# Question #a14c2

##### 1 Answer

Jul 20, 2017

#### Explanation:

All you need to know in order to answer this question is that the radius of an orbit depends on the *principal quantum number*,

If you take *Bohr orbit*, you can say that you have

#r_n = n^2 * a_0#

In your case, the third orbit is characterized by

#n = 3#

so you will have

#r_3 = 3^2 * a_0 = 9 * a_0#