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UNIT 2

UNIT 2. Atoms, Ions, Electronic Structure, and Periodic Properties of Atoms and Ions. A HISTORY OF THE STRUCTURE OF THE ATOM. John Dalton’s Atomic Theory. Each element is composed of extremely small particles called atoms .

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UNIT 2

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  1. UNIT 2 Atoms, Ions, Electronic Structure, and Periodic Properties of Atoms and Ions

  2. A HISTORY OF THE STRUCTURE OF THE ATOM

  3. John Dalton’s Atomic Theory • Each element is composed of extremely small particles called atoms. • All atoms of a given element are identical. The atoms of different elements are different and have different properties (including different masses). • Atoms of an element are not changed into different types of atoms by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. This is theLaw of Conservation of Mass. • Compounds are formed when atoms of more than one element combine. A given compound always has the same relative number and kind of atoms. This is theLaw of Constant Composition.

  4. John Dalton’s Atomic Theory Almost right. A good start. very small Structure of the atom after Dalton (ca. 1810)

  5. J.J. Thomson: Cathode Rays Atoms subjected to high voltages give off cathode rays.

  6. J.J. Thomson: Cathode Rays Cathode rays can be deflected by a magnetic field.

  7. Cathode Rays = ELECTRONS* high voltage Electron (-) + (originally known as a cathode ray) Positive ion Atom Cathode rays are electrons. Electrons are in atoms. *Thomson wanted the name to be “corpuscle.”

  8. J.J. Thomson’s Model of the Atom “Plum pudding” model: Negative electrons are embedded in a positively charged mass. Electrons (-) Unlike electrical charges attract, and that is what holds the atom together. Positively charged mass Structure of the atom after Thomson (ca. 1900)

  9. Henri Becquerel – Natural Radioactivity Some atoms naturally emit one or more of the following types of radiation: alpha (α) radiation (later found to be He2+ - helium nucleus) beta (β) radiation (later found to be electrons) gamma (γ) radiation (high energy light) α Alpha particles Electrons (-) γ γ Positively charged mass α Somehow gamma radiation is in there, too. Structure of the atom incorporating radioactivity

  10. Ernest Rutherford – The Nucleus Scattering experiment: firing alpha particles at a gold foil

  11. Ernest Rutherford – The Nucleus Scattering experiment: firing alpha particles at a gold foil some are deflected a little stream of α particles most pass through the foil some are deflected a LOT detector gold foil (very thin) Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged!

  12. Ernest Rutherford – The Nucleus and the Proton The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus. The positively charged particles in the nucleus are protons. Electrons (-) are now outside the nucleus. Structure of the atom after Rutherford (1910)

  13. James Chadwick – The Neutron In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons. Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics) The positively charged particles in the nucleus are protons. n + n Structure of the atom after Chadwick (1932)

  14. Structure of the Atom proton (+) Electrons occupy the sphere. neutron electrons (-)responsible for the volume and size of the atom, negatively charged 10-10 m nucleus - responsible for the mass of the atom, positively charged 10-14 m 10-15m

  15. Atomic Facts 1 amu = 1 atomic mass unit = 1.66054 x 10-24 g n + n Electrons are outside the nucleus in quantized energy states called orbitals.

  16. Atomic Number Carbon atom • The number of protons in the nucleus is called the atomic number Z. • Z determines the identity of an element. • Saying “the atomic number of an element is 6” is the same as saying “carbon.” • The number of electrons in the atom is also Z (because atoms have no net electric charge). • How many neutrons are in C? - proton - neutron

  17. Isotopes A 12C Z 6 • The number of protons and neutrons (nucleons) in an element is called the mass number A. A = Z + number of neutrons. • An element may have different numbers of neutrons but NOT different numbers of protons. • Atoms of an element with different numbers of neutrons are called isotopes of that element. - proton - neutron How many neutrons are in C? The answer is “it depends on the isotope.”

  18. Isotopes 12 6 12C or C-12 6 14 6 14C or C-14 6 16 8 16O or O-16 8 238 92 238U or U-238 92

  19. Atomic Masses Atomic masses are based on 12C. The mass of 12C (or C-12) is defined to be exactly 12 amu.

  20. Atomic Masses The mass (weight) shown in the periodic table is the mass of the element as its occurs naturally. If the element has more than one isotope, the mass shown is the weighted average of the masses of the isotopes. Mg has 3 isotopes. 24Mg 78.99% 23.985 amu 25Mg 10.00% 24.986 amu 26Mg 11.01% 25.983 amu weighted average of Mg: 0.7899x23.985 18.946 0.1000x24.986 2.499 0.1101x25.983 +2.861 24.31 amu atomic weight of Mg based on natural abundance: 24.31 amu

  21. Isotopes • Are there isotopes in • the food you eat? • the air you breathe? • Are all isotopes radioactive?

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