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Atomic Structure Syllabus Ref: 7.1— 7.1—The atom 7.1.1: Describe a model of the atom that features a small nucleus surrounded by electrons. 7.1.2: Outline the evidence that supports a nuclear model of the atom 7.1.3: Outline one limitation of the simple model of the nuclear atom. 7.1.4: Outline evidence for the existence of atomic energy levels. (emission/absorption spectra) “Plum Pudding” model Electron discovery in 1890’s (J.J. Thomson) Atomic structure First visualized as a homogenous sphere of positive charge w/ negative charges within. Like a plum pudding. Rutherford, Geiger, Marsden 19111911-Shot a beam of α-particles at a thin gold foil. Most passed through unaffected or deflected at small angles – Atom must be mostly empty space Some deflected at very large angles! – Must be a small, dense, positively charged “nucleus” nucleus” Rutherford model Further electron scattering showed the nucleus to have a very small radius – r ~ (A1/3) 1.2 x 10-15 m – Compare to 10-10 m for atom “Planetary” Planetary” or “Nuclear” Nuclear” model Positive nucleus containing 99.9% of the mass of the atom Surrounded at a great distance by orbiting electrons – 666,000 times the scale of the solar system Not a perfect model 1 Problems with planetary model Orbiting electrons are accelerating and should give off EM radiation – Should decrease energy and spiral into the nucleus (in a few ns). – Frequency should increase also Should observe (but don’ don’t): – Continuous spectrum of radiation Actually find discrete spectra – Unstable atoms Many are stable Atomic Spectra Spectral Lines Emission and absorption spectra Atomic Spectra Balmer and Rydberg discovered mathematical relationship Niels Henrick David Bohr model Planetary model is good, but should incorporate the recent quantum theories of Planck and Einstein, and should explain atomic spectra. 2 Bohr Model Postulates: Perhaps electrons only move in quantum fashion from one “orbital” orbital” to another and they give discrete amounts of energy when they do this Bohr Model: Complies with observed spectra of Hydrogen and made predictions of lines later to be discovered Assumptions: – Fixed electrons do not emit radiation – Angular momentum is quantized – Can’ Can’t explain how electrons move from one energy state to the next Won Nobel Prize in 1922 (37 yrs old!) Limitations of Bohr’s Model Doesn’t work with atoms with multiple electrons Didn’t explain relative brightness of lines Didn’t explain bonding No experimental evidence for the postulates Does the nucleus have structure? Complicated, still not entirely understood 30,000 carbon nuclei would stretch across a single carbon atom 1930’s, understood to be made of both protons and neutrons Protons and Neutrons Charge and Mass ~2000 times more massive than electrons. Essentially mass of the nucleus = mass of the atom 3 Vocabulary: Nucleons Nuclear Symbols – constituents of the nucleus (protons and neutrons) Nuclides – the different types of nuclei collectively Atomic Number (Z) – number of protons in the nucleus Atomic Mass Number (A) – total number of nucleons in the nucleus Isotope – Nuclei that contain the same number of protons, but different amounts of neutrons Isotopes and mass Discovered w/ mass spectrometers Isotopes are atoms with different numbers of neutrons – Same chemical properties, different mass and nuclear properties Atomic Mass Unit (u) Atomic Structure Syllabus Ref: 7.1— 7.1—The atom 7.1.1: Describe a model of the atom that features a small nucleus surrounded by electrons. 7.1.2: Outline the evidence that supports a nuclear model of the atom 7.1.3: Outline one limitation of the simple model of the nuclear atom. 7.1.4: Outline evidence for the existence of atomic energy levels. (emission/absorption spectra) – 1/12 of a neutral 12C atom 4