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The atomic composition of a molecule is given by its formula. However, the fact that we can write a formula for a compound does not imply the existence of molecules having that composition. Gases and most liquids consist of molecules, but many solids exist as extended lattices of atoms or ions electrically charged atoms or molecules.
For example, there is no such thing as a "molecule" of ordinary salt, NaCl see below. Confused about the distinction between molecules and compounds? Maybe the following will help: A molecule but not a compound - Ozone, O3, is not a compound because it contains only a single element.
This well-known molecule is a compound because it contains more than one element. It is built from interpenetrating lattices of sodium and chloride ions that extend indefinitely. Structure and properties Composition and structure lie at the core of Chemistry, but they encompass only a very small part of it.
It is largely the properties of chemical substances that interest us; it is through these that we experience and find uses for substances, and much of chemistry-as-a-science is devoted to understanding the relation between structure and properties. For some purposes it is convenient to distinguish between chemical properties and physical properties, but as with most human-constructed dichotomies, the distinction becomes more fuzzy as one looks more closely.
Take some time to look it over and make sure you understand all the terms and the relations between them. For a more in-depth treatment of much of the material covered here, please see The basics of atoms, moles, formulas equations, and nomenclature.
On a microscopic basis it can be thought of as a re-arrangement of atoms. As with most human-created dichotomies, this begins to break down when examined closely. This is largely because of some ambiguity in what we regard as a distinct "substance".
Elemental chlorine exists as the diatomic molecule Cl2 in the gas, liquid, and solid states; the major difference between them lies in the degree of organization. In the gas the molecules move about randomly, whereas in the solid they are constrained to locations in a 3-dimensional lattice.
In the liquid, this tight organization is relaxed, allowing the molecules to slip and slide around each other. Since the basic molecular units remain the same in all three states, the processes of melting, freezing, condensation and vaporization are usually regarded as physical rather than chemical changes.
Because the ions in the solid, the hydrated ions in the solution, and the molecule Na2Cl2 are really different chemical species, the distinction between physical and chemical change becomes a bit fuzzy. Most chemical reactions proceed to some inermediate point that yields a mixture of reactants and products.
For example, if the two gases phosphorus trichloride and chlorine are mixed together at room temprature, they will combine until about half of them have changed into phosphorus pentachloride: The result, in any case, will be an equilibrium mixture of reactants and products.
The most important question we can ask about any reaction is "what is the equilibrium composition"?
If the answer is "all products and negligible quantities of reactants", then we say the reaction can takes place and that it "goes to completion ".
If the answer is "negligible quantities of products", then we say the reaction cannot take place in the forward direction, but that the reverse reaction can occur. If the answer is "significant quantities of all components" both reactans and products are present in the equilibrium mixture, then we say the reaction is "reversible" or "incomplete".
The aspect of "change" we are looking at here is a property of a chemical reaction, rather than of any one substance. But if you stop to think of the huge number of possible reactions between the more than 15 million known substances, you can see that it would be an impossible task to measure and record the equilibrium compositions of every possible combination.
One or two directly measurable properties of the individual reactants and products can be combined to give a number from which the equilibrium composition at any temperature can be easily calculated.Find freelance Scientific Writing work on Upwork.
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Don’t chemists spend their time turning knobs, mixing reagents, and collecting data?
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Guide for Writing in Chemistry About Writing in Chemistry this guideScientific discovery depends as much on the development of innovative ideas as it does on a scientist’s ability to communicate those ideas within the scientific community and to the public as a whole.