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Chapter 4.1 Atomic Theory Democritus Positives World made of two things 1. Empty space 2. Small particles 2. Atoms were the smallest possible particles 3. Different atom for each substance 1. Democritus Negatives 1. 2. Changes in matter were the result of changes in the groupings of atoms and not changes in the atoms themselves Very general, not supported experimentally Aristotle Contradicted Democritus on the theory that empty space could not exist Since Democritus could not experimentally support his theory, Aristotle's reputation killed the idea of the atom for 1600 years Isaac Newton & Robert Boyle Renewed support of Democritus Still no experiments or predictions Dalton’s Atomic Theory Positives 1. All matter composed of atoms 2. All elements have different atoms Dalton’s Atomic Theory 3. 4. Atoms combine in simple whole number ratios to form compounds In a reaction atoms are separated, combined or rearranged Dalton’s Atomic Theory Negatives 1. Atoms are the smallest unit of matter 2. All atoms within an element are the same Nanotechnology Manipulating individual atoms in order to create new products Chapter 4.2 Subatomic Particle History Electron Thomson Discovered the electron by using a cathode ray tube to compare the ratio of charge to mass Determined mass was less than Hydrogen Electron – Importance of Thomson’s Experiment 1. There must be particles smaller than the atom 2. Dalton’s theory was wrong Not believed at first, but further experiments proved him right Electron Mendeleev’s Response No internal structure but Thomson had discovered a new element Thomson’s New Element 1. 2. 3. Atomic mass 1/1,000,000 of H Traveled at 1398 miles/second Capable of moving freely through space Electron Millikan Determined the electron had a negative charge Electron Millikan Determined the mass of the electron Mass of electron is 1/1840 of H -28 g mass equals 9.1x10 Atomic Models Thomson’s Plum Pudding Model Rutherford’s Model Beginning of the modern model of the atom Atom is mostly empty space Rutherford’s Model Nucleus (Gold foil experiments) Centrally located in the atom Positively charged Extremely dense – virtually all the mass of the atom Electrons are held to the nucleus by the electromagnetic force Questions still to be resolved 1. 2. 3. How are the electrons arranged around the nucleus? How do atoms of different elements differ in structure? What accounts for the mass of the elements since the mass of an electron is so small? Proton Discovered by Henry Moseley Shot x-rays at samples of elements and found that wavelength decreased with increasing atomic mass Decreased wavelength was caused by an increase in number of electrons Proton Carries the opposite but equal charge to the electron Neutron Bothe & Chadwick Energy particles in the atom with no charge Mass identical to the proton Modern Atomic Theory 1. 2. Atoms are composed of a nucleus with electron(s) circling and held in place by protons Protons and electrons are identical in number in a neutral atom Modern Atomic Theory 3. Nucleus 1. Positively charged 2. Composed of protons and neutrons 3. 99.97% of the mass of the atom Chapter 4.3 Periodic Table Information Atomic Number Number of protons in the nucleus Moseley determined atomic number for each element Re-ordered Mendeleev’s table Experimental methods were used to support/refute claims of new elements Isotopes Atoms of an element with the same number of protons but different numbers of neutrons (p.100) Most elements exist as a mixture of isotopes All react the same chemically and physically Mass Number Sum of the protons and neutrons in an element Recorded in the upper left quadrant i.e. K-39 = 39K # N0 = Mass # – Atomic # i.e. K-39 = 39-19 = 20 N0 Atomic Mass Weighted average of the mass of all the isotopes of an element Majority of the mass is in the nucleus Atomic Mass Measured in atomic mass units(amu’s) 1 amu = 1/12 mass of a C-12 atom 1 amu = approximate mass of a proton or neutron Calculating Atomic Mass 1. 2. Mass of each isotope multiplied by the percentage of the time it occurs Add the products derived for each isotope Cl-35 = 76% 26.6 Cl-37 = 24% 8.9 Cl = 35.5 amu Chapter 4.4 Radioactive Decay Nuclear Reaction Any reaction that involves a change in the nucleus of an atom Changes the identity of the element Radioactivity 1. 2. 3. Spontaneous emission of radiation Atoms gain stability by losing energy through radioactive decay Radioactive decay continues until a non-radioactive stable element is formed Types of Radiation Alpha • Equivalent to a He nucleus + 0 2 P and 2 N emitted Overall 2+ charge Types of Radiation Alpha Mass number decreases by 4 Atomic number decreases by 2 Types of Radiation Beta E- emitted from the nucleus Generated by the decay of a N0 N0 decay every 930 s Types of Radiation Beta Mass number does not change Atomic number increases by 1 Types of Radiation Gamma High energy x-rays Usually accompanies the other two types of radiation Accounts for the bulk of the energy emitted Nuclear Equation 1. 2. 3. Representation of radioactive decay 1. 226Ra 222Rn + 2. 14C 14N + (-1 ) 3. 238U 234Th + + 2 Conservation of mass and atomic numbers Stability determined by ratio of N0/ P+ Strong Force Force that holds N0 & P+ in the nucleus Effective over only very short distances ( 10-3 pm) 100,000 times stronger than gravity