How does a scientist know where on the periodic table to put a newly discovered element?

Periodic table has vertical columns and horizontals rows.

Periods: The horizontal rows are named as periods in a periodic table. In a period elements with same numbers of principle quantum number ('n') or total number of shells (energy levels) are present. For example : Sodium comes in third period which means that sodium has three shells in total.

Groups: The vertical columns are named as group in a periodic table. In a group elements with same number electrons in their outermost shell are present. For example: In group one all the elements Li, Na, K etc have same number of valence electrons in their outermost shell.

So, in order to place a newly discovered element in a periodic table scientist must have the information regarding the number of valence electrons and total number of shells present in an atom of that element. One can easily look for valence number of electrons and total number of shells from the atomic number of that particular element.

By the knowledge of valence electrons and total number of shells the scientist will able to decide the group number and period number for newly discovered element.