Which of the following species would be expected to have the lowest ionization energy?

• A. Na
• B. Al
• C. Cl
• D. Mg

Answer: (A)

Verified step by step guidance

1. Recall that ionization energy is the energy required to remove an electron from a gaseous atom or ion. It generally increases across a period (left to right) on the periodic table and decreases down a group (top to bottom).
2. Identify the position of each element in the periodic table: Na (Sodium) is in Group 1, Period 3; Mg (Magnesium) is in Group 2, Period 3; Al (Aluminum) is in Group 13, Period 3; Cl (Chlorine) is in Group 17, Period 3.
3. Since all elements are in the same period (Period 3), compare their ionization energies based on their group positions. Ionization energy tends to increase from left to right across a period due to increasing nuclear charge and decreasing atomic radius.
4. Recognize that Group 1 elements like Na have the lowest ionization energies in their period because they have a single electron in their outermost shell that is relatively easy to remove.
5. Conclude that among Na, Mg, Al, and Cl, sodium (Na) will have the lowest ionization energy because it is furthest to the left in Period 3 and has the least nuclear attraction on its valence electron.