Elements fixed ratio

These isotope masses and their ratio of abundances are characteristic of carbon. Similarly, the isotopes of other elements that occur naturally have fixed ratios of isotopes, as given in Tables 47.1 and 47.2 at the end of the accompanying full text. 

Observations that compounds have fixed compositions and that therefore their atoms are combined in fixed ratios led to the determination of atomic masses and later to the concept that the atoms of a given element have a characteristic combining power that is, each atom can form a certain number of bonds called its valence. Because a hydrogen atom does not... 

In a solution, the ratio of solvent to solute is not fixed, and it can vary over a wide range, unlike compounds that are composed of definite, fixed ratios of the elements that make them up. 

Q Compounds are formed when atoms of different elements combine. Atoms will combine in certain fixed ratios with other atoms. 

Components of a mixture have their respective chemical properties. Mixtures may be composed of different elements and compounds. There is no fixed ratio among the components. 

The importance (or not) of the distinction between N and P as a model currency is closely related to the concept of the Redfield ratio (Redfield et al. 1963). Because many models impHcidy or expHcitly enforce fixed elemental ratios, the distinction between N and P hmitation is often of fittle importance in models. In most cases a fixed ratio (e.g., Redfield ) model will behave almost identically in terms of primary production, plankton abundance, and carbon fluxes, whether N or P is specified to be the primary fimiting nutrient (the exception is when initial or boundary conditions are drawn from observations in which deviations from the Redfield N P ratio are present). 

So far we have examined single units without a reaction occurring in the unit. How is the count for Nd affected by the presence of a reaction in the unit The way Nv is calculated does not change. As to Nr, all restrictions and constraints are deducted from N that represent independent restrictions on the unit. Thus the number of material balances is not necessarily equal to the number of species (H2O, O2, CO2, etc.) but instead is the number of independent material balances that exist determined in the same way as we did in Secs. 2.2 to 2.4, usually (but not always) equal to the number of elemental balances (H, O, C, etc.). Fixed ratios of materials such as the O2/N2 ratio in air or the CO/CO2 ratio in a product gas would be a restriction, as would be a specified conversion fraction or a known molar flow rate of a material. If some degrees of freedom exist still to be specified, improper specification of a variable may disrupt the independence of equations and/or specifications previously enumerated in the unit of Nr, so be carefiil. 

Figure 1-9 Diagram of the decomposition of calcium carbonate to give a white solid A (56.0% by mass) and a gas B (44.0% by mass). This decomposition into simpler substances at a fixed ratio proves that calcium carbonate is a compound. The white solid A further decomposes to give the elements calcium (71.5% by mass) and oxygen (28.5% by mass). This proves that the white solid A is a compound it is known as calcium oxide. The gas B also can be broken down to give the elements carbon (27.3% by mass) and oxygen (72.7% by mass). This establishes that gas B is a compound it is known as carbon dioxide.

Furthermore, we may say that a compound is a pure substance consisting of two or more different elements in a fixed ratio. Water is 11.1% hydrogen and 88.9% oxygen by mass. Similarly, carbon dioxide is 27.3% carbon and 72.7% oxygen by mass, and calcium oxide (the white solid A in the previous discussion) is 71.5% calcium and 28.5% oxygen by mass. We could also combine the numbers in the previous paragraph to show that calcium carbonate is 40.1% calcium, 12.0% carbon, and 47.9% oxygen by mass. Observations such as these on innumerable pure compounds led to the statement of the Law of Definite Proportions (also known as the Law of Constant Composition) ... 

Molecule versus Mixture Molecules are compounds with elements in definite, fixed ratios. Those atoms are held together usually by one of the three bonds discussed above. For example water, glucose, ATP. Mixtures are compounds... 

The crucial parameters are the concentration ks at which uptake is half of the maximum rate and the fixed ratio (or yield) q l at which nutrient is converted to biomass. The yield may be the Redfield ratio or some other optimum composition. Tilman etal. (1982) used the model to show how freshwater phytoplankters of different optimum composition or different half-saturation concentrations, might succeed to different extents depending on the ambient ratios of nutrient elements. Although the assumption of constant yield may be appropriate for pelagic heterotrophs, it is now seen to be too simple for accurate prediction of the growth of phytoplankters (Droop, 1983 Sommer, 1991 Ducobu etal., 1998). 

