Final answer:
Cesium is more reactive than sodium because its outermost electron is farther from the nucleus and experiences a lower effective nuclear charge, leading to easier removal and higher reactivity.
Step-by-step explanation:
Cesium is more reactive than sodium due to differences in atomic structure and the energy required to remove valence electrons. In cesium, the outermost electron is in the 6s orbital, which is farther from the nucleus compared to sodium's outermost electron in the 3s orbital. Because of the greater distance and the shielding effect from the inner electrons, cesium's outermost electron is less tightly held by the nucleus. As a result, less energy is required to remove this electron, making cesium more reactive.
The concept that helps explain this behavior is called effective nuclear charge (ENC). The ENC felt by the valence electron in cesium is much lower than its actual nuclear charge due to the shielding effect from inner electrons. With cesium's lower ENC, the valence electron is more easily lost, and cesium is thus more reactive in chemical reactions.