Modern Periodic Law (Moseley, 1913): Physical and chemical properties of elements are periodic functions of their atomic number. The table has 7 periods (horizontal) and 18 groups (vertical). Blocks: s-block (Groups 1-2, $ns^{1-2}$), p-block (Groups 13-18, ns^{2np}^{1-6}), d-block (Groups 3-12, (n-1)$d^{1-10}$ $ns^{0-2}$), f-block (lanthanoids Ce-Lu and actinoids Th-Lr, (n-2)$f^{1-14}$). Period number = number of shells occupied. Group number relates to valence electrons: for s-block, group = valence electrons; for p-block, group = 10 + valence electrons. Hydrogen is anomalous — placed in Group 1 ($ns^1$) but shares properties with Group 17. Helium (1$s^2$) is in Group 18 (noble gas behaviour) despite s-block configuration.