Mass number defined

Elements are defined by their atomic number. An element s atomic number is the number of protons in the atom and is sometimes written as a subscript of the elemental symbol (i.e., nNa). Because the mass number defines the elemental symbol (sodium always has 11 protons and carbon always has 6 protons), the atomic number is frequently omitted. 

EHEKES The number of protons and the mass number define the type of atom. 

The comparison of the magnitude of the two resistances clearly indicates whether tire metal or the slag mass transfer is rate-determining. A value for the ratio of the boundary layer thicknesses can be obtained from the Sherwood number, which is related to the Reynolds number and the Schmidt number, defined by... 

Visually, failure was mostly eohesive within the adhesive (see Figs. 34 and 46). However, there was a small area of apparent interfacial failure ( initiation zone ) located at one end of each substrate. Line scans were eondueted aeross the initiation zone, from the edge of the substrate to the area of cohesive failure within the adhesive. From the line scans, it was apparent that there were patehes of polymer present in the initiation zone, even when failure appeared to be interfaeial (see Fig. 46). SIMS images of the initiation zone were constructed for various mass numbers (see Figs. 47-49). The images showed well-defined cation-rieh... 

Naturally, there are two more Peclet numbers defined for the transverse direction dispersions. In these ranges of Reynolds number, the Peclet number for transverse mass transfer is 11, but the Peclet number for transverse heat transfer is not well agreed upon (121, 122). None of these dispersions numbers is known in the metal screen bed. A special problem is created in the monolith where transverse dispersion of mass must be zero, and the parallel dispersion of mass can be estimated by the Taylor axial dispersion theory (123). The dispersion of heat would depend principally on the properties of the monolith substrate. Often, these Peclet numbers for individual pellets are replaced by the Bodenstein numbers for the entire bed... 

Thus, a polyester sample (1-3 g, exactly weighed) is dissolved in 25 mL of a titrated solution of acetic anhydride in dry pyridine (10% mass). The solution is heated to reflux for 1 h. After cooling, 50 mL pyridine and 10 mL water are added. The excess acetic acid present in the resulting solution is titrated by aqueous potassium hydroxide (0.5 mol/L) using a potentiometric titrator. The determination must be carried out in duplicate and a blank titration must be performed under the same conditions. The mass of polyester and the concentration of reactants should be adjusted to ensure that at least a fourfold excess of acetic anhydride is used. The final result (OH content) is expressed in mmol OH/g polyester or as the hydroxyl number, defined as the number of milligrams of KOH required to neutralize the acetic acid consumed per gram of polyester. [Hydroxyl number = (number of mmol OH/g polyester) x 56.106.]... 

The secondary flows from natural convection can become larger than the primary flow, so it seems likely that the secondary flows might become turbulent or nonsteady. Shown in Tables 1 and 2 are the dimensionless groups at the inlet and outlet, based on cup-average quantities, as well as the Reynolds numbers for the primary and secondary flows (Reynolds numbers defined in terms of the respective total mass flowrate, the viscosity and the ratio of tube perimeter to tube area). 

If it is not possible to include a particular element in the calibration solutions, it is possible to perform a semiquantitative analysis. This uses the response of those elements which are in the calibration solution, but predicts the sensitivity (defined as cps/concentration) for the missing element(s) by interpolating between the sensitivities of known elements. By plotting sensitivity against mass for all the elements present in the calibration solutions (Fig. 9.7) and fitting a curve through the points, it is possible to predict the sensitivity of the instrument for any particular mass number, and hence use this sensitivity to convert cps to concentration at that mass number. As can be seen from the figure, however, this is a very crude approximation, and any data produced in this way must be treated with some caution. 

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

Modifier D is used to show the mass number of the atom being considered, this being the total number of neutrons and protons considered to be present in the nucleus. The number of protons defines the element, but the number of neutrons in atoms of a given element may vary. Any atomic species defined by specific values of atomic number and mass number is termed a nuclide. Atoms of the same element but with difierent atomic masses are termed isotopes, and the mass number can be used to designate specific isotopes. 

A particular element is defined by its atomic number - the number of protons in the nucleus (which will equal the number of electrons surrounding the nucleus in a neutral atom). For example, iron is the element of atomic number 25, meaning that every iron atom will have 25 protons in its nucleus. Chemists use a one or two-letter symbol for each element to simplify communication iron is given the symbol Fe, from the old Latin word for iron, ferrum. The sum of the protons plus neutrons found in a nucleus is called the mass number. For some elements only one mass number is found in nature. Fluorine (atomic number 9, mass number 19) is an example of such an element. Other elements are found in nature in more than one mass number. Iron is found as mass number 55 (91.52%), 54 (5.90%), 57 (2.245%), and 58 (0.33%). These different mass numbers of the same element are called isotopes, and vary in the number of neutrons found in the nucleus. Atomic weight refers to the average mass found in nature of all the atoms of a particular element the atomic weight of iron is 55.847. For calculation purposes, these... 

