Neutral atom

We imagine the bare nucleus of atomic number Z and then feed into the available orbitals Z electrons one after the other. The first two rules of the building-up principle are  

According to the Pauli exclusion principle, each orbital may accommodate up to two electrons. 

The order of occupation is approximately the order of energies of the individual orbitals because in general the lower the energy of the orbital, the lower the total energy of the atom as a whole when that orbital is occupied. An s subshell is complete as soon as two electrons are present in it. Each of the three p orbitals of a shell can accommodate two electrons, so a p subshell is complete as soon as six electrons are present in it. A d subshell, which consists of five orbitals, can accommodate up to 10 electrons. 

Electrons occupy different orbitals of a given subshell before doubly occupying cuiy one of them. 

It follows that a nitrogen atom (Z= 7) has the configuration [He]2s 2pi2p 2p . Only when we get to oxygen (Z = 8) is a 2p orbital doubly occupied, giving the configuration [He]2s 2pj2pj2p.  

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Positive ions also fomi readily by adding a proton to a neutral atom or molecule [120]... 

A gas phase ionic cluster can be described as a core ion solvated by one or more neutral atoms or molecules... 

The electron distribution, p(r), has been computed by quantum mechanics for all neutral atoms and many ions and the values off(Q), as well as coefficients for a useful empirical approximation, are tabulated in the International Tables for Crystallography vol C [2]. In general,is a maximum equal to the nuclear charge, Z, lor Q = 0 and decreases monotonically with increasing Q. 

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Phillips W D 1998 Laser cooling and trapping of neutral atoms Rev. Mod. Rhys. 70 721... 

Metcalf H and van der Straten P 1994 Cooling and trapping of neutral atoms Phys. Rep. 244 203-86... 

Walker T, Sesko D and Wieman C 1990 Collective behavior of optically trapped neutral atoms Phys.Rev.Lett. 64 408-11... 

Migdall A L, Prodan J V, Phillips W D, Bergman T H and Metoalf H J 1985 First observation of magnetioally trapped neutral atoms Phys.Rev.Lett. 54 2596-9... 

Pritohard D E 1983 Cooling neutral atoms in a magnetio trap for preoision speotrosoopy Phys.Rev.Lett. 51 1336-9... 

Bagnato V S, Lafyatis G P, Martin A C, Raab E L, Ahmad-Bitar R and Pritohard D E 1987 Continuous stopping and trapping of neutral atoms Phys.Rev.Lett. 58 2194-7... 

Two recent reviews recount subsequent research in the physics of neutral-atom cooling and trapping [3, 4],... 

A simple example would be in a study of a diatomic molecule that in a Hartree-Fock calculation has a bonded cr orbital as the highest occupied MO (HOMO) and a a lowest unoccupied MO (LUMO). A CASSCF calculation would then use the two a electrons and set up four CSFs with single and double excitations from the HOMO into the a orbital. This allows the bond dissociation to be described correctly, with different amounts of the neutral atoms, ion pair, and bonded pair controlled by the Cl coefficients, with the optimal shapes of the orbitals also being found. For more complicated systems... 

As mentioned above, HMO theory is not used much any more except to illustrate the principles involved in MO theory. However, a variation of HMO theory, extended Huckel theory (EHT), was introduced by Roald Hof nann in 1963 [10]. EHT is a one-electron theory just Hke HMO theory. It is, however, three-dimensional. The AOs used now correspond to a minimal basis set (the minimum number of AOs necessary to accommodate the electrons of the neutral atom and retain spherical symmetry) for the valence shell of the element. This means, for instance, for carbon a 2s-, and three 2p-orbitals (2p, 2p, 2p ). Because EHT deals with three-dimensional structures, we need better approximations for the Huckel matrix than... 

HyperChem tjuantum tn ech an ics calcu lation s tn ust start with the number of electrons (N) and how many of them have alpha spins (th e remain in g electron s have beta spin s ). HyperCh em obtain s th is in form ation from the charge an d spin m u Itiplicity th at you specify in th e Sem i-em pirical Op lion s dialog box or. Ab Initio Option s dialog box. is th en computed by coun ting the electron s (valence electrons in sem i-em pirical methods and all electrons in a/ irti/io m ethod) associated with each (assumed neutral) atom and... 

Heitler W and F London 1927. Wechselwirkung neutraler Atome und Homoopolare Bindung nach der Quantenmecharuk. Zeitschrift fur Physilc 44-.455M72. 

Valence electrons of neutral atom Electron count Formal charge... 

Section 1 1 A review of some fundamental knowledge about atoms and electrons leads to a discussion of wave functions, orbitals, and the electron con figurations of atoms Neutral atoms have as many electrons as the num ber of protons m the nucleus These electrons occupy orbitals m order of increasing energy with no more than two electrons m any one orbital The most frequently encountered atomic orbitals m this text are s orbitals (spherically symmetrical) and p orbitals ( dumbbell shaped)... 

Formal charge = (number of electrons m neutral atom)... 

Count the number of electrons assigned to each atom and subtract that number from the number of electrons in the neutral atom the result IS the oxidation number... 

Formal charge (Section 1 6) The charge either positive or negative on an atom calculated by subtracting from the number of valence electrons in the neutral atom a number equal to the sum of its unshared electrons plus half the elec trons in its covalent bonds... 

The attraction for two neutral atoms separated by more than four Angstroms is approximately zero. The depth of the potential wells is minimal. For the AMBER force field, hydrogen bonds have well depths of about 0.5 kcal/mol the magnitude of individual van der Waals well depths is usually less. 

The minimum amount of energy required to remove the least strongly bound electron from a gaseous atom (or ion) is called the ionization energy and is expressed in MJ moE. Remember that 96.485 kJ = 1.000 eV = 23.0605 kcal. In Table 4.2 the successive stages of ionization are indicated by the heading of each column I denotes first spectra arising from a neutral atom viz.,... 

The light regions in the discharge result from electron collisions with neutral atoms in the gas and from recombination of electrons and positive ions to give atoms. 

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