Answer: K Potassium is an alkali metal while all others are halogens.
Answer: Magnesium Magnesium is an alkaline earth metal while all others are alkali metals.
Answer: Beryllium Beryllium is an alkaline earth metal while all others are noble gases.
Answer: Calcium Except Calcium, all other elements were undiscovered when Mendeleev prepared his periodic table. Thus, he had left vacant places in his table for these elements.
Answer: Gallium Gallium is a metal while all others are metalloids.
Answer: Moseley Moseley classified the elements on the basis of their atomic numbers while all the other tried to classify the elements on the basis of their atomic mass numbers (atomic weights).
Answer: Boron Boron belongs to group 13 while all the others are halogens belonging to group 17.
Answer: Neon Neon is an inert gas while all the others are chemically active elements.
Answer: Potassium Potassium is an alkali metal while all the others are alkaline earth metals.
Answer: Carbon The valency of Carbon is 4, while the valency of all the others is 2.