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Introduction
Published in Armen S. Casparian, Gergely Sirokman, Ann O. Omollo, Rapid Review of Chemistry for the Life Sciences and Engineering, 2021
Armen S. Casparian, Gergely Sirokman, Ann O. Omollo
The atomic masses are given for each element in the periodic table in amus or atomic mass units. The mass number of a given isotope of an element is the sum of the number of protons and the number of neutrons in its nucleus. Recall that in Example 1.1, the mass numbers of the three isotopes of uranium were given as U-234, U-235, and U-238. However, this is not equivalent to the atomic weight. Most elements have more than one isotope, so the natural distribution of the isotopes of an element must also be taken into consideration. Using the carbon-12 isotope as the standard for mass, atomic masses can then be assigned to all the elements. For each element, this number must be a number that is averaged over all of its isotopes according to their relative percent natural abundance. The atomic weight of an element, then, is the average atomic mass of all of the element’s naturally occurring isotopes. The molecular mass then becomes the sum of the atomic weights comprising the molecule, according to the number of each kind of atom occurring in the molecule. In other words, the molecular mass is the sum of the weights of the atoms represented in a molecular formula. Molecular masses (also called molecular weights) are the masses of molecules, which consist of essentially covalent compounds, while formula masses (also called formula weights) are the masses of formula units, which are essentially ionic compounds. The unit, in either case, is the amu, but often converted to the more useful grams/mole, which has the same numerical value.
Preformulation of New Biological Entities
Published in Sandeep Nema, John D. Ludwig, Parenteral Medications, 2019
Riccardo Torosantucci, Vasco Filipe, Jonathan Kingsbury, Atul Saluja, Yatin Gokarn
Small-molecule pharmaceuticals are characterized by a defined chemical formula and structure, in a molecular mass range of 100–900 Da. Unlike peptides and proteins, small molecules generally do not feature higher order structures essential for their action. Biologics, on the other hand, can be much larger and almost always are composed of heterogeneous mixtures of species resulting from post-translational modifications (PTMs), myriad chemical degradation pathways, as well as the physical degradation process of aggregation. A key differentiating feature between the two is that the quality of small molecules, being defined by a unique chemical formula, is generally independent of the manufacturing process. Different manufacturing routes/processes can yield a small-molecule product of the identical quality. In contrast, for biologics, even minor process modifications can cause substantial changes to the product quality [26].
Fossil Fuels
Published in Efstathios E. Michaelides, Energy, the Environment, and Sustainability, 2018
The chemicals that constitute the petroleum reserves are extracted as an almost homogeneous mixture of more than 500 chemical compounds, primarily hydrocarbons. In their pure form, these compounds span all three states of matter: under ambient temperature and atmospheric pressure, they would be gases, liquids, or solids. In general, the higher the molecular mass of a chemical compound, the higher its boiling point is, with the heavier compounds being solids. For example, the light methane and ethane in the crude oil mixture are gases; heptane and octane are liquids; and the much heavier waxes and tars are solids. When mined, the crude oil mixture has the properties of a very viscous (thick) liquid that may be transported in pipelines with high pressure drop per unit length.
The Adam–Gibbs relation and the TIP4P/2005 model of water
Published in Molecular Physics, 2018
Philip H. Handle, Francesco Sciortino
For completeness, we compare the TIP4P/2005 data also with the excess entropy scaling [31,41–47] proposition which relates D with the excess entropy . The rationale behind this hypothesis is that dynamics is controlled by the total number of accessible microstates in phase space. The validity of such a scaling can be tested if an adimensional diffusion constant is plotted against . is commonly defined as [42] to scale out the the trivial thermal velocity contribution . Here ρ is the molecule number density and M the molecular mass.
Investigation of ortho↔para hydrogen conversion by collisions with neutrons
Published in Journal of Nuclear Science and Technology, 2018
Longwei Mei, Cong Liu, Fei Shen, Songlin Wang, Zhiliang Hu, Bin Zhou, Tianjiao Liang
In the table, ac and ai, respectively, indicate the coherent and incoherent scattering lengths. The outer summation index J in Equations (4) and (5) represents the rotational quantum number of the initial state before scattering. For pH, i.e. when S = 0, the total angular momentum J can only take even values. For oH, i.e. when S = 1, only odd J numbers occur. The inner summation index J′ indicates the final rotational state after scattering, even and odd J′ values correspond to pH and oH [13], respectively. PJ is a statistical weight factor [14,15], is the energy transfer for the rotational transition, jl(y) is the spherical Bessel function of order l, and C(JJ′l; 00) is a Clebsch–Gordan coefficient. The parameter y is given by , where a is the interatomic distance in the molecule, and M is molecular mass. The translational weight w is 1/2 for H2 and 1/4 for D2. Based on the assumption by Young and Koppel, i.e. the molecular translations are free, Equations (4) and (5) contain a term Sf of the following form:
Biomedical applications of polyurethane materials and coatings
Published in Transactions of the IMF, 2018
J. Joseph, R. M. Patel, A. Wenham, J. R. Smith
PUs, unlike many other synthetic and natural biodegradable polymers, have mechanical and physical properties comparable to natural tissue.36 This, matched with low platelet adhesion and in vitro protein adsorption allows many uses of PU in the biomedical industry.34 Critically, consideration of the biodegradation of PUs, as with other biomaterials, needs to be considered. Tailoring these properties, in the case of PUs, can be easily achieved through variation of the chemical composition, ratio of hard-to-soft segments and the molecular mass. Generally, the rate of PU biodegradation is mainly dependent on the soft segment structure, which is controlled by the polyol chemistry.36 This is in keeping with the observation that PUs with amorphous structures degrade more rapidly than those with semi-crystalline segments, since they allow permeation of water through the amorphous regions. PPGs, PEGs, PCLs and glycolic acid are common polyols used in biodegradable PUs. Non-toxic degradation products are one of the main attractions in PEGs as well as hydrophilicity, solubility in water and organic solvents and absence of antigenicity and immunogenicity. PCLs also produce nontoxic degradation products, although are often more hydrophobic and so reduce degradation rates. Increasing this property can be affected through the introduction of hydrolysable chain extenders into the hard segments. BD, 1,2-ethanediamine and 1,2-ethanediol are often used as chain extenders. Both aromatic and aliphatic diisocyanates are used in making biodegradable PU systems, although the former (e.g. TDI and MDI) have been found to degrade into toxic byproducts and are being replaced with aliphatic diisocyanates (e.g. IPDI and HDI).