Stoichiometry: 3.11 - The nature of matter (2023)

pure substances and mixtures

Chemistry is the study of all matter, its physical and chemical properties. However, most of the matter we experience in everyday life is not pure, but a mixture of pure substances that are not chemically bound together.

Mixtures are impractical to study because they usually show the properties of their individual components. It is difficult to attribute a specific property to a particular component without knowing how pure substances behave.

This is leading chemists to develop separation techniques designed to provide substances in their pure form for study. Mixtures can be heterogeneous or homogeneous, and the nature of the mixture determines the method used for separation.

Chemistry is the study of all matter

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Simple molecular structures

Simple molecular structures consist of molecules that are not held together by formal bonds. The molecules themselves are constructed by covalent (shared pair of electrons) bonds between nonmetal atoms. Molecular structures can usually be identified by the absence of metallic elements in the formula, however, they are some metal compounds that are covalently bonded and consequently form simple molecular structures.

Examples are aluminum chloride, anhydrous ferric chloride and beryllium chloride. These anomalies are mostly metal ions with a high charge density bound to chlorine.

The smallest particles in simple molecular structures can consist of individual atoms in the simplest case, as in the noble gases argon and neon etc. As the name suggests, the noble gases are always gases at room temperature.

Simple molecular structure

As the molecules get larger and more complex, the forces between them increase the boiling and melting points. The larger molecular substances are solids at room temperature.

Note: There is a correlation between molecular weight and melting point, although this is not necessarily true for boiling point.

We can see that sulfur and iodine with similar Mr values ​​have similar melting points although the boiling points are different. This is accomplished by altering the sulfur molecules of S₈Crowns to S₈Chains after the melting point, which have a larger surface area and hence larger van der Waal forces than the simple I₂molecules.

Consequently, sulfur requires more energy to vaporize than iodine.

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Huge molecular structures

The entire network (lattice) is a construction of atoms, all held together by formally covalent bonds (shared pair of electrons). This effectively means the structure is one giant molecule.

Huge molecular structure

examples

The relative masses in these cases are only the masses of the simplest formula units. The table shows that no predictions can be made regarding the melting and boiling points. When these substances melt, it either occurs with accompanying decomposition (chemical bonds are broken) or with conversion into a simpler molecular substance.

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ionic structures

Ionic compounds are formed between metallic and non-metallic elements. The metal atoms lose electrons and form positively charged ions while the nonmetal atoms gain electrons that form negatively charged ions. These oppositely charged ions are attracted by strong omnidirectional electrostatic forces, and the structure is a giant lattice made up of repeating units of oppositely charged ions. Water molecules can also be found within the structure to help build the lattice (crystal water).

examples

The melting and boiling points are determined by the electrostatic forces within the crystal lattice. The electrostatic force in turn depends on the charge density of the ions; small ions with large charges create stronger electrostatic forces. Again, formula relative mass is really just the mass of ions in the simplest formula unit (the simplest ratio of ions in the compound)

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metallic structures

Metallic structures are formed by atoms of metallic elements either alone or mixed with other metallic atoms (alloys). These are giant structures of atoms in which the outer electrons are delocalized throughout the structure, effectively leaving the atoms as positively charged ions.

examples

The melting and boiling points are a function of the attractive forces between the delocalized electrons and the ions within the metal lattice. Again, the electrostatic forces are greater when the charge density of the ion is greater. Aluminum, with a three-plus charge Al³⁺has a high charge density and a much higher melting point than sodium, which consists of singly charged and larger ions.

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Simplest formula units

For the sake of simplicity, it is easier to deal with huge structures than if the atoms were not connected. Metals and giant molecular structures can be viewed as a mass of individual atoms, where we assume that the simplest relationship between the different types of atoms is the formula of the substance in question.

That makesNotmeans that there are simple molecules containing only one silicon and two oxygen atoms, this merely gives us a logical way to represent the compound for convenience and calculations.

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summary

There are four basic types of structures. These are shown below.

simple molecular structure	huge molecular structure	ionic structure	huge metallic structure
e.g. water	e.g. silica	z.b. Natriumchloride	z.b. Natrium

The smallest particle of any type of structure is either an atom, a molecule, or an ion. Huge structures require a bit more flexible thinking from us since they are effectively one giant molecule.

We define the simplest formula unit as the simplest ratio of atoms within the giant structure. Likewise, ionic structures have to be described according to a simplest formula unit, which consists of the simplest ion ratio.

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