Classification of Elements the Periodic Table — Telangana (SSC) Class 10 Science Solutions (Free)

Modern periodic law: Properties of elements repeat periodically when arranged in order of increasing atomic number. Mendeleev's law: Properties repeat in order of increasing atomic mass (had gaps, some elements out of order). Improvement: Moseley discovered atomic number is more fundamental than mass. Correcting by atomic number fixed anomalies (e.g., Ar before K, although heavier). Modern law is universal and explains all known elements

Period: Horizontal row in periodic table. Elements have same number of electron shells. Group: Vertical column. Elements have same number of valence electrons, similar chemical properties. Example trend - Atomic radius: Decreases from left to right across period (increasing nuclear charge with same shell) Increases down a group (increasing number of shells). Atom becomes larger down a group, smaller across period

Electronegativity: Ability of atom to attract electrons in a chemical bond. Trend: • Increases from left to right across period (more protons, stronger attraction) • Decreases down a group (more shells, electrons farther from nucleus) • Most electronegative: Fluorine (F) = 4.0 • Least electronegative: Francium (Fr) and Cesium (Cs) ≈ 0.7 Used to predict bond type: large difference → ionic, small difference → covalent

Representative elements: Elements in Groups 1, 2, 13-18 (s and p block). Also called main group or normal elements. Block: s-block (Groups 1, 2) and p-block (Groups 13-18) Characteristic: Valence electrons in outermost s or p subshell. Examples: Na (Group 1), Mg (Group 2), Al (Group 13), O (Group 16), F (Group 17) These are most abundant and important elements in chemistry

Atomic radius decreases from left to right across period. All four are in period 2. Atomic number order: C(6) < N(7) < O(8) < F(9) As atomic number increases, nuclear charge increases → electrons pulled closer → radius decreases Order of increasing radius: F < O < N < C Opposite order of increasing nuclear charge