Final answer:
Cesium (Cs) has the smallest first ionization energy of the elements listed because it is located at the bottom left of the periodic table. The correct answer is option a.
Step-by-step explanation:
To determine which of the listed elements has the smallest first ionization energy, we need to consider their positions on the periodic table. Ionization energy generally decreases as we move from top to bottom in a group and increases from left to right across a period.
Given the elements Sb (antimony), O (oxygen), Si (silicon), Sr (strontium), and Cs (cesium), cesium (Cs) is located at the bottom left of the periodic table, within the alkali metals group. This position indicates that Cs will have the lowest first ionization energy among the choices provided.
Ionization energy tends to be lowest for elements in this region because they have larger atomic radii and a greater distance of the outermost electrons from the nucleus, which makes it easier for these electrons to be removed.