Question

Identify the group which is not a Dobereiner triad
(a) Li, Na, K
(b) Be, Mg, Cr
(c) Ca, Sr, Ba
(d) Cl, Br, I

Answer 1

Answer:

Here the correct answer to your question is Option b). Be, Mg, Cr.

Explanation:

Speaking of Dobereiner Traits, these are special groups of elements which are placed in Groups of three each.

As per the Law, the elements have to be organized in their groups in order of their increasing Atomic masses. These groups are then termed as common traits. These are elements that within their organized groups also exhibit similar chemical properties.

There is something more to it. The element in the group that is placed in the middle position is the one who has AM (arithmetic mean ) is equal to the sum of atomic masses of the first and the third element in the group.

So, when understanding this, we also known that not all elements exhibit this trait. Not all elements in the periodic table can be termed as Doberiner traits.

So if there are any of the two elements in the group that has a very low and very high atomic mass number, then these elements do not exhibit Doberiner traits.

Here for the options provided by you,  

a).  Li, Na, K  

Li (7) , Na (23) , K (39)  = Li + K = 7 + 39 = 46 / 2 = 23 = Na (middle element) so this is a Dobereiner traits.

d). Cl (35.5) , Br (80) , I (127) = Cl + I = 35.5 + 127 = 162.5 / 2 =81.25 = Br (appx.) is a Dobereiner trait.

c). Ca (40) , Sr (88) , Ba (137) = Ca + Ba = 40 + 137 = 177 / 2 = 88.5 = Sr (appx.) is a Dobereiner trait.

But the last one B). Be (9) , Mg (24.3) , Cr (51.9) = Be + Cr = 9 + 51.9 = 60.9 / / = 30.45 is not equal to Mg . so it is not.

Here the correct answer to your question is Option b). Be, Mg, Cr.  

Answer 2

Answer:

Option B Be, Mg, Cr is the correct answer

Explanation:

Since the mass of the middle one is net average of other two.

Therefore, Be,Mg,Cr is the correct answer