Well, academically, across Period 2, increasing nuclear charge pulls electrons closer. Sodium starts Period 3, adding a new electron shell, so shielding wins and radius jumps. But from this Delhi tuition bench? I see students *memorize* the jump, but don't *feel* the 'shell' change. They forget it's like my marigold rows: you can only pack plants so tight in one plot (a period). For the next row, you must start a whole new, wider bed. That’s sodium. So I tell them, "Bhool mat jaao, naye khet ki shuruaat hai.
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