What is the atomic mass of the first 30 elements of periodic table

Q. Write the symbols of the first ten elements in the periodic table with the atomic numbers.

Q. In Mendeleev's periodic table, some elements with higher atomic masses were placed before the elements with lower atomic masses.

Q.

From the standpoint of atomic structure, what determines which element will be first and which the last in a period of the periodic table ?

Q. Question 30
In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reason for the same.