Question

Choose the correct option.

In which of the following systems will the radius of the first orbit of the electron be smallest?

Options

• Hydrogen

• Singly ionized helium

• Deuteron

• Tritium

Answer 1

Singly ionized helium

Explanation:

According to Bhor's theory, the radius of an atomic number z atom's n^th orbit is: `r prop "n"^2/"Z"`. 

Where n is the orbit number.

(i) For hydrogen,

∴ n = 1 and Z = 1

Therefore, `r prop "n"^2/"Z"`

⇒ r ∝ 1

(ii) For Helium,

∴ n = 1 and Z = 2

Therefore, `r prop "n"^2/"Z"`

`=> "r" prop 1/2`

(iii) For Deuteron.

∴ n = 1 and Z = 1

Therefore, `r prop "n"^2/"Z"`

`=> "r" prop 1`

(iv) For Tritium,

∴ n = 1 and Z = 1

Therefore, `r prop "n"^2/"Z"`

`=> "r" prop 1`

Hence, the smallest is Helium. The correct option is Singly ionized helium.