Question

which order of relative size amongst the following is incorrect?
(A) $ Li < Na < K $
(B) $ C < Si < Al $
(C) $ Mg > Al < Na $
(D) $ F < Cl < Br $

Answer 1

Hint: the answer to the question above can be given by studying the trends in atomic size in the periodic table while moving down a group and moving across the period. All the options given above are either in the same period or in the same group.

Complete step by step answer:
Trends in atomic size while moving down a group:
When we see down the group the size of the atom increases this is because by moving down a group by each element one shell increases and therefore the size of the atom increases while moving down.
Trends in atomic size while moving across a period:
When we see across the period the size of the atom decreases this is because by moving across a group by each element the shell remains the same but the proton count increases gradually and therefore the attraction between the nucleus and electron increases thus decreasing the size of an atom while moving across the period.
In option third we are provided with magnesium, aluminium and sodium all the three elements are present in period third, aluminium having the smallest size then magnesium and then sodium but this order is not correctly mentioned in the option that is the relative atomic size amongst the following option which is incorrect is option C.
Correct answer is option (C) .

Note:
The atomic size in period decreases because per head attraction of proton on electron increases, this is because the proton is stronger than the electron and there is no increment in the shell which results in decreasing of size while going across the period.