and can be precisely controlled through doping, which is exactly why silicon and germanium form the technological backbone of computer chips, transistors, and solar cells.

Metals are generally good conductors of both heat and electricity, a property arising from their unique metallic bonding structure, in which outer valence electrons are not tightly bound to individual atoms but instead form a delocalized sea of electrons that can move freely throughout the metal's structure. This same free electron mobility explains both properties simultaneously: electrons carry electric charge to conduct electricity, and they also efficiently transfer kinetic energy as heat throughout the material. This shared electron sea is also responsible for metallic luster, malleability, and ductility, distinguishing metals fundamentally from the more rigidly, covalently or ionically bonded nonmetals.

Compared to metals, nonmetals are generally poor conductors of heat and electricity, since their electrons are typically tightly bound within covalent or ionic bonds rather than existing as a freely moving electron sea, leaving few or no charge carriers available to transport electric current or thermal energy efficiently through the material. This is why nonmetal materials like rubber, glass, and wood are commonly used as electrical and thermal insulators. Notable exceptions exist, such as graphite, a form of carbon that conducts electricity well due to its unique layered structure with delocalized electrons, despite carbon otherwise being classified as a nonmetal.

The property of metals that allows them to be hammered or rolled into thin sheets without breaking is called malleability, arising from the metallic bonding structure in which layers of positively charged metal ions can slide past one another while remaining held together by the surrounding sea of delocalized electrons, absorbing the applied stress without fracturing the material. Gold is exceptionally malleable, capable of being hammered into sheets so thin they become translucent, while metals like tin and aluminum are also prized for this property in manufacturing foil and sheet metal. Malleability contrasts with brittleness, seen in most ionic and covalent solids, which fracture rather than deform under stress.

The property of metals that allows them to be drawn or stretched into thin wires without breaking is called ductility, which, like malleability, results from the metallic bonding structure allowing layers of metal atoms to slide past one another while the delocalized electron sea maintains the overall bonding integrity throughout the deformation process. Copper and gold are both highly ductile, which is why copper is the standard material for electrical wiring, capable of being drawn into extremely fine strands. Ductility and malleability often occur together in the same metals, though the specific degree of each property varies considerably between different metallic elements.

Out of all known chemical elements, metals make up the clear majority found on the periodic table, comprising roughly three-quarters of all elements, including the alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and several post-transition metals. Nonmetals and metalloids together account for a comparatively small remaining portion, clustered primarily in the upper right region of the table. This numerical dominance of metals reflects the periodic table's structure, where the vast middle block of transition metals alone spans ten groups, contributing significantly to the overall preponderance of metallic elements across the full range of known chemistry.

Transition metals are generally located in the middle block of the periodic table, spanning groups 3 through 12, and include familiar elements like iron, copper, zinc, nickel, and gold, known for typically forming multiple different oxidation states, producing colorful compounds, and often acting as effective catalysts in chemical reactions. Their partially filled d-orbitals, a distinctive feature of this block, give rise to these characteristic variable valencies and vivid coloration, seen for example in the different colored solutions formed by various oxidation states of manganese or chromium compounds, a property less commonly seen among main-group metals in groups 1 and 2.

The lanthanides and actinides, placed separately below the main body of the periodic table to keep it a manageable width, are together known as the inner transition metals, since both series involve the progressive filling of inner f-orbitals rather than the d-orbitals filled across the main transition metal block. The lanthanides, sometimes called rare earth elements, are used in magnets, lasers, and phone screens, while the actinides include naturally occurring radioactive elements like uranium and thorium as well as synthetically created elements like plutonium, many of which are significant in nuclear technology and weapons applications.

Radioactive elements are generally characterized by unstable nuclei that spontaneously decay over time, releasing energy in the form of particles or electromagnetic radiation, such as alpha particles, beta particles, or gamma rays, as the nucleus transforms into a more stable configuration, sometimes becoming an entirely different element in the process. This decay occurs at a statistically predictable rate described by an element's half-life, the time required for half of a given sample to decay, ranging from fractions of a second to billions of years depending on the isotope. Marie and Pierre Curie pioneered early research into radioactivity, a term Marie Curie herself coined.

Uranium and plutonium are the two radioactive elements commonly used as fuel in nuclear power reactors, both capable of undergoing nuclear fission, in which their heavy nuclei split apart after absorbing a neutron, releasing tremendous energy along with additional neutrons that sustain a controlled chain reaction. Uranium-235 occurs naturally, though it requires enrichment to increase its concentration from the more abundant uranium-238, while plutonium-239 is typically produced artificially within reactors when uranium-238 absorbs a neutron and undergoes subsequent radioactive decay. Both elements were also central to the development of nuclear weapons during and after World War II.