Expanded shell

There are examples with even more electrons around the central atom, such as IF7 (14 electrons), [TaFg] (16 electrons), and [XeFg] (18 electrons). There are rarely more than 18 electrons (2 for s, 6 for p, and 10 for d orbitals) around a single atom in the top half of the periodic table, and crowding of the outer atoms usually keeps the number below this, even for the much heavier atoms with / orbitals energetically available. 

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Characteristic of fluidized bed reactors is the large wind box to equalize pressure. This is a primary requirement to get even flow through the bed. The expanding shell at the upper part is there to retain as much solid as possible in the reactor. 

Figure 10-22A. Construction details of two-pass expanding shell-side baffle. (Used by permission Struthers-Wells Corp., Bui. A-22.)...

Fig. 2. VLA detection of 3He+ in the PN J 320. We have modeled the radio continuum and line emission using the radiative transfer code NEBULA [1], assuming an expanding shell of ionized gas. The dashed line is the model including the H171 7 and 3He+ transitions. The solid line shows the observed spectrum and only includes the 3He+ transition. The model fits the data reasonably well even though the morphology is bipolar as indicated by the HST image [6]...

The shock wave of fresh debris from a supernova explosion travels a great distance. For example, it has been calculated that the Crab Nebula will attain a diameter of 70 light years 23,000 years from now 140 light years in 260,000 years 210 light years in 1.3 million years and 280 light years in 4 million years. After that the expanding shell will begin to dissipate because the velocity of expansion drops to about 1-2 miles/sec. (3,600-7,200 miles/hr.), which is the velocity of the molecules of interstellar gas. 

According to Opik (13), the supernova explosion sweeps up and compresses into an expanding shell a mass of interstellar gas of the order of 15,000 solar masses, 1000 times greater than the original mass of the supernova. This means that the fresh debris from a supernova is diluted 1000 times by old debris. 

When it is impossible to draw a structure consistent with the octet rule, it is necessary to increase the number of electrons around the central atom. An option limited to elements of the third and higher periods is to use d orbitals for this expansion, although more recent theoretical work suggests that expansion beyond the i and p orbitals is unnecessary for most main group molecules. In most cases, two or four added electrons will complete the bonding, but more can be added if necessary. Ten electrons are required around chlorine in CIF3 and 12 around sulfur in SFg (Figure 3-2). The increased number of electrons is described as an expanded shell or an expanded electron count. 

Some molecules have satisfactory electron-dot structures with octets, but have better structures with expanded shells when formal charges are considered. In each of the cases in Figure 3-6, the observed structures are consistent with expanded shells on the central atom and with the resonance structure that uses multiple bonds to minimize formal charges. The multiple bonds may also influence the shapes of the molecules. 

The more massive post-AGB stars (M > 0.6M ) with higher luminosity contract more quickly (L > 1O4L0, tK 10000y). When their surface temperature exceeds 30 000 K, surface hydrogen is fully ionized, the Lyman continuum opacity drops and ultraviolet photons are able to ionize the much-expanded shell, which now becomes visible as a planetary nebula. Schematically we might represent the evolution as follows ... 

From Figures 5.22 and 5.23, it is seen that there is room to accommodate the thermal expansion of the tubes. In the case of the finned heat exchanger, the tubes can expand in an expandable shell, whereas in the case of the U-tube heat exchanger, expansion room is provided because of the existence of free fixed tubes in a firm shell. 

Experimental studies of voidage in the vicinity of bubbles rising in a fluidized bed have shown that these bubbles are surrounded by an expanded shell where the void fraction is considerably in excess of that in the remote bulk of the bed [61,62]. Such voidage variations had been reported in earlier experiments conducted using two-dimensional model beds [63,64]. Because these observations are at variance with the available theories of steady bubble motion in fluidized beds [65], they have been used by a number of researchers to reexamine these theories. Thus, Collins [66] fits an equation to the results of reference [63],... 

Where Cx is the electromechanical conversion efficiency of the transducer, assumed here to be itsed both for transmission and reception hence raised to a power of two, S is the strength of the scatterer (how much incident energy it reflects back), and r is the range to the scatterer, assuming an omnidirectional transducer in a deep ocean. The power of the transmitter is spread out over a spherically expanding shell of area as it travels to the... 

Supernova remnants are the result of much more violent disruptions of (more massive) stars the shock waves produced by the stellar explosion excite emissions from the gas both in the castoff envelope and in the surrounding interstellar gas, which is swept up in the expanding shell. Hence, the elemental compositions of supernova remnants are intermediate between those of H II regions and planetary nebulae. 

The CO(l-O) maps of NGC 7027 show the kinematic signature of a uniformly expanding shell with v xp = 14 km (Graham et d. 1993a). For uniform expansion the central velocity channel corresponds to a spatial slice in the plane of the sky through the CO shell at the position of the star. If we compare the H2 and the CO central channel (Fig. 1) we find a striking relationship. The H2 sits within the CO cavity, and the CO delineates the outer boundary of the H2. 

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