In medical experiments, which group receives placebos?
the experimental group
the control group
both the experimental and control groups
neither the experimental nor control group

the control group

Explanation:

you use the control group so you have something to compare your results to

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.