Elements of the modern periodic table are classified into how many blocks based on the differentiating electron?

**Four blocks** based on which subshell the differentiating electron enters: - **s-block**: Groups 1 and 2 (Li, Na, K, Be, Mg, Ca, etc.) - **p-block**: Groups 13 to 18 (B, C, N, O, F, halogens, noble gases) - **d-block**: Groups 3 to 12 (transition metals) - **f-block**: Lanthanides and actinides (separated below the main table) No g-block has been confirmed experimentally yet, though it's predicted for superheavy elements ($Z > 120$).