An atom has 18 protons and 8 valance electrons. Which statement would best identify this elementa A. The element is highly reactive with eight electrons in its outer shell. B. The element has an atomic mass of 18. C. The element is "happy" or stable because it outer energylevel is filled. D. The element is a metal.

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.