Unit_1_Notes - Unit 1 Atomic Structure 4.1 Defining the...

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Unit 1 – Atomic Structure 4.1 Defining the Atom I. Atomic Theory A. Modern Atomic Theory 1. All matter is made up of very tiny particles called atoms 2. Atoms of the same element are chemically alike 3. Individual atoms of an element may not all have the same mass. However, the atoms of an element have a definite average mass that is characteristic of the element 4. Atoms of different elements have different average masses 5. Atoms are not subdivided, created, or destroyed in chemical rxns II. Sizes of Atoms A. Atomic radius 1. 40 to 270 picometers (pm) a. 1 pm = 10 -12 m 2. Most of the atomic radius is due to the electron cloud B. Nuclear radius 1. 0.001 pm 2. density is 2x10 8 metric tons/cm 3 a. 1 metric ton = 1000kg 4.2 Structure of the Nuclear Atom I. The Electron A. Discovery 1. Joseph John Thomson (1897) a. Cathode ray tube produces a ray with a constant charge to mass ratio b. All cathode rays are composed of identical negatively charged particles (electrons) B. Inferences from the properties of electrons 1. Atoms are neutral, so there must be positive charges to balance the negatives 2. Electrons have little mass, so atoms must contain other particles that account for most of the mass II. The Nucleus A. The Rutherford Experiment (1911) 1. Alpha particles (helium nuclei) fired at a thin sheet of gold a. Assumed that the positively charged particles were bounced back if they approached a positively charged atomic nucleus head-on (Like charges repel one another) 2. Very few particles were greatly deflected back from the gold sheet a. nucleus is very small, dense and positively charged b. most of the atom is empty space
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III. Structure of the Nucleus 1. Protons a. Positive charge, mass of 1.673x10 -27 kg b. The number of protons in the nucleus determines the atom's identity and is called the atomic number 2. Neutrons a. No charge, mass of 1.675x10 -27 kg 3. Nuclear Forces a. Short range attractive forces: neutron-to-neutron, proton-to-proton, proton-to-neutron Particle Symbols Relative charge Mass Number Electron e - e 0 1 -1 0 Proton p + 1 1
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This note was uploaded on 11/12/2011 for the course CHEM CHEM 2B taught by Professor Cabrera during the Spring '09 term at San Jose City College.

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Unit_1_Notes - Unit 1 Atomic Structure 4.1 Defining the...

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