Here's why:

* Ionization Energy: Ionization energy is the energy required to remove an electron from an atom. Cesium has a much lower ionization energy than silicon. This means it's easier to remove an electron from cesium, indicating that its outermost electron is held less tightly and has a higher energy level.

* Electron Configuration: Cesium has a single valence electron in its outermost shell (6s1), while silicon has four valence electrons (3s2 3p2). This means cesium's valence electron is further away from the nucleus and experiences weaker electrostatic attraction, making it higher in energy.

* Atomic Size: Cesium is a much larger atom than silicon. This larger size also contributes to the weaker attraction between the nucleus and the valence electron in cesium, resulting in a higher energy level.

In summary, cesium's lower ionization energy, single valence electron, and larger atomic size all contribute to its higher energy level compared to silicon.