A molecule is an aggregate of at least two atoms in a definite arrangement held together by chemical forces (also called chemical bonds). hmo c x may contain atoms of the same element or atoms of two or more elements joined in a fixed ratio, in accordance with the law of definite proportions stated in Section 2.1. Thns, a molecule is not necessarily a compound, which, by definition, is made up of two or more elements (see Section 1.2). Hydrogen gas, for example, is a pure element, bnt it consists of molecnles made np of two H atoms each. Water, on the other hand, is a molecular compound that contains hydrogen and oxygen in a ratio of two H atoms and one O atom. Like atoms, molecules are electrically neutral. 

A formula represents a fixed ratio of the elements in a compound, so a different ratio represents a different compound. 

Redfield stoichiometry Redfield and colleagues noted that organisms in the sea consistently removed nutrient elements from the water in a fixed ratio (C N P=106 16 1). Subsequent workers have found that nutrient concentrations in the sea typically are present in those same ratios. 

A compound is matter formed by the combination of two or more elements in fixed ratios. Let s consider an example. Hydrogen peroxide (H Oj) is a compound composed of two elements, hydrogen and oxygen, in a fixed ratio. 

A compound is composed of two or more elements combined in fixed ratios or proportions. 

The postulates also account for the law of definite proportions. Com-poimds are made of elements in fixed or definite proportions. Since the atoms have fixed mass, compounds should have elements in a fixed ratio with respect to mass. Finally, these postulates predict what is known as the law of multiple proportions. According to this law, if two elements form two or more different compounds, the ratio of the mass of one element of these compounds to a fixed mass of the other element is a simple whole number. 

The various isotopes of an element are usually present in relatively fixed ratios throughout nature, but in some cases the ratio can depend on the environment or molecule in which they are found. For example, the element carbon most commonly exists with 6 protons, 6 neutrons, and 6 electrons (referred to as Carbon-12, the total number of particles in the nucleus). A small fraction, however, have 6 protons, 7 neutrons, and 6 electrons (Carbon-13). Roughly 99% of carbon atoms have 6 neutrons, but most of the remaining 1% have 7 neutrons. 

Chemical compounds Pure substances composed of atoms of different elements combined in definite, fixed ratios... 

Stoichiometry is largely explained in terms of the electronic configurations of the atoms of the elements, as stable species are those where the atomic cores are well shielded from other species by the outer layer of electrons. This tends to happen in ions and molecules, or sometimes atoms, with particular patterns of valence electrons (Chapter 3). This leads to atomic cores combining in fixed ratios. The evenness (symmetry) of the pattern of electron density is more important than the neutrality of the species, so ions such as Na and are found in common materials, whereas the atoms Na and O are not. However, it is important to realise that the stability of ions is only possible because they are usually fotmd in neutral lattices or surrounded by solvent molecules isolated ions are usually less stable than the corresponding atom. 

In the next 2,000 years, the alchemists discovered more and more elements. Till to eighteenth century, Lavoisier named the elements of oxygen and hydrogen, and proved the mass conservation in chemical reactions (Lavoisier et al. 1783). This milestone delivered the birth of chemistry. At the begiiming of nineteenth cenmry, Dalton proposed that each molecule contains a fixed ratio of atoms among several elements (Dalton 1808). This theory was another milestone that opened the gate to modem chemistry. Since then, the atomic and molecular theory became the main stream of chemistry. 

The second son of a modest Quaker weaver in England s Lake District, John Dalton s contribution to chemistry was to reintroduce a systematic atomic theory based on the elements of Lavoisier. We say reintroduce because the concept of atoms was certainly nothing new Democritus postulated atoms in pre-Aristotelian Greek philosophy, and atoms were proposed by Descartes and Hooke. In 1738 Daniel Bernoulli correctly derived Boyle s law by assuming gases consisted of collections of particles that continuously collided with the container walls. But Dalton did not propose atoms as an abstraction or mathematical device Dalton s atoms were physical. They had characteristic masses (atomic weights) and combinations of these atoms in fixed ratios made up the range of chemical compounds. 

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