Which pair lists the element with the lower first ionization energy first?

• A. Ne, F
• B. Mg, Na
• C. Cl, S
• D. Na, Mg

Answer: (D)

Verified step by step guidance

1. Understand that the first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms.
2. Recall that ionization energy generally increases across a period (left to right) in the periodic table and decreases down a group (top to bottom).
3. Compare each pair by locating the elements on the periodic table and determining which has the lower first ionization energy based on their positions.
4. For the pair Ne (Neon) and F (Fluorine), Neon is to the right of Fluorine in the same period, so Neon has a higher ionization energy; thus, F has the lower ionization energy.
5. For the pair Mg (Magnesium) and Na (Sodium), both are in the same period, but Sodium is to the left of Magnesium, so Sodium has the lower ionization energy; therefore, the correct order with the lower ionization energy first is Na, Mg.