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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 (1$s^2$ = noble gas, not Group 2). Number of elements per period follows 2$n^2$: 2, 8, 8, 18, 18, 32, 32.