Final answer:
An atom of Krypton (Kr) in its ground state contains electrons in the fourth principal energy level. Neither Neon (Ne), Helium (He) nor Argon (Ar) has electrons in the fourth principal energy level in their ground states. Option 1 is correct.
Step-by-step explanation:
You asked which element in the ground state has electrons in the fourth principal energy level. The options provided were Kr (Krypton), Ar (Argon), He (Helium), and Ne (Neon). To determine this, we need to consider the atomic number and electron configurations of these elements.
Neon (Ne) has 10 electrons, with the first 2 electrons in the first energy level and the remaining 8 in the second energy level. Therefore, it does not have any electrons in the fourth energy level in its ground state.
Helium (He) has 2 electrons, which completely fill its first and only energy level. Argon (Ar) has 18 electrons, with electrons filling the first three energy levels but not reaching the fourth.
The correct answer is Krypton (Kr), which has 36 electrons. Krypton's electron configuration shows that it has electrons in the first, second, third, and fourth energy levels. So, an atom of Krypton in its ground state contains electrons in the fourth principal energy level.