4. Which of the following elements is found in the same group on the periodic table as hydrogen (H)?
A. aluminum (AI)
B. calcium (Ca)
C. magnesium (Mg)
D. sodium (Na)
5. Which of these elements is found in its natural state as a gas?
A. argon (Ar)
B. boron (B)
C. phosphorus (P)
D. potassium (K)

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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