Particles in the Atom Electrons (-) charge no mass located outside the nucleus Protons (+) charge 1 amu located inside the nucleus Neutrons no charge 1 amu located inside the nucleus Atom – the smallest unit of an element that retains its chemical properties. Atoms can be split into smaller parts.

Discovery of the Neutron + + Lord Rutherford predicted the existence of the neutron is 1920.  Walter Bothe obtained evidence of the neutron in 1930.   However it was James Chadwick, who repeated Bothe's work, who is known as the discoverer of the neutron.  He found these uncharged particles with essentially the same mass as the proton.  He was awarded the Nobel Prize in physics in 1935. Chadwick is credited with the discovery of the neutron as a result of this transmutation experiment. When Ernest Rutherford bombarded the gold foil with alpha particles...we said four possible things may happen. (a) the particle will pass through the foil (b) the particle will be deflected while passing through the gold foil (c) the particle is deflected back towards the source (d) the alpha particle is absorbed by the gold foil It is this last event that is occurring above as beryllium is changed into carbon. Notice this is a nuclear reaction - the nucleus is changed in the atom. James Chadwick bombarded beryllium-9 with alpha particles, carbon-12 atoms were formed, and neutrons were emitted. Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 764 *Walter Boethe

Subatomic particles Actual e- -1 p+ +1 no mass (g) Relative mass Name Symbol Charge Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron no 1 1.67 x 10-24

Subatomic Particles equal in a neutral atom Atomic Number NUCLEUS ELECTRONS equal in a neutral atom PROTONS NEUTRONS Negative Charge QUARKS Atomic Number equals the # of... Positive Charge Neutral Charge Most of the atom’s mass. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem