Chemical compound

Water, the most common chemical compound in nature. The model of rods and spheres of the molecule shows the spatial association of two parts of hydrogen (white) and one of oxygen (red)

A chemical compound is a substance formed by the chemical combination of two or more elements from the periodic table. Compounds are represented by a chemical formula. For example, water (H₂O) is made up of two hydrogen atoms and one oxygen. The elements of a compound cannot be divided or separated by physical processes (decantation, filtration, distillation), but only by chemical processes.

Compounds are made up of molecules or ions with stable bonds that do not obey arbitrary human selection. Therefore, they are not mixtures or alloys like bronze or chocolate. A chemical element bonded to an identical chemical element is not a chemical compound, since only one element is involved, not two different elements.

There are four types of compounds, depending on how the constituent atoms are held together:

• molecules linked by covalents
• Ionic compounds linked by ionic links
• Intermetallic compounds joined by metallic links
• Certain complexes that remain united by covalents coordinated

Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): their CAS number.

Formula

In inorganic chemistry, compounds are represented by chemical formulas. A chemical formula is a way of expressing information about the proportions of atoms that make up a particular chemical compound, using standard abbreviations for chemical elements and subscripts to indicate the number of atoms involved. For example, water is made up of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H₂O. In the case of non-stoichiometric compounds, the ratios may be reproducible with respect to their preparation and give fixed ratios of their component elements, but ratios that are not integral [for example, for palladium hydride, PdH _x (0.02 <x <0.58)].

The order of elements in the formula of inorganic compounds starts from the left with the least electronegative element, to the right with the most electronegative. For example, in NaCl, chlorine, which is more electronegative than sodium, goes on the right. For organic compounds, there are several other rules and skeletal or semi-developed formulas are used to represent them.

Definitions

Any substance consisting of two or more different types of atoms (chemical elements) in a fixed stoichiometric ratio can be called a chemical compound. The concept is best understood when considering pure chemical substances. From the composition of fixed proportions of two or more types of atoms, it follows that chemical compounds can be converted, through a chemical reaction, into compounds or substances, each with fewer atoms.

Chemical compounds have a unique, defined chemical structure that is held together in a spatial arrangement defined by chemical bonds. Chemical compounds can be molecular compounds, held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or the subset of chemical complexes held together by coordinate covalent bonds. Pure chemical elements are generally they are not considered chemical compounds, since they do not meet the requirement of two or more atoms, although they often consist of molecules composed of multiple atoms (as in the diatomic molecule H₂, or the polyatomic molecule S ₈, etc.) Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): their CAS number.

There is variable and sometimes inconsistent nomenclature for differentiating substances, including truly non-stoichiometric examples of chemical compounds, which require that the proportions be fixed. Many solid chemicals, for example many silicate minerals, do not have simple formulas that reflect the chemical bonding of elements with each other in fixed ratios; even so, these crystalline substances are often called "non-stoichiometric compounds". It can be argued that they are related to such products, rather than being proper chemical compounds, insofar as the variability in their compositions is often due to the presence of foreign elements trapped within the crystalline structure of a chemical compound true, or due to disturbances in its structure relative to the known compound that arises due to an excess or deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of the Earth's crust and mantle. Other compounds considered chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, slightly changing the mass ratio of the elements.

Classification

They can be classified according to the type of chemical bond or their composition. Based on the type of chemical bond, they can be divided into:

• molecules
• Ionic compounds
• Intermetallic compounds
• Complex

Due to their composition, they can be divided into two large groups:

1. Inorganic compounds:
   • Basic oxides. Also called metal oxides, which are formed by a metal and oxygen. Examples: Plumbic oxide, lithium oxide.
   • Acid oxides. Also called non-metallic oxides, formed by a non-metal and oxygen. Examples: hypochlorous oxide, selenious oxide.
   • Drubs, which can be both metallic and non-metallic. They are composed of an element and hydrogen. Examples: aluminum hydro, sodium hydrour.
   • Hydracids, they are non-metallic hydra that, when dissolved in water, acquire acid character. For example, iodhydric acid.
   • Hydroxides, composed of the reaction between a basic oxide and water, which are characterized by the oxydrile group (OH). For example, sodium hydroxide, or caustic soda.
   • Oxy acids, compounds obtained by the reaction of acid oxide and water. Their molecules are formed by hydrogen, non-metal and oxygen. For example, chloric acid.
   • binary salts, composed of a hydrogen plus a hydroxide. For example, sodium chloride.
   • Oxisals, formed by the reaction of an oxacyte and a hydroxide, such as sodium hypochlorite.
2. Organic compounds:
   • Alphatic compounds are organic compounds made up of carbon and hydrogen whose character is not aromatic.
   • Aromatic compounds, is a conjugated cyclic organic compound that possesses greater stability due to electronic delocalization in π links.
   • Heterocyclic compounds are cyclic organic compounds in which at least one of the components of the cycle is of a different carbon element.
   • Organometallic compounds, it is a compound in which carbon atoms form covalent bonds, that is, they share electrons, with a metallic atom.
   • Polymers are macromolecules formed by the union of smaller molecules called monomers.

