The structure of the atom

1. Chapter 3.2 The structure of the atom

2. 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.

3. 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

4. Properties of subatomic particles

5. 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 2 – Click Here Visual Concept 1 – Click Here

6. 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

7. Composition of the Atomic Nucleus • All atomic nuclei are made of protonsand 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