Question

If 21 electrons are present in the 4th orbit then the atomic number of that element is:

• A. 58
• B. 59
• C. 56
• D. 60

Answer 1

The correct option is B 59

The total number of electrons in the element is equal to sum of electrons in all orbits$=2\left(1^{st}\right)+8\left(2^{nd}\right)+18\left(3^{rd}\right)+21\left(4^{th}\right)=49$. Thus total number of electrons is $49$. Hence the atomic number of the element is $49$