Question

22.Put the following atoms/ions in decreasing order:

a) K, Se, Ca, Br

b) K+, S-2, Ca2+, Cl-

I know the answer but please explain to me why and how you get to the answer... the thought process behind it and how you know.

Answer 1

a)

size decrease in order : K > Ca > Se > Br

these are all same period eleemtns those are 4th period elements . we know that in a period from left to right atomic size decreases it is due to electrons enters into the same shell when we moves from left to right.

b)

size decrease in order : S-2 > Cl- > K+ > Ca+2

explanation :

if negative charge increases on an atom there are more electrons than protons . suppose in S-2 there are 18 electrons and 16 protons . 16 protons cannot hold good 18 electrons . so it has more ionic size

and cation charge increases size decreases . it is due less number of electrons than protons.

in Ca+2 , there are 20 protons but electrons are only 18. so 20 protons strongly attacks 18 electrons . ionic size will be reduced