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History and Progression of Atomic Theory. Democritus. 400 BC. This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago.
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Democritus 400 BC • This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. • He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?
Democritus’ Atomic Theory 400BC Democritus asserted that space contained an infinite number of particles Named atomos, or "indivisible” Atoms are eternal and invisible; absolutely small, so small that their size cannot be diminished; totally full and incompressible. Atoms are homogeneous, differing only in shape, arrangement, position, and number
Atomos • To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. • Atoms were infinite in number, always moving and capable of joining together.
Dalton’s Theory (early 1800’s) • He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compoundsare formed by the joining of atoms of two or more elements in specific ratios.
J.J. Thomson (English 1897) • Used Cathode Ray tube to determine the presence of – (electrons) and + (protons) particles.
J.J. Thomson (English 1897) • Plum Pudding Model -- the structure of an atom is something like pudding. He assumed that the basic body of an atom is a spherical object containing electrons & protons randomly confined in homogeneous jellylike material. Positive charges cancel the negative charges.
Nagaoka (Japanese 1904) • The Japanese Nagaoka imagined the atom to be like the planet Saturn. He thought it had a positively charged center, with electrons in orbit around it’s middle. There was a problem with this picture of the atom. A charged particle, such as an electron, moving in a circle, radiates electromagnetic waves. As it radiates, it loses energy. Unless it can get energy from some other source, it will slow down and follow a spiral path into the center of the circle. A Saturnian atom would have collapsed.
Ernest Rutherford (Eng.1911) • Atoms have a central positive nucleus surrounded by negative orbiting electrons. • This idea was the result of his famous Gold Foil Experiment(see next slide). This experiment involved the firing of radioactive alpha particles through gold foil. • This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. • Most of particles passed straight through the foil but approximately 1 in 8000 were deflected.
Nucleus-Positive Charged Center The nucleus is far too large in this drawing.
Niels Bohr (Danish 1913) • The Bohr Model is probably familiar to us as the "planetary model" of the atom is used to symbolize atomic energy. • Electrons orbit the nucleus much like planets orbiting the Sun. • However, the orbits are not confined to a plane as is approximately true in the Solar System).
Chadwick (English 1932) • James Chadwick discovered a third type of particle, which he named the Neutron. • Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. • Neutronsalways reside in the nucleus of atoms and they are about the same mass and size as protons. • Neutrons do not have any electrical charge; they are electrically neutral
Cloud Theory • Based on the work of many scientists • Based on the mathematical approach of Quantum Mechanics • Electrons are assigned regions of space (Orbitals) not pathways (Orbits) • Electrons are moving around the nucleus rapidly in no predictable path producing a cloud of e-’s over time. Think of a rapidly moving fan blade.
Cloud Theory of Today Electron Cloud
How to find Protons • Number of protons in an atom is ALWAYS equal to the Atomic Number
Number of Electrons (e-) • Atoms – Protons and electrons are equal • Ions • Loss of electron makes positive ions • Gain of electrons makes negative ions
Loss or Gain of Electrons • Loss of Electrons • Atom Ion • Na Na+1 + 1e- (1e- removed) • +11 +11 • -11 -10 • 0 net +1 NET CHARGE • Gain of Electrons • Cl2 + 2e- 2Cl-(2e- added to Cl2) • +17 +17 • -17 -18 • 0 net -1 NET CHARGE
How to calculate Neutrons • Atomic Mass (rounded to integer) • - Atomic Number • ---------------------------------------------------- • Number of Neutrons in the nucleus • Atomic Mass – Atomic Number = # Neutrons
Atom Contents • Protons (P+) – always equal to Atomic # • Electrons (e-) • Atoms – Same as the # of Protons • Ion – Net charge after e-’s have been lost or gained in an attempt to become stable (HAPPY NUMBERS!) • Loss of e-’s = Positive charge • Gain of e-’s = Negative charge • Neutrons (N±) = Atomic Mass – Atomic #
Energy Levels • Maximum • K Shell = 2 e-’s • L Shell = 8 e-’s • M Shell = 8 e-’s • N Shell = 18 e-’s • When you get to Chemistry, you will find that this gets more involved than shown here.
Oxygen Atom • How many protons(+) are present? • How many electrons(-) are present? • How many neutrons(±) are present? • In what energy levels are the e-’s found?
Oxygen Atom • How many protons(+) are present? • Atomic number = 8 = protons • How many electrons(-) are present? • In atoms, p+ = e- therefore 8 e-’s • How many neutrons(±) are present? • At. Mass – At. # = 16 – 8 = 8 neutrons • In what energy levels are the e-’s found? • 8e-’s total = 2 in K shell, 6 in L shell
Magnesium Ion • How many protons(+) are present? • How many electrons(-) are present? • What is the charge of the ion? • How many neutrons(±) are present? • In what energy levels are the e-’s found?
