Classification of Elements and Periodicity - Periodic Trends in Physical Properties

Atomic Radius: It is the distance from the center of the nucleus to the outermost shell containing electrons. Across a period, atomic radius decreases because the effective nuclear charge ($Z_{eff}$) increases. Down a group, it increases due to the addition of a new energy shell.

Ionic Radius: The distance between the nucleus and the electron in the outermost shell of an ion. Cations are smaller than their parent atoms ($r_{cation} < r_{atom}$), while anions are larger ($r_{anion} > r_{atom}$).

Ionization Enthalpy ($\Delta_i H$): The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state. It increases across a period and decreases down a group.

Electron Gain Enthalpy ($\Delta_{eg} H$): The enthalpy change when an electron is added to an isolated gaseous atom to form a negative ion. Halogens have highly negative $\Delta_{eg} H$ because they achieve a stable noble gas configuration.

Electronegativity ($\chi$): A qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself. Fluorine is the most electronegative element with a value of $4.0$ on the Pauling scale.

Screening Effect (Shielding Effect): The reduction in the nuclear pull on valence electrons due to the presence of inner shell electrons. Effective nuclear charge is given by $Z_{eff} = Z - \sigma$, where $\sigma$ is the shielding constant.

Periodic Trends in Chemical Reactivity: Reactivity is high at the extremes of a period (alkali metals and halogens) and lowest in the center and for noble gases. Metallic character decreases across a period and increases down a group.