General Science MCQs – Page 21 (Q201-210)

The small, dense central part of an atom is called the nucleus, discovered by Ernest Rutherford in 1911 through his famous gold foil experiment, in which most alpha particles passed straight through thin gold foil while a small fraction deflected sharply, revealing that an atom's mass and positive charge are concentrated in a tiny, dense core. Although the nucleus contains almost all of an atom's mass, it occupies only a minuscule fraction of its total volume, with the rest of the atom being mostly empty space occupied by electrons. This discovery replaced the earlier plum pudding model proposed by J.J. Thomson, who had discovered the electron in 1897.

The nucleus of an atom contains two types of subatomic particles, protons and neutrons, collectively called nucleons, which are held together tightly despite their close proximity and, in the case of protons, mutual electrical repulsion, by the strong nuclear force, one of the four fundamental forces of nature and by far the strongest at very short ranges. Protons carry a positive charge while neutrons carry no charge, and together they account for nearly all of an atom's mass, since electrons, which orbit the nucleus rather than reside within it, are roughly 1,836 times lighter than a proton.

Electrons in an atom are generally found revolving around the nucleus in defined regions called shells or energy levels, arranged at increasing distances from the nucleus, with each shell able to hold a specific maximum number of electrons following patterns like the 2, 8, 8 rule for the first three shells. This shell model, refined by Niels Bohr in 1913 building on Rutherford's nuclear discovery, was later replaced in modern quantum mechanics by the more precise concept of electron orbitals, probability clouds describing where an electron is likely to be found rather than a fixed orbital path. Electron arrangement determines an element's chemical behavior.

A proton carries a positive electric charge, equal in magnitude but opposite in sign to the negative charge carried by an electron, and it is this balance between the number of protons and electrons in a neutral atom that results in the atom having no overall net charge. The number of protons in an atom's nucleus, called the atomic number, uniquely defines which chemical element the atom is, since changing the proton count changes the element entirely, unlike changing the neutron count, which merely creates a different isotope of the same element.

An electron carries a negative electric charge, equal in magnitude to the positive charge of a proton, and it is the movement of these negatively charged electrons through a conductor that constitutes electric current in most everyday electrical phenomena, even though current is conventionally described as flowing in the opposite direction, from positive to negative. Electrons occupy the outer regions of an atom in shells or orbitals, and it is specifically the electrons in the outermost shell, called valence electrons, that determine how an atom bonds chemically with other atoms, forming the basis of nearly all chemical reactions.

A neutron carries no electric charge, making it electrically neutral, as its name suggests, and this lack of charge allows neutrons to penetrate deeply into atomic nuclei without being repelled the way a positively charged particle would be by the nucleus's protons. James Chadwick discovered the neutron in 1932, completing the basic picture of atomic structure alongside the previously known proton and electron. Neutrons contribute significantly to an atom's mass without affecting its charge or chemical identity, and varying neutron counts within the same element produce isotopes, some of which are radioactive and undergo nuclear decay.

The atomic number of an element is equal to the number of protons in its nucleus, a value that uniquely identifies each element and determines its position in the periodic table, since elements are arranged in order of increasing atomic number. This is distinct from the mass number, which includes both protons and neutrons, and from atomic mass, which is a weighted average accounting for all naturally occurring isotopes of an element. Hydrogen, the simplest element, has an atomic number of 1, meaning a single proton, while uranium, one of the heaviest naturally occurring elements, has an atomic number of 92.

The mass number of an atom is equal to the total number of protons plus neutrons in its nucleus, representing the atom's approximate total mass in atomic mass units, since electrons contribute negligibly to overall mass. Mass number is typically written as a superscript before an element's symbol, such as carbon-14, indicating 14 total protons and neutrons combined. This differs from atomic number, which counts only protons, and from atomic weight, the average mass of all naturally occurring isotopes of an element weighted by their relative abundance, a value listed on the periodic table that is often a non-whole number.

Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes, meaning they share identical chemical properties and atomic number but differ in mass number and physical properties like stability. Carbon-12 and carbon-14 are well-known isotopes of carbon, with carbon-14 being radioactive and used in radiocarbon dating to determine the age of organic archaeological and geological samples up to about 50,000 years old. Isotopes should not be confused with isomers, which are compounds with the same molecular formula but different structural arrangements, or allotropes, different structural forms of the same element in the same physical state.

An element is fundamentally defined and distinguished from other elements by its atomic number, the fixed number of protons found in the nucleus of every atom of that element, since altering the proton count fundamentally transforms the atom into a different element entirely, a process called transmutation that occurs naturally in radioactive decay or artificially in nuclear reactors and particle accelerators. Physical properties like melting point, color, and physical state can vary between different forms or conditions of the same element, or overlap between different elements, making atomic number, rather than these variable characteristics, the only truly definitive identifying feature.