Suppose that an electron starts in the n=4 shell of a neutral hydrogen atom. How many photons will be emitted once it has fallen to the n=1 shell?

If an Atom has 12 Electrons then if it is a Neutral atom then the Atomic Number of the Atom will be 12.

The Electronic Configuration of Element with Atomic Number 12 will be :

⇒ 1s² 2s² 2p⁶ 3s²

We can notice from the Electronic Configuration that :

Given atom has 2 electrons in the 1st shell.

⇒ In the Shell : n = 1

Number of Electrons = 2

In the similar way : Number of Electrons in 2nd shell = 2 + 6 = 8

⇒ In the Shell : n = 2

Number of Electrons = 8

In the similar way : Number of Electrons in 3rd shell = 2

⇒ In the Shell : n = 3

Number of Electrons = 2

Explanation: