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The Modern Periodic Law (Moseley, 1913) states properties are periodic functions of atomic number. The table has 7 periods and 18 groups, organized into four blocks based on the subshell being filled.

s-block (Groups 1-2): ns^1-2, reactive metals with low IE. p-block (Groups 13-18): ns^2np^1-6, most diverse — metals, metalloids, non-metals, noble gases. d-block (Groups 3-12): (n-1)d^1-10 ns^0-2, transition metals with variable OS, coloured compounds, catalytic properties. f-block: (n-2)f^1-14, inner transition metals (lanthanoids and actinoids).

Period number = highest principal quantum number of occupied orbital. Group number determination: s-block = valence electrons; p-block = 10 + valence electrons; d-block = sum of ns + (n-1)d electrons.

Anomalous placements: H in Group 1 (but resembles Group 17 too), He in Group 18 (1s^2 = noble gas, not Group 2). Number of elements per period follows 2n^2: 2, 8, 8, 18, 18, 32, 32.