One example would be ice melting or methane hydrate dissociation when rising in seawater. Convective melting rate may be obtained by analogy to convective dissolution rate. Heat diffusivity k would play the role of mass diffusivity. The thermal Peclet number (defined as Pet = 2aw/K) would play the role of the compositional Peclet number. The Nusselt number (defined as Nu = 2u/5t, where 8t is the thermal boundary layer thickness) would play the role of Sherwood number. The thermal boundary layer (thickness 8t) would play the role of compositional boundary layer. The melting equation may be written as... 

The mass number gives the total number of protons and neutrons in an atom of an element, but it does not convey the absolute mass of the atom. To work with the masses of elements, we use comparative masses. Initially, Dalton and the other pioneers of the atomic theory used the lightest element hydrogen and compared masses of other elements to hydrogen. The modern system uses C-12 as the standard and defines one atomic mass unit (amu) as 1/12 the mass of one C-12 atom. One amu is approximately 1.66 X 10 g. This standard means the masses of individual protons and neutrons are slightly more than 1 amu as shown in Table 4.6. 

The fact that both the mole and the mass of an element are based on carbon-12 enables us to relate mole and mass. A molar mass is defined as the mass in grams of one mole of a substance, and it can be obtained directly from an element s atomic mass. We can use the elements hydrogen and nitrogen to illustrate this concept. Periodic table entries for both elements are shown below. The whole number above the element is the atomic number and gives the number of protons in the nucleus. The number below the element s symbol is the molar mass (as well as the atomic mass) ... 

The number of nucleons is equal to the sum of the number of protons (Z = atomic number) and number of neutrons (N) in the nucleus and is defined as the mass number (A = nucleon number)... 

A nuclide may be defined as a species of atoms, wilh specified atomic number and mass number. The term nuclide should be used, not isotope. Different nuclides having the same atomic number wv isotopes. Different nuclides having the same mass number are isobars. 

The atomic number may be defined as the number of protons in an atomic nucleus, or the positive charge of the nucleus, expressed in terms of the electronic charge. Atomic number usually is denoted by the symbol Z. In the symbolic designation of individual nuclides, the atomic number sometimes is written as a subscript lo Ihe left of the chemical symbol of the atomic species, such as x160 for the oxygen isotope of mass number 16. This usage is redundant, in that the chemical symbol per se specifies the atomic number of the nuclide. 

Because the mass of an atom s electrons is negligible compared with the mass of its protons and neutrons, defining 1 amu as 1/12 the mass of a atom means that protons and neutrons each have a mass of almost exactly 1 amu (Table 2.1). Thus, the mass of an atom in atomic mass units—called the atom s isotopic mass—is numerically close to the atom s mass number. A jH atom, for instance, has a mass of 1.007 825 amu a 292U atom has a mass of 235.043 924 amu and so forth. 

Elements differ from one another according to how many protons their atoms contain, a value called the atomic number (Z) of the element. The sum of an atom s protons and neutrons is its mass number (A). Although all atoms of a specific element have the same atomic number, different atoms of an element can have different mass numbers, depending on how many neutrons they have. Atoms with identical atomic numbers but different mass numbers are called isotopes. Atomic masses are measured using the atomic mass unit (amu), defined as 1/12 the mass of a 12C atom. Because both protons and neutrons have a mass of approximately 1 amu, the mass of an atom in atomic mass units (the isotopic mass) is numerically close to the atom s mass number. The element s atomic mass is a weighted mass average for naturally occurring isotope mixtures. 

Owen Thomson (1963) analysed heat and mass transfer from rough surfaces in terms of a parameter B, termed the sublayer Stanton number, defined by the equation... 

The mole can be realized in a similar way but, of course, there are millions of different types of mole. It is more appropriate to speak of realizing a mole and this can be done by measuring a specified entity and making use of chemical stoichiometry and atomic mass values to relate the measured property to mass, as defined in the definition of the mole, i.e., Mx = Nx/An = m/ M, where Mx = number of moles of entity X Nx = number of entities of X An = Avogadro s number m = mass of X M = atomic mass of X. 

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