Final answer:
Lithium, with an atomic number of 3, has two energy levels in its ground state. The electron configuration is 1s² 2s¹, with the third electron in the 2s orbital. Lithium does not require a third energy level for its electrons.
Step-by-step explanation:
The number of energy levels needed for a lithium atom's three electrons when the atom is in its ground state is two. In a lithium atom, which has an atomic number of 3, the electrons are arranged such that the first two fill the 1s orbital, due to the 1s subshell being able to hold a maximum of two electrons. Consequently, the electron configuration for the ground state of lithium is 1s²2s¹, indicating that the third electron occupies the next lowest energy level's orbital, which is the 2s orbital.
The arrangement of electrons across the energy levels follows the principle that electrons fill up orbitals in order of increasing energy. Therefore, lithium does not need a third energy level in its ground state. Understanding the energy levels and electron configuration is crucial for explaining many chemical properties and reactions of elements such as lithium.