The effective nuclear charge of an element with three valence electrons is 2.60. What is the minimum atomic number of the element?

Explanation

Since there are three electrons in the valence shell , therefore screening constant `(sigma)` of two other electrons on the third electron `2 xx 0. 35 = 0. 70`
If there are three electrons is the valence shell (i.e., `2s^(2) 2p^(1))` then the inner shell i.e., must contain two other electrons. the screening constant `(sigma)` of these two electrons in the inner shell i.e., `1s^(2) = 2xx 0. 85` = 1.70
Total screening constant `(sigma) = 1.70 = 0. 70 = 2.40 `
But `Z_(ef) = Z - sigma ` or `Z_(eff ) = 2.60 ` (Given )
`:. 2.60 = Z - 2.4 ` or `Z= 2.6 + 2.4 = 5.0`
Thus atomic number (Z ) of the element = 5