In the periodic table, elements with identical valence electron configurations are grouped together. These are referred to as groups, vertical columns, or chemical families, as they exhibit similar chemical properties due to their shared electronic structure in the outermost shell.

Based on standard chemical properties: (a) Recently named (e.g., Copernicium), (b) Iron (variable valency), (c) Fluorine (max electronegativity), (d) Chlorine (max electron affinity). The mapping aligns with option D.

Atomic number increases as you move from left to right across a period. Beryllium (Be) has an atomic number of 4, and Boron (B) has an atomic number of 5. Therefore, the sequence Be, B represents an increase in atomic number. The other options (Ca=20, Cl=17; Na=11, Ne=10; He=2, H=1) show a decrease in atomic number.

Cesium (Cs) is the most reactive alkali metal. Fluorine (F) is the most reactive non-metal. Osmium (Os) has the highest density among metals. Mercury (Hg) is the only transition metal that is liquid at room temperature. Thus, the correct mapping is a-2, b-1, c-4, d-3.

Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself within a covalent bond. It is a dimensionless quantity often measured on the Pauling scale. Higher electronegativity values indicate a stronger attraction for electrons, which significantly influences the polarity of chemical bonds and the overall reactivity of molecules in various chemical environments.

As you move down a group, the number of electron shells increases, leading to a larger atomic radius. This increased distance between the nucleus and the valence electrons reduces the effective nuclear attraction, making it easier for the atom to lose electrons. Consequently, the metallic character, which is the tendency to lose electrons, increases as you descend a group in the periodic table.

Periodic properties are those that vary in a predictable manner based on atomic number and electronic configuration. Atomic size, valency, and electronegativity all show clear trends. Radioactivity, however, depends on the stability of the atomic nucleus and does not follow a predictable periodic trend across the table.

Elements in the same group share similar chemical properties because they possess the same number of valence electrons in their outermost shells. Since chemical reactivity is primarily determined by valence electron configuration, elements in a group undergo similar chemical reactions and form compounds with similar stoichiometry.

Group 18 elements, known as noble gases, possess a stable electronic configuration with a complete octet (except Helium, which has a complete duplet). This stable arrangement makes them chemically inert under standard conditions, as they have little tendency to gain, lose, or share electrons.

The configuration 2, 8, 1 indicates three occupied electron shells, placing the element in the third period. Since there is only one electron in the outermost shell, it belongs to Group 1 (the alkali metals). This element is sodium (Na).