Answer : 3. "68"
Explanation :

Mendeleev's periodic table Mendeleev published the periodic table in 1869 but not all the elements were discovered by that time, so he left some spaces under Aluminium and Silicon. He examined the relationship between atomic masses and the physical and chemical properties of elements. While preparing the periodic table there were a few instances where he had to place elements with a slightly greater atomic mass before an element with a slightly lower atomic mass as he wanted to place elements with similar chemical properties together. Periodic law states that "properties of elements are periodic function of their atomic masses.