What is element #35? Describe its atomic mass, configuration number, and charges. Please explain J.J. Thomson's experiments.

Element #35 is Bromine, an member of the halogen family  Its standard atomic mass is 79.904 amu, and it possesses 10 isotopes.  The most stable and widespread form of Bromine has 45 neutrons, for an atomic mass of 70; the standard atomic mass is higher due to the weighted average of the 10 isotopes,  which contain additional neutrons.   Since it has 7 electrons in its outer shell, Bromine, like all the Halogens, is reactive to fill in that last electron slot, reacting most readily with the Akaline Earth elements.

Joseph John Thompson was an  English physicist who, with improved vacuum tubes, was able to show in 1897 the deflection of cathode rays in an electric field.  This confirmed the rays as particulate in nature, and he is therefore credited with discovering the electron. He also devised a model of the atom where the electrons were embedded amongst the positively-charged protons, known as the "Raisins in a Pound Cake" model.  Thompson was awarded the 1906 Nobel Prize in Physics for his work.  Seven of his research assistants went on to win Nobel Prizes themselves!

Asimov's Biographical Encyclopedia of Science and Technology, I. Asimov, pg. 402, 1964.