The first ionization energy is the energy required to remove the most loosely bound electron from a neutral gaseous atom. It is influenced by several factors:

• Atomic size: Smaller atoms have higher ionization energy because the electron is closer to the nucleus and experiences a stronger attraction.
• Nuclear charge: A higher nuclear charge (more protons) increases ionization energy due to stronger attraction to electrons.
• Shielding effect: Inner electrons shield the outer electrons from the nucleus, reducing ionization energy.
• Stability of half-filled or fully filled orbitals: Atoms with stable configurations (like half-filled or fully filled subshells) have higher ionization energy.

All the given elements (K, Sc, Rb, Na) are in Group 1 (alkali metals) except Scandium (Sc), which is a transition metal in Group 3. Let's analyze them:

Step 1: Compare Group 1 elements (K, Rb, Na)

In Group 1, ionization energy decreases down the group because atomic size increases and shielding effect increases, making it easier to remove an electron. The order of ionization energy is:

$\mathrm{Na}>K>\mathrm{Rb}$

So, among Group 1 elements, Na has the highest ionization energy.

Step 2: Compare Sc with Group 1 elements

Scandium (Sc) has atomic number 21 and electronic configuration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^1$. It is smaller in size compared to Group 1 elements due to higher effective nuclear charge. Also, removing an electron from Sc (which has a stable half-filled d-subshell after losing one electron: ${\mathrm{Sc}}^{+}:3d^1$) requires more energy. Therefore, Sc has a higher ionization energy than all Group 1 elements.

Final Answer: Scandium (Sc) has the highest first ionization energy among the given atoms.

Related Topics

Ionization Energy Trends: Ionization energy generally increases across a period and decreases down a group in the periodic table. Exceptions occur due to stable electron configurations.

Periodic Properties: Understanding atomic radius, effective nuclear charge, and shielding effect is crucial for predicting ionization energy.

Formulae

Ionization energy is measured in kJ/mol. While there is no simple formula to calculate it directly, it can be determined experimentally and trends are predicted based on periodic table position.

The energy required for the first ionization is represented as:

$X\left(g\right)\to X^{+}\left(g\right)+e^{-} \Delta H={\mathrm{IE}}_1$