Answer :
The least possible uncertainty in an electron's velocity is, 
The percentage of the average speed is, 33 %
Explanation :
According to the Heisenberg's uncertainty principle,
 ...........(1)
 ...........(1)
where,
 = uncertainty in position
 = uncertainty in position
 = uncertainty in momentum
 = uncertainty in momentum
h = Planck's constant
And as we know that the momentum is the product of mass and velocity of an object.

or,
 .......(2)
      .......(2)
Equating 1 and 2, we get:


Given:
m = mass of electron = 
h = Planck's constant = 
radius of atom =  
     
 = diameter of atom =
 = diameter of atom = 
Now put all the given values in the above formula, we get:


The minimum uncertainty in an electron's velocity is, 
Now we have to calculate the percentage of the average speed.
Percentage of average speed = 
Uncertainty in speed = 
Average speed = 
Percentage of average speed = 
Percentage of average speed = 33.2 % ≈ 33 %
Thus, the percentage of the average speed is, 33 %