The element with smallest value of ionization energy is

  • A. Li
  • B. Al
  • C. Ca
  • D. Ba

Explanation

Ionisation energy falls down a group as the outer electron lies further from the nucleus and is better shielded, and barium is the lowest of these four in the periodic table. Lithium is small and holds its outer electron tightly by comparison. Across a period the trend runs the other way, which is why aluminium is not the answer despite being a metal.

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