Explain why Na has higher ionization energy than K.

Sodium (Na) has a higher ionization energy than potassium (K) because sodium has a smaller atomic radius and its valence electron is closer to the nucleus. Potassium, being in the 4th period, has a larger atomic radius, and its valence electron is farther from the nucleus. This makes it easier for potassium to lose its valence electron, requiring less energy for ionization compared to sodium.