Understanding Ionization Energy Trends in Groups

Ionization energy (IE₁) is the energy required to remove the first electron from an atom. Generally, IE₁ decreases down a group due to increasing atomic size and shielding effect. However, irregularities occur due to factors like poor shielding by d and f orbitals and stable electronic configurations.

Step 1: Analyze Each Option

Option 1: B, Al, Ga (Group 13)

Expected trend: IE₁ decreases from B to Al, but Ga has higher IE₁ than Al due to poor shielding by d-electrons in Ga (atomic number 31, electron configuration: [Ar] 3d¹⁰ 4s² 4p¹). Irregular trend observed: B > Ga > Al.

Option 2: Sc, Y, La (Group 3)

All are d-block elements. Regular decreasing trend down the group due to increasing size and shielding. No significant irregularity.

Option 3: Ni, Pd, Pt (Group 10)

Transition metals. Generally decreasing trend with minor variations due to half-filled/full-filled stability, but no major irregular trend like in Group 13.

Option 4: C, Si, Ge (Group 14)

Expected decreasing trend: C > Si > Ge. Ge has higher IE₁ than Si due to poor shielding by d-electrons (Ge: [Ar] 3d¹⁰ 4s² 4p²), but irregularity is less pronounced than in Group 13.

Step 2: Identify the Group with Clear Irregularity

Group 13 (B, Al, Ga) shows the most prominent irregular trend: IE₁(B) > IE₁(Ga) > IE₁(Al). This is due to the presence of d-electrons in Ga causing poor shielding and increased effective nuclear charge.

Final Answer

The irregular trend of first ionization energy is observed in B, Al, Ga (Group 13).

Related Topics

• Periodic Trends: Atomic Size, Ionization Energy, Electron Affinity
• Shielding Effect and Effective Nuclear Charge
• d-Block and p-Block Element Properties
• Exceptions in Periodic Table Trends

Key Formulae and Theory

Ionization Energy Equation (simplified):

Where $Z{\mathrm{eff}}_{}$ is effective nuclear charge and $n$ is principal quantum number. Irregularities arise when shielding is incomplete (e.g., d and f electrons).