Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
History of the Atomic Model Early Greek Theories Aristotle (350 B.C.) • 4 Elements Democritus (400 B.C) • Atoms and a void (empty space) • Atoms are indivisible • Based on philosophical arguments, NOT experimental evidence Dalton’s Billiard Ball Model (1805) • All matter is made of tiny, indivisible particles called atoms. • Each element is made up of atoms that are unique and unlike atoms of any other element. • Atoms of an element are identical. • Matter is composed of indestructible, indivisible atoms Thomson’s Raisin Bun Model (1897) • Materials, when rubbed, can develop a charge difference. • This electricity was called “cathode rays” • These rays have a small mass and are negatively charged. • Thomson noted that these negative subatomic particles (electrons) were a fundamental part of all atoms. Rutherford’s Nuclear Model (1911) • Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Most particles passed through. So, atoms are mostly empty space. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive (protons) & holds most of an atom’s mass. Table 1, p. 26 Practice • P. 26 #1-5 Atomic numbers, Mass numbers Elements are often symbolized with their mass number (A) and atomic number (Z) 16 E.g. Oxygen: 8 O Z = # of protons = # of electrons A - Z = # of neutrons Calculate # of e–, n0, p+ for Ca, Ar, and Br Atomic Mass p+ e- no Ca 20 40 20 20 20 Ar 18 40 18 18 22 Br 35 80 35 35 45 Bohr - Rutherford diagrams • Putting all this together, we get B-R diagrams • To draw them you must know the # of protons, neutrons, and electrons (2,8,8,18 filling order) • Draw protons (p+), (n0) in circle (i.e. “nucleus”) • Draw electrons around in shells He p+ 2 2 n0 Li Li shorthand 3 p+ 4 n0 3 p+ 2e– 4 n0 1e– Draw Be, B, Al and shorthand diagrams for O, Na Be B Al 4 p+ 5 n° O 5 p+ 6 n° 13 p+ 14 n° Na 8 p+ 2e– 6e– 8 n° 11 p+ 2e– 8e– 1e– 12 n° Isotopes and Radioisotopes Isotopes: Atoms of the same element that have different numbers of neutrons – Due to isotopes, mass #s are not round #s. – E.g. Li (6.9) is made up of both 6Li and 7Li. – Often, at least one isotope is unstable.It breaks down, releasing radioactivity.These types of isotopes are called radioisotopes Q- Sometimes an isotope is written without its atomic number - e.g. 35S (or S-35). Why? Q- Draw B-R diagrams for the two Li isotopes. A- The atomic # of an element doesn’t change Although the number of neutrons can vary, atoms have definite numbers of protons. 6Li 7Li 3 p+ 3 n0 2e– 1e– 3 p+ 4 n0 2e– 1e– Practice • P. 29 #1-7 Limitations to Rutherford’s Model • Orbiting electrons should emit light, losing energy in the process • This energy loss should cause the electrons to collapse into the nucleus • However, matter is very stable, this does not happen Bohr’s Planetary Model • Electrons orbit the nucleus in energy “shells” • An electron can travel indefinitely within a shell without losing energy • The greater the distance between the nucleus and the shell, the greater the energy level • An electron cannot exist between shells, but can move to a higher, unfilled shell if it absorbs a specific quantity of energy, or to a lower, unfilled shell if it loses energy (quantized) • When all the electrons in an atom are in the lowest possible energy levels, it is in its ground state. • An atom becomes excited when one of its electrons absorb energy • If enough energy is absorbed then the electron can make a quantum leap to the next energy level, if there is room • When the electron returns to a lower energy state the energy is released in the form of a photon, which we see as visible light • The energy of the photon determines its wavelength or color • Each element has its own frequencies of color, so it emits its own distinctive glow Summary of Atomic Models 1) Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. 2) Thomson’s “Raisin bun” model (1900) Negative electrons in a positive framework. 3) Rutherford’s “Nuclear” model (~1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus. 4) Bohr’s “Planetary” model (~1920) Negative electrons orbit a positive nucleus. Quantized energy shells 5) Quantum Mechanical model (~1930) Electron probabilities (orbitals) Practice • Pre-lab: Atomic Spectra (p. 40) • p. 42 #1-3 • p. 45 #6-8