Question

When Mendeleev organized elements in his periodic table in order of increasing mass, elements with similar properties were in the same __________.

Answer 1

Mendeleev arranged the elements in rows in order of increasing mass so that elements with similar properties were in the same column. 

So, the answer will be When Mendeleev organized elements in his periodic table in order of increasing mass, elements with similar properties were in the same Column.

Answer 2

Answer: Elements with similar properties were in the same group.

Explanation:

A Russian scientist named as Dmitri Mendeleev arranged the elements according to increasing atomic weights.

He arranged elements in horizontal rows known as periods and vertical columns known as groups.

In his periodic table, elements having similar properties lied in the same vertical columns or groups because all the elements had same number of valence electrons.

Hence, elements with similar properties were in the same group.