orbitals-universityofnorthcarolinawilmington軌道-北卡羅來那大學wilming-文庫吧
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【正文】 ? Electrons ( charge) are considered to be a cloud of charge around the nucleus. Orbitals describe where the electrons are. Electrons have very little mass pared to protons and neutrons. ? Electrons are found in s orbitals (spherical), p orbitals (dumbbell), or d orbitals (various shapes) ? Electrons are grouped in different layers or shells. 9 Atomic Structure ? Structure of an atom ? Positively charged nucleus (very dense, protons and neutrons) and small (1015 m) ? Negatively charged electrons are in a cloud (1010 m) around nucleus ? Diameter is about 2 ? 1010 m (200 pieters (pm)) [the unit Angstrom (197。) is 1010 m = 100 pm] 10 Atomic Structure: Orbitals ? Quantum mechanics: describes electron energies and locations by a wave function, ? ? A plot of ? 2 describes the region where electrons are most likely to be ? An electron cloud has no specific boundary so we represent its shape by the region of highest probability of finding an electron. ? Solutions of the wave equation give rise to regions of electron density on each atom of specific shapes (atomic orbitals) 11 Shapes of Atomic Orbitals ? Four different kinds of orbitals occupied by electrons ? Denoted s, p, d, and f (listed in increasing energy) ? s and p orbitals are most important in anic chemistry ? s orbitals: spherical, with the nucleus at center ? p orbitals: dumbbellshaped, with the nucleus at the center 12 pOrbitals ? There are three perpendicular p orbitals, px, py, and pz, of equal energy ? Lobes of a p orbital are separated by region of zero electron density, called a node. 13 Atomic Structure: e Configuration ? The lowest energy electron configuration of an atom of any element can be predicted by following three rules: ? The aufbau principle: Electrons are filled into the lowest energy orbitals first (1s, then 2s, then 2p, then 3s, then 3p, then 4s, then 3d) ? The Pauli exclusion principle: Only two electrons may occupy an orbital。 they must have opposite spin orientations. ? Hund’s rule: If there are two or more equal energy (degenerate) orbitals available, the electrons will spread out among the orbitals with parallel spins, only pairing up after the orbitals are halffilled. 14 Examples of Electron Configuration 1s 2s 2p x 2p y 2p zHCNOFat . 167893s 3p x 3p y 3p zN e 1 0C l 1715 The Nature of the Chemical Bond ? Atoms form bonds because the pound that results is more stable than the separate atoms. ? Ionic bonds in salts form as a result of electron transfers, followed by electrostatic attraction between opposite charges. ? Organic pounds form covalent bonds by sharing electrons (G. N. Lewis, 1916). ? Lewis structures show valence electrons of an atom as dots. ? Hydrogen has on
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