the electron in hydrogen atom doesn't occupy the second, third or fourth energy level. how does it move to a higher energy level then returns to the 2nd, 3rd, 4th energy level?

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.  

d : a person who learns about the world by observing it. no that is not a smiley face

explanation: