General Science MCQs – Page 29 (Q281-290)

Carbon-14, used in radiocarbon dating, has six protons in its nucleus, exactly the same as ordinary, stable carbon-12, since atomic number, and therefore the identity of an element, is determined solely by proton count rather than neutron count. Carbon-14 differs from carbon-12 only in having two additional neutrons, eight rather than six, making it an isotope of carbon rather than a different element entirely, and this extra neutron mass makes its nucleus unstable, causing it to undergo radioactive beta decay over time. This proton-neutron distinction is a frequently tested concept differentiating atomic number from mass number in general science exams.

An ion is formed when a neutral atom gains or loses electrons, disrupting the normally equal balance between its positively charged protons and negatively charged electrons and resulting in a net electric charge. Atoms tend to gain or lose electrons in ways that achieve a stable, full outer electron shell resembling the nearest noble gas configuration, following the octet rule, which is why metals with few valence electrons tend to lose them and form positive ions, while nonmetals with nearly full outer shells tend to gain electrons and form negative ions. Ion formation is fundamental to ionic bonding and countless chemical reactions.

A positively charged ion, formed when an atom loses one or more electrons, is called a cation, a term coined by English scientist Michael Faraday, derived from the Greek word for descending, referencing how cations move toward the negatively charged cathode during electrolysis. Metals typically form cations by losing their loosely held valence electrons to achieve a stable electron configuration, such as sodium losing one electron to become a sodium ion with a plus-one charge. Cations are essential in biology and chemistry, including sodium, potassium, and calcium ions, which play critical roles in nerve signaling and muscle contraction.

A negatively charged ion, formed when an atom gains one or more extra electrons, is called an anion, a term also coined by Michael Faraday, derived from the Greek word for ascending, referencing how anions move toward the positively charged anode during electrolysis. Nonmetals typically form anions by gaining electrons to complete their outer shell, such as chlorine gaining one electron to become a chloride ion with a minus-one charge. Anions and cations attract each other through electrostatic forces to form ionic compounds, such as sodium and chloride ions combining to form common table salt, sodium chloride.

When a sodium atom loses one electron to form an ion, the resulting sodium ion carries a plus-one charge, since sodium naturally has eleven protons and eleven electrons in its neutral state, but after losing a single valence electron, it retains eleven protons while having only ten electrons, leaving one unbalanced positive charge. This electron loss allows sodium to achieve a stable, full outer shell resembling the noble gas neon, which is energetically favorable and explains why sodium so readily loses this single valence electron during chemical reactions, such as when reacting with chlorine to form the stable ionic compound sodium chloride, common table salt.

When a chlorine atom gains one electron to form an ion, the resulting chloride ion carries a minus-one charge, since chlorine naturally has seventeen protons and seventeen electrons in its neutral state, but after gaining an additional electron, it retains seventeen protons while having eighteen electrons, leaving one unbalanced negative charge. This electron gain allows chlorine to achieve a stable, full outer shell resembling the noble gas argon, which is energetically favorable and explains chlorine's high reactivity as a halogen, readily accepting an electron from metals like sodium to form stable ionic compounds such as sodium chloride, common table salt.

Magnesium is the metal element that forms the central atom of the chlorophyll molecule in green plants, coordinated at the center of a large ring-shaped organic structure called a porphyrin ring, similar in overall architecture to the iron-centered heme group found in hemoglobin. This magnesium-centered chlorophyll molecule is essential for photosynthesis, absorbing light energy, primarily in the red and blue wavelengths while reflecting green light, which is why plants appear green to the human eye. Magnesium deficiency in soil can cause visible yellowing between leaf veins in plants, a condition called chlorosis, directly linked to impaired chlorophyll production.

Iron is the metal element that forms the central atom of the hemoglobin molecule in human blood, coordinated within a ring-shaped organic structure called a heme group, four of which are incorporated into each hemoglobin protein found in red blood cells. This centrally positioned iron atom is what allows hemoglobin to reversibly bind oxygen molecules in the lungs and release them to tissues throughout the body, and it is also responsible for blood's characteristic red color. Iron deficiency reduces the body's ability to produce functional hemoglobin, leading to iron-deficiency anemia, one of the most common nutritional deficiencies worldwide.

Nitrogen gas makes up approximately 78 percent of Earth's atmosphere by volume, making it by far the most abundant atmospheric gas, followed by oxygen at about 21 percent and much smaller amounts of argon, carbon dioxide, and trace gases. Nitrogen's relative chemical inertness, due to the strong triple covalent bond within its diatomic N2 molecule, allows it to remain largely unreactive in the atmosphere, diluting the more reactive oxygen and helping to moderate the rate of combustion on Earth's surface. Despite its atmospheric abundance, most living organisms cannot directly use nitrogen gas and depend on nitrogen-fixing bacteria to convert it into biologically usable compounds.

Oxygen gas makes up approximately 21 percent of Earth's atmosphere by volume, the second most abundant atmospheric gas after nitrogen, and this concentration is essential for supporting aerobic respiration in humans, animals, and most other complex life forms, as well as enabling combustion processes like fire. Atmospheric oxygen levels have not always been this high, having risen dramatically roughly 2.4 billion years ago during the Great Oxidation Event, driven by photosynthetic cyanobacteria releasing oxygen as a byproduct, a shift that dramatically transformed Earth's atmosphere and enabled the eventual evolution of oxygen-dependent complex life.