Across a period, the first ionisation energy generally
Correct answer: B. increases, because the atoms get smaller and the nuclear charge increases
- A. decreases, because atomic size increases
- B. increases, because the atoms get smaller and the nuclear charge increases
- C. remains constant
- D. decreases, because shielding increases sharply
Explanation
A greater nuclear charge acting on a shell at nearly constant distance holds the outer electrons more tightly, so more energy is needed to remove one. The trend is not perfectly smooth, dipping at boron and at oxygen because of the start of the p subshell and the first pairing of p electrons respectively. Those small irregularities are themselves strong evidence for subshell structure.
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About Periodic Properties and Trends
Periodic properties arise from electron configuration and effective nuclear charge, producing trends in atomic and ionic radius, ionization energy, electron affinity, electronegativity, metallic character and reactivity. Comparisons run across periods and down groups, with attention to common exceptions. The topic also relates these trends to the behavior of s-block and p-block elements.
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