Final answer:
The most chemically reactive metal used in photoelectric cells and atomic clocks for its accuracy is cesium (Cs), a member of Group 1: The Alkali Metals.
Step-by-step explanation:
The most chemically reactive metal that is mentioned for use in photoelectric cells and atomic clocks, known for its greater accuracy compared to mechanical or electrical clocks, is cesium (Cs). Cesium, along with lithium, sodium, potassium, rubidium, and francium, is part of Group 1: The Alkali Metals. These metals are characterized by their softness, shininess, and high reactivity, with cesium being the most reactive metal in the periodic table. While lithium, rubidium, and cesium are relatively rare, and francium is highly unstable and radioactive, sodium and potassium are much more abundant in the Earth's crust.