COMEDK Exam 2019 Solved Paper

Radius of n th-orbit in hydrogen like atom is given by ‌‌r=‌

n2h2

4π2mkZe2

i.e., ‌‌r∝‌

n2

Z

For hydrogen, Z=1,n=1 in ground state ⇒‌‌‌

n2

Z

=‌

12

1

=1
For Beryllium (Be+++,Z=4. orbital is same
‌∴‌‌‌

n2

Z

=1
‌⇒‌‌n2=1×Z⇒n2=4
‌⇒‌‌n=2 ‌
Thus, the second level of triply ionised Be+++has same radius as the ground state of hydrogen.