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The Atom

The Atom. Smallest particle of an element that retains the properties of the element. The beginning. Democritus first suggested the existence of atoms in the 4 th century BC. John Dalton was the first to perform scientific experiments to test and correct his idea of the atom.

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The Atom

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  1. The Atom Smallest particle of an element that retains the properties of the element

  2. The beginning • Democritus first suggested the existence of atoms in the 4th century BC. • John Dalton was the first to perform scientific experiments to test and correct his idea of the atom. • He created the first “Atomic Theory”

  3. Dalton’s Atomic Theory • All elements are composed of tiny indivisible particles called atoms. • Atoms of the same element are identical. • Atoms of different elements can combine physically or chemically (in whole number ratios to form compounds). • Chemical reactions occur when atoms are separated, joined, or rearranged. • However, not all holds true today…

  4. The atom diversifies… • Electrons – negatively charged subatomic particles • Discovered by J.J. Thomson in 1897 • Conducted Cathode Ray Experiment

  5. Electron continued • Robert Millikan measured the quantity of charge and the ratio of charge to mass of the electron • Calculated the electron’s mass • Oil Drop Experiment

  6. Things we know about charges • Atoms have no net electric charge • Electric charges are carried by particles of matter • Electric charges always exist in whole number ratios • When a given number of negatives combines with the same number of positives, a neutral atom is formed. So…we know there must be something POSITIVE inside the atom.

  7. Protons • Goldstein (1886) used cathode ray tube • observed a ray in the opposite direction (canal rays) • named particle the proton • Its mass was ~ 1840 times the mass of the electron

  8. neutrons • Chadwick (1932) confirmed the existence of the neutron • Approx. same mass as the proton

  9. Structure of the Atom • Thomson’s “Plum Pudding” model • Electrons are interspersed throughout positive material

  10. Plum pudding under attack • Rutherford tested Thomson’s theory • Gold Foil Experiment

  11. Gold foil experiment

  12. Gold foil experiment explained • Beam of Alpha particles directed at gold foil. • Particles were scattered suggesting a dense mass of positive charge • Rutherford proposed the idea of the atomic nucleus. • Most of the atom is empty space • Nucleus contains protons and neutrons • Space contains electrons

  13. Where does that leave us? • In the atom, we have: • Protons (p+) • Neutrons ( n0) • Electrons (e-) • Protons and Neutrons are in the nucleus. • Electrons are outside the nucleus. • Identities of elements are determined by the number of protons in the nucleus.

  14. The atom by the numbers • Atomic Number – number of protons (whole number) • Mass Number – Protons plus Neutrons (number of particles in the nucleus) • Atomic Mass – weighted average of the masses of all isotopes of the element (decimal number) 6 C 12.011

  15. Same element…different numbers • Isotope – atoms of the same element with different numbers of neutrons • Atomic Number will be the SAME • Mass Number will be DIFFERENT • Atomic Mass will be the SAME • In an Isotope, • Protons – SAME • Neutrons - DIFFERENT • Electrons – SAME • Ex. Carbon – 12 and Carbon - 14

  16. Protons, neutrons, electrons and the atom

  17. Protons, neutrons, electrons and the atom

  18. Atoms vs. ions • Atoms can also gain and lose electrons. • An atom that has lost or gained an electron is an ION. • Positive Ions (CATIONS) form when atoms lose electrons • Ex. Calcium-41 atom 20 p+ 20 e- 21 n0 Calcium-41 +2 ION 20 p+ 18 e- 21 n0 • Negative Ions (ANIONS) form when atoms gain electrons. • Ex. Oxygen-16 atom 8 p+ 8 e- 8 n0 Oxygen-16 -2 ION 8 p+ 10 e- 8 n0

  19. Ions • In an ION, • Protons – Same • Neutrons – Same • Electrons – Different • More electrons means a NEGATIVE ion. • Anion (O2-) • Less electrons means a POSITIVE ion. • Cation (Ca2+)

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