To determine which element has atoms with valence electrons in a higher energy level than those of calcium (Ca), we need to look at the electron configuration of calcium and compare it with the electron configurations of the other elements listed.
Calcium (Ca) has an atomic number of 20, and its electron configuration is \( [Ar] 4s^2 \). The valence electrons of calcium are in the 4th energy level (n=4).
Now let's look at the other elements:
- Bromine (Br): Atomic number 35. Electron configuration: \( [Ar] 4s^2 3d^{10} 4p^5 \). Valence electrons are in the 4th energy level (n=4).
- Lithium (Li): Atomic number 3. Electron configuration: \( 1s^2 2s^1 \). Valence electrons are in the 2nd energy level (n=2).
- Oxygen (O): Atomic number 8. Electron configuration: \( 1s^2 2s^2 2p^4 \). Valence electrons are in the 2nd energy level (n=2).
- Cesium (Cs): Atomic number 55. Electron configuration: \( [Xe] 6s^1 \). Valence electrons are in the 6th energy level (n=6).
From this information, we can see that cesium (Cs) has valence electrons in a higher energy level (n=6) compared to calcium (n=4).
Thus, the correct answer is cesium (Cs).