In the periodic table, which of the following elements has the lowest first ionization energy (that is, it loses an electron most easily)?

Introduction / Context:
Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom. Trends in ionization energy across the periodic table are very important for understanding chemical reactivity, metallic character, and the formation of ions. This question asks you to identify which listed element has the lowest first ionization energy, meaning it can lose an electron most easily and therefore behaves as a very reactive metal.

Given Data / Assumptions:

• We are dealing with first ionization energy, the energy needed to remove the first electron from a neutral atom in the gaseous state.
• The elements listed are helium (He), neon (Ne), francium (Fr), and rubidium (Rb).
• Helium and neon are noble gases in periods 1 and 2, respectively.
• Rubidium and francium are alkali metals in groups 1, located in lower periods of the table.

Concept / Approach:
Ionization energy generally increases across a period from left to right and decreases down a group as atomic size increases. Alkali metals, located in group 1, typically have low ionization energies because they have a single valence electron that is relatively far from the nucleus. As you move down group 1 from lithium to sodium to potassium, rubidium, caesium, and francium, the atomic radius increases and the outer electron is held less tightly, so ionization energy continues to decrease. Noble gases like helium and neon have very high ionization energies due to their stable closed shell configurations. Among the options, francium is the lowest in its group and is expected to have the lowest first ionization energy.

Step-by-Step Solution:
Step 1: Identify helium and neon as noble gases with filled electron shells and very high ionization energies. Step 2: Recognise rubidium and francium as alkali metals in group 1, which have low ionization energies compared with most elements. Step 3: Recall the trend that within a group, ionization energy decreases from top to bottom as atomic size increases. Step 4: Note that francium lies below rubidium in group 1, meaning its valence electron is even farther from the nucleus and more shielded by inner electrons. Step 5: Conclude that francium, among the given options, has the lowest first ionization energy and select francium (Fr) as the answer.

Verification / Alternative check:
Periodic trends diagrams show that group 1 metals have the smallest ionization energies in each period. As you move from hydrogen down to francium, there is a steady decrease in ionization energy because the effective nuclear attraction on the outermost electron becomes weaker. Noble gases, despite being on the far right of each period, have high ionization energies because removing an electron from a stable closed shell requires a lot of energy. Rubidium, while already reactive, is higher than francium in the group and thus has a slightly higher ionization energy. These trend considerations confirm francium as the element with the lowest ionization energy among the options.

Why Other Options Are Wrong:
Helium has one of the highest ionization energies in the entire periodic table due to its small size and stable 1s^2 configuration. Neon also has a high ionization energy as a noble gas with a complete outer shell. Rubidium, although a reactive alkali metal with low ionization energy, is above francium in group 1 and therefore does not have the absolute lowest among the given choices. These elements cannot match the extremely low ionization energy expected for francium at the bottom of group 1.

Common Pitfalls:
Some students mistakenly pick a noble gas because they focus on the right side of the periodic table without remembering that noble gases have high ionization energies, not low ones. Others may select rubidium simply because it is more familiar and may not recall francium clearly. To avoid these mistakes, remember that the most reactive metals with the lowest ionization energies are at the bottom of group 1, and that noble gases have high ionization energies due to their stable configurations. This simple rule helps answer many periodic trend questions correctly.

Final Answer:
Among the elements listed, the lowest first ionization energy is for francium (Fr), making it the easiest to ionize.