When is it most necessary for a student to read the instructions for a lab apparatus? O while using the apparatus during an experiment
O before using the apparatus in an experiment
O after the experiment fails for unclear reasons
O while writing a report about the experiment's results

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.  

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