Glossary

Term	Definition
Atomic Number (Z)	Number of protons in the nucleus of an atom; unique identifier of each element
Modern Periodic Law	Physical and chemical properties of elements are periodic functions of their atomic number (Moseley, 1913)
Period	Horizontal row in the periodic table; all elements in a period have the same highest principal quantum number (n)
Group	Vertical column in the periodic table; elements share similar valence electron configuration and chemical properties
s-block	Elements whose last electron enters an s-subshell; Groups 1 and 2 (+ He)
p-block	Elements whose last electron enters a p-subshell; Groups 13–18
d-block	Elements whose last electron enters a d-subshell; Groups 3–12 (transition metals)
f-block	Elements whose last electron enters an f-subshell; lanthanoids and actinoids
Effective Nuclear Charge (Zeff)	Net positive charge experienced by an electron = $Z - \sigma$ ; determines atomic properties
Shielding Effect	Reduction in nuclear attraction on valence electrons due to inner-shell electrons; denoted by $\sigma$
Atomic Radius	Half the distance between nuclei of two adjacent identical atoms in a molecule or crystal
Ionic Radius	Radius of a monatomic ion in an ionic crystal; differs from atomic radius
Isoelectronic Species	Atoms or ions with the same number of electrons but different nuclear charges
Ionization Enthalpy (IE)	Energy required to remove the outermost electron from a gaseous atom in its ground state; always endothermic
Successive IE	$IE_{1}$ < $IE_{2}$ < $IE_{3}$ … ; the large jump reveals the outermost shell
Electron Gain Enthalpy (EGE)	Enthalpy change when an electron is added to a gaseous atom; mostly exothermic for non-metals
Electronegativity (EN)	Relative tendency of a bonded atom to attract shared electrons toward itself; Pauling scale: F = 4.0 (max)
Pauling Scale	Dimensionless electronegativity scale devised by Linus Pauling; F = 4.0 (reference point)
Diagonal Relationship	Similarity in properties between two elements diagonally placed (one period apart, one group to the right) in the periodic table; e.g., Li–Mg, Be–Al, B–Si
Charge-to-Size Ratio (Polarizing Power)	Ratio of ionic charge to ionic radius; governs diagonal relationship similarities
Amphoteric Oxide	Oxide that reacts with both acids and bases; e.g., $Al_{2}O_{3}$ , BeO
Half-Filled Subshell Stability	Extra stability when each orbital in a subshell contains exactly one electron (e.g., N: 2 $p^{3}$ ) — minimizes electron repulsion (Hund's rule)
Fully Filled Subshell Stability	Extra stability when all orbitals in a subshell are completely filled (e.g., Be: 2 $s^{2}$ )
Lanthanoid Contraction	Steady decrease in atomic/ionic radius across the lanthanoid series due to poor shielding by f-electrons
Noble Gas Configuration	Completely filled outermost shell ( $ns^{2}$$np^{6}$ ); associated with chemical inertness and positive EGE
Valence Electrons	Electrons in the outermost shell that participate in bonding; determine chemical properties