As compared to K, Na has

The correct option is higher ionization potential.

To understand why sodium (Na) has a higher ionization potential compared to potassium (K), we can analyze their positions in the periodic table and the resulting periodic trends:
1. Position in the Periodic Table: Both sodium (Na, atomic number 11) and potassium (K, atomic number 19) belong to Group 1 (alkali metals) of the periodic table. Sodium is in Period 3, while potassium is in Period 4, directly below sodium.
2. Electronic Configuration:
The valence electron configuration of sodium is [Ne] 3s¹.
The valence electron configuration of potassium is [Ar] 4s¹.

3. Atomic Size and Shielding Effect: As we move down a group in the periodic table, the number of energy shells increases. This causes the atomic radius to increase, meaning the valence electron of potassium is located farther from the nucleus than that of sodium. In addition, potassium has more shielding from inner shell electrons, which reduces the effective nuclear charge felt by its outer electron.

4. Ionization Potential: Ionization potential (or ionization energy) is the energy required to remove the outermost valence electron from a gaseous atom. Because sodium's valence electron is closer to the nucleus and experiences less shielding, it is held more tightly by the nucleus than potassium's valence electron. Therefore, it requires more energy to remove the outermost electron from sodium, meaning sodium has a higher ionization potential than potassium.

Let's also look at why the other options are incorrect:
- Melting Point: Metallic bond strength decreases as the size of the metal atoms increases. Since sodium atoms are smaller than potassium atoms, sodium has stronger metallic bonds and therefore a higher melting point than potassium.
- Electronegativity: The tendency to attract shared electrons decreases down the group as atomic size increases. Thus, sodium has a higher electronegativity compared to potassium.
- Atomic Radius: As we go down Group 1, atomic radius increases. Therefore, sodium has a smaller atomic radius compared to potassium.