The structure of the atom Chapter 3.2 The structure of the atom

Objectives Summarize the observed properties of cathode rays that led to the discovery of the electron. Summarize the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus. List the properties of protons, neutrons, and electrons. Define atom.

Structure of the Atom Define atom: smallest particle of an element that retains the chemical properties of that element. Nucleus very small region located at the center of an atom Contains Protons – positive charged particle Neutron – particle with NO charge Area surrounding nucleus Electron cloud Electrons – negative charged particle Subatomic particles Visual Concept – Click Here

Properties of subatomic particles

Discovery of the Electron Late 1800’s – J.J. Thomson discovered the electron through experiments with cathode- ray tubes. Plum Pudding Model of atom Robert A. Millikan – 1909 – Oil drop experiment Electron is 1/1837 the mass of hydrogen atom Visual Concept 1 – Click Here Visual Concept 2 – Click Here

Discovery of the Atomic Nucleus In 1911 – Ernest Rutherford – Gold Foil Experiment Showed that there was a small – dense positively charged nucleus of an atom. Visual Concept – Click Here

Composition of the Atomic Nucleus All atomic nuclei are made of protons and neutrons. Atoms are electrically neutral - contain equal numbers of protons and electrons Nuclear Forces – Short range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together Sizes of Atoms Nucleus compared to rest of atom - approximately size of marble compared to a large football stadium. Atom is mostly empty space