The periodic table of the elements is used to predict the valencies and chemical properties of the elements. eight pairs of elements are outlined by larger squares in the periodic table below. which pair of elements from those outlined has the greatest difference in chemical properties? a. thallium (tl) and gallium (ga) b. potassium (k) and francium (fr) c. neon (ne) and radon (rn) d. krypton (kr) and strontium (sr)

D. krypton (Kr) and strontium (Sr)

Explanation:

Valency is the number of electrons present in the outer shell of an atom.

The vertical column of the periodic table is called the Group.

The elements present in the same group will have same number of valence electrons (outer shell electrons)  

For example  

Fluorine, Chlorine, Bromine are all present in the same Group

The electronic configurations  are  

" alt="F-9 \Rightarrow 1s^2 2s^2 2p^5\\\\Cl-17 \Rightarrow 1s^2 2s^2 2p^6 3s^2 3p^5\\\\Br-35 \Rightarrow 1s^2 2s^2 2P^6 3s^2 3p^6 4s^2 3d^{10} 4p^5" />" />

We see here the number of valence electrons of F, Cl and Br present in the same group is 7.

Hence the chemical properties of the elements present in the same group will be the same .

Thus we conclude

A. thallium (Tl) and gallium (Ga)

B. potassium (K) and francium (Fr)

C. neon (Ne) and radon (Rn)

All these 3 pairs have similar chemical properties as they are present in the same group.

Option D Krypton present in the VIIIA group and Strontium present in IIA group will have completely different properties as they are in different groups.

i just learnt this recently but i need to look through my notes again