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“Atomic Structure”

“Atomic Structure”. Defining the Atom. The Greek philosopher Democritus was among the first to suggest the existence of atoms (from the Greek word “ atomos ”) He believed that atoms were indivisible and indestructible . Dalton’s Atomic Theory (experiment based!).

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“Atomic Structure”

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  1. “Atomic Structure”

  2. Defining the Atom • The Greek philosopher Democritus was among the first to suggest the existence of atoms (from the Greek word “atomos”) • He believed that atoms were indivisible and indestructible

  3. Dalton’s Atomic Theory (experiment based!) All elements are composed of tiny indivisible particles called atoms Atoms of the same element are identical. Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. John Dalton

  4. Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

  5. Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. J. J. Thomson

  6. Ernest Rutherford’sGold Foil Experiment - 1911

  7. The Rutherford Atomic Model • Based on his experimental evidence: • The atom is mostly empty space • All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” • The electrons distributed around the nucleus, and occupy most of the volume • His model was called a “nuclear model” • The neutron was discovered by Chadwick in 1932

  8. Bohr Model • Bohr refined Rutherford's model of the atom by finding that electrons were found in distinct energy levels around the nucleus. When electrons move to different energy levels energy is released.

  9. Quantum Atomic Model • This is the current view of atoms, it is a mathematical model rather than a physical model. It describes the electrons as being found in regions of probability around the nucleus, often called an “electron cloud”

  10. Changes in Atomic model

  11. Subatomic Particles

  12. Atomic Number • Elements are different because they contain different numbers of PROTONS • The “atomic number” of an element is the number of protons in the nucleus • In a neutral atom the # protons is equal to the # electrons

  13. Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

  14. Complete Symbols Mass number X Superscript → Atomic number Subscript →

  15. Symbols • Find each of these: • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number 80 Br 35

  16. Dalton was wrong about all elements of the same type being identical • Atoms of the same element can have different numbers of neutrons. • Thus, different mass numbers. • These are called isotopes.

  17. We can also put the mass number after the name of the element: • carbon-12 carbon-14 Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons.

  18. Isotope Examples

  19. Examples of Isotopes:

  20. Atomic Mass • How heavy is an atom of oxygen? • It depends, because there are different kinds of oxygen atoms. • We are more concerned with the average atomic mass. • This is based on the abundance (percentage) of each variety of that element in nature. • We don’t use grams for this mass because the numbers would be too small.

  21. Instead of grams, the unit we use is the Atomic Mass Unit(amu) • It is defined as one-twelfth the mass of a carbon-12 atom. • Carbon-12 chosen because of its isotope purity. • Each isotope has its own atomic mass, thus we determine the average from percent abundance.

  22. To calculate the average: • Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. • If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

  23. Atomic Masses Atomic mass is the average of all the naturally occurring isotopes of that element. Carbon = 11.87 + 0.14 + .0014 =12.011

  24. The Periodic Table:A Preview • Each horizontal row (there are 7 of them) is called a period • Each vertical column is called a group, or family

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