Answer :
The same atomic number. The periodic table assembles all of the recognized elements appropriate increasing atomic number. Order usually accords with increasing atomic mass. The number of electrons in a period rises as one it moves down the periodic table; so, as the energy level of the atom rises, the number of energy sub-levels per energy level rises.
Answer;
C. Similar chemical properties
Explanation;
-The vertical columns on the periodic table are called groups or families because of their similar chemical behavior. All the members of a family of elements have the same number of valence electrons and similar chemical properties. The horizontal rows on the periodic table are called periods.
-For example; all of the 1A elements have one valence electron. This is what causes these elements to react in the same ways as the other members of the family. They are all very reactive and form compounds in the same ratios with similar properties with other elements. Because of their similarities in their chemical properties, these elements are in the same group. These elements are called alkali metals.
