Answer:
[tex]n_{fn}[/tex]= 4
Explanation:
To solve this exercise we will use Bohr's atomic model
[tex]E_{n}[/tex] = - 13.606 / n² [eV]
The transition from level n = 2 to level n = 1 is valid
[tex]E_{21}[/tex] = - 13.606 [¼ -1/1]
[tex]E_{21}[/tex] = 10.2045 eV
Bohr's model for atoms with only one electron is
[tex]E_{n}[/tex] = -13.606 Z² / n²
Where Z is the atomic number of the atom.
In this case the helium atom has an atomic number of Z = 2 from the level n₀ = 2 let's look up to what level it reaches
ΔE = -13.606 [4 / [tex]n_{fn}[/tex]² - 4/4]
4 / [tex]n_{fn}[/tex]² = -ΔE / 13.606 + 1
4 / [tex]n_{fn}[/tex]² = -10.2045 / 13.606 +1 = -0.75 +1
4 / [tex]n_{fn}[/tex]² = 0.25
[tex]n_{fn}[/tex] = √ 4 / 0.25
[tex]n_{fn}[/tex]= 4