Final answer:
Given the energy of the photon emitted from a hydrogen atom, we determine the final energy level of the electron by using the Rydberg formula. For the problem provided, with an initial electron level at n=4 and an emitted photon energy of 2.042 x 10^-18 J, the final energy level of the electron is n=2.
Step-by-step explanation:
To find the final energy level (n) of an electron in a hydrogen atom that emits a photon with a given energy, we can use the Rydberg formula for energy levels in a hydrogen atom. This formula relates the photon's energy to the energy levels the electron transitions between:

Where E is the energy of the emitted photon, E₁ and E₂ are the energy levels of the initial and final states, R_H is the Rydberg constant (2.18 \times 10^{-18} J), and n₁ and n₂ are the principal quantum numbers of the initial and final states. We are given that E = 2.042 \times 10^{-18} J and n₁ = 4 (the starting level). Our task is to find n₂ (the final level).
First, we rearrange the Rydberg formula to solve for n₂^2:

Then, plug in the given values:

Calculate n₂^2:

Find n₂ by taking the square root:

Since n₂ must be an integer, we can infer that the actual final state is n = 2, which is the closest integer value below n₂ calculated (you cannot have a fractional principal quantum number). Therefore, the final level (n) of the electron is n = 2.