An element's atomic number defines it. An element with 17 protons will always be chlorine. However an element's mass numbers can vary, which means that it can have different numbers of neutrons.

Photo: Niels Bohr's research notes for his new atomic theory Rutherford's find came from a very strange experience. Everyone at that time imagined the atom as a "plum pudding." That is ...

Elements are arranged in the periodic table in order of increasing atomic number ... Example: sodium has the electron arrangement 2,8,1. An atom of sodium has 11 electrons. The first two fill ...