Correct order of radius of the Ist orbit of H, He + , Li 2+ , Be 3+ is:
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and 
contain one electron like hydrogen, so they are called H -like particles.
Radius of the first orbit of the hydrogen atom is called Bohr's radius, i.e. 
.
The expression for the radius of 
orbit for H -like particles is given as
Where, Z is the atomic number of H -like particles.
We have to find the order of the radius of the 1st orbit, i.e. 
.
Now, the radius of the first orbit for 
and 
becomes
Since, the value of 
is constant, i.e. 52.9 pm , therefore, we can say that radius r is inversely proportional to the atomic number Z of the H -like particle, i.e.
Atomic number, Z for 
and 
is 2,3 and 4 , respectively.
Since, atomic number Z increases from H to 
, radius r being inversely proportional decreases from 
to 
. This implies that the radius of the first orbit is largest for 
and smallest for 
. Therefore, the correct order of radius of the 1st orbit of 
and 
is:
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