Answer:
re = 2.09 Å.
Explanation:
As the electron is moving around the stationary proton along a circular path at a constant speed, there exists a force that keeps the electron within this trajectory, which is the centripetal force.
This centripetal force, is no other than the attractive electric force between the proton and the electron, so we can write the following equality:
Fe = Fc ⇒ k*Qp*Qe / re² = me*ve²/re
⇒ re = k*Qp*Qe / m*ve², where:
k = 9*10⁹ N, Qp =-Qe= 1.6*10⁻¹⁹ coul, me =9.1*10⁻³¹ kg, and ve= 1.1*10⁶m/s
⇒ re = 9*(1.6)²*10⁻²⁹ / 9.1*(1.1)²*10⁻¹⁹ m
⇒ re = 2.09*10⁻¹⁰ m = 2.09 Å.