Which of the noble gases has the greatest ionization energy

Analysis

The question asks for the noble gas with the greatest ionization energy. Ionization energy is the amount of energy required to remove the most loosely held electron from a neutral gaseous atom.

Concept Explanation

1. Periodic Trend: Ionization energy generally increases across a period (from left to right) and decreases down a group.
2. Atomic Size: As you move down a group in the periodic table, the number of electron shells increases. This means the outermost electrons are further away from the nucleus and are shielded by more inner-shell electrons.
3. Nuclear Attraction: Because the outermost electrons are further from the nucleus in larger atoms, the electrostatic attraction between the nucleus and these electrons is weaker. Therefore, it requires less energy to remove an electron from a larger atom than from a smaller one.
4. Noble Gases (Group 18): Helium (He) is at the top of the group, meaning it has the smallest atomic radius. Because its electrons are closest to the nucleus and experience the strongest nuclear pull, it requires the most energy to remove an electron.

Step-by-Step Solution

• Step 1: Identify the positions of the elements in Group 18: • Helium (He) - Period 1 • Neon (Ne) - Period 2 • Argon (Ar) - Period 3 • Krypton (Kr) - Period 4 • Xenon (Xe) - Period 5 • Radon (Rn) - Period 6
• Step 2: Apply the trend. Since ionization energy decreases as you go down the group, the element at the very top (Helium) will have the highest ionization energy.
• Step 3: Compare the options provided. Helium (He) is the smallest noble gas, making its electrons the most tightly bound to the nucleus.

Conclusion

Helium has the smallest atomic radius among the noble gases, which results in the highest effective nuclear charge felt by its valence electrons. Consequently, it has the highest first ionization energy of all elements in the periodic table.

Exam Tip: Always remember the mnemonic "Up and Right": Ionization energy, electronegativity, and electron affinity generally increase as you move up a group and right across a period.