Choose the correct option. In which of the following systems will the radius of the first orbit of the electron be smallest?

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KollegeApply · Accepted Answer

Singly ionized helium Explanation: According to Bhor's theory, the radius of an atomic number z atom's nth orbit is: r prop "n"^2/"Z". Where n is the orbit number. (i) For hydrogen, $\therefore$ n = 1 and Z = 1 Therefore, r prop "n"^2/"Z" $\Rightarrow$ r ∝ 1 (ii) For Helium, $\therefore$ n = 1 and Z = 2 Therefore, r prop "n"^2/"Z" => "r" prop $\frac{1}{2}$ (iii) For Deuteron. $\therefore$ n = 1 and Z = 1 Therefore, r prop "n"^2/"Z" => "r" prop 1 (iv) For Tritium, $\therefore$ 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.

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