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Internal Structure of Atoms Chapter 3-Section 2 1 Subatomic Particles Electrons, protons, and neutrons Electricity – J.J.Thomson – Electrons Metal plates – electrodes –Cathode –Anode + 2 Cathode Ray Tube 3 Electrons The wheel in the cathode ray tube-mass Originate at negatively charged side – negatively charged electrons Magnet 4 Mass Further experiments found the mass of an electron to be 5.485799 x 10-4 amu 2000 times lighter than an atom The charge was found to be -1.602189 x 10-19 C C = Coulombs – SI unit 5 Charges An atom is not the simplest form of matter Since electrons (e-) are negative, how is an atom neutral? 6 The Nucleus Thompson’s plum pudding model This is no longer a valid model 7 Rutherford’s Nucleus Gold foil experiment –Alpha particles –Some passed through and some were repelled away from the foil 8 Rutherford’s Nucleus 2 years later… Must be a concentrated tiny space with a mass larger than the α particle The rest of the gold foil would be empty space. 9 The modern nucleus The nucleus is a dense central portion of the atom that is positively charged. The nucleus has most of the mass of the atom, but is very small compared to the entire atom. Marble and football 10 Protons and Neutrons Protons =+ -19 Charge = 1.602189 x 10 C Mass = 1.0073 amu (2000 times the mass of an electron) 11 Protons and Neutrons A neutral atom has equal numbers of protons and electrons Masses of all atoms (except H) were greater than the mass of e- and p+ combined so where does this mass come from? 12 Neutrons Joliot-Curie and Chadwick experiment with beryllium –Alpha particles –Beam of power not deflected by electric or magnetic fields These particles must be neutral Mass of 1.0087 amu 13 Coulomb’s Law States that the force between two charged particles is inversely proportional to the square of the distance between them. 14 Coulomb’s Law Force = 0.58 N Distance = 0.020 pm Force = 2.32 N Distance = 0.010 pm 15 Strong Force The force that holds the protons and neutrons in a nucleus. It is greater than the repulsive forces at close distances. Must be neutrons in nucleus if there is more than one proton. 16 Atomic Number Atomic number – the number of protons in the nucleus of an atom # of protons =#of electrons Always whole numbers Same for all atoms of the same element. 17 Mass Number The total number of protons and neutrons in the nucleus of an atom. # of Neutrons=Mass# - Atomic# Can vary for atoms of the same element. 18 Boron The atomic number of boron is 5. The mass number of boron is 11. The number of protons and electrons are equal to the atomic number. 19 Atomic Number Element Symbol Boron Mass Number 20 Boron Electrons =5 Protons = 5 Neutrons = 11 – 5 = 6 21 Copper How many protons, neutrons, and electrons are present in copper which has an atomic number of 29 and a mass number of 64? 22 Copper Electrons = 29 Protons = 29 Neutrons = 35 23 Atomic Structure Atomic and mass numbers can be written beside the elements symbol Mass Number 9 Be 4 Atomic Number 24 Concept Check What three subatomic particles are important to chemistry and what are their electrical charges? 25 Isotopes Isotope – An atom that has the same # of protons as other atoms of the same element but has a diff. # of neutrons Many elements exist as mixtures of isotopes. 26 Lead Isotopes Name # of Mass Amt Neutrons (amu) (%) Lead - 122 203.973 1.4 204 -206 124 205.974 24.1 -207 125 206.976 22.1 -208 126 207.977 52.4 27 Radioisotopes Some elements have only one stable isotope and will degrade and are called radioisotopes. Unstable nuclear configurations. Unstable atoms that undergo radioactive decay. 28 Unstable in Nature Some elements have no stable isotopes. Technetium has no stable nucleus. Was the first element to be made artificially in a laboratory. 29 Section Review Complete numbers 1-6 in the section review on page 89. 30