Molecules

A molecule is an electrically neutral group of two or more atoms joined by chemical bonds. A molecule can be homonuclear, that is, be made up of atoms of the same chemical element, as is the case with two atoms in the oxygen molecule (O₂); or it can be heteronuclear, that is, a chemical compound composed of more than one element, such as water (two hydrogen atoms and one oxygen atom; H₂O). The bonded atoms and complexes by non-covalent bonds such as hydrogen bonds or ionic bonds are not usually thought of as individual molecules.

Ionic Compounds

An ionic compound is a chemical compound made up of anions held together by electrostatic forces called ionic bonding. The compound is generally neutral, but it consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions like sodium(Na⁺) and chloride (Cl⁻) in sodium chloride, or polyatomic species like ammonium (NH+
4) and carbonate ( CO 2−
3) in ammonium carbonate. Individual ions within an ionic compound generally have multiple nearest neighbors, so they are not considered part of molecules, but part of a continuous three-dimensional lattice, usually in a crystalline structure.

Ionic compounds that contain basic hydroxide (OH⁻) or oxide (O²⁻) ions are classified as bases. Ionic compounds without these ions are also known as salts and can be formed by acid-base reactions. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation, freezing, a solid-state reaction, or the electron transfer reaction of reactive metals with reactive nonmetals, such as halogen gases.

Ionic compounds often have high melting and boiling points, and are hard and brittle. As solids, they are almost always electrically insulating, but when they melt or dissolve they become highly conductive, because ions are mobilized.

Intermetallic compounds

An intermetallic compound is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good mechanical properties at high temperatures. They can be classified as either stoichiometric or non-stoichiometric intermetallics.

Chemical complexes

A coordination complex consists of a central atom or ion, which is usually metallic and is called the coordination center, and a surrounding matrix of bound molecules or ions, which in turn are known as ligands or complexing agents. Many metal-containing compounds, especially those of transition metals, are coordination complexes. A coordination complex whose center is a metal atom is called a metal complex or d-block element.

Links and forces

Compounds are held together by different types of bonds and forces. The differences between the types of bonds in compounds depend on the type of element present in the compound.

London dispersion forces are the weakest of the intermolecular forces. They are temporary attractive forces that form when the electrons in two adjacent atoms are positioned in a way that creates a temporary dipole. Furthermore, these forces are responsible for the condensation of nonpolar substances in liquids and subsequent freezing to a solid state depending on the ambient temperature.

A covalent bond, also known as a molecular bond, involves the exchange of electrons between two atoms. Primarily, this type of bond occurs between elements that appear near each other on the periodic table of elements, although it is seen between some metals and nonmetals. This is due to the mechanism of this type of link. Nearby elements on the periodic table tend to have similar electronegativities, meaning they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons such that both elements have a more stable octet.

Ionic bonding occurs when valence electrons are fully transferred between elements. Unlike covalent, this chemical bond creates two oppositely charged ions. Metals in ionic bonds generally lose their valence electrons, becoming cations, positively charged. The nonmetal will gain the electrons from the metal, making the nonmetal an anion, that is, negatively charged. That is, ionic bonds occur between an electron donor, usually a metal, and an electron acceptor, which tends to be a nonmetal.

Hydrogen bonding occurs when a hydrogen atom attached to an electronegative atom forms an electrostatic connection with another electronegative atom through dipoles or interacting charges.

Reactions

A compound can be converted into a different chemical composition (products) by interacting with a second chemical compound (reactants) through a chemical reaction. In this process, the bonds between the atoms are broken in both interacting compounds, and then the bonds are reformed so that they obtain new associations between the same atoms. Schematically, this reaction could be described as AB + CD → AD + CB, where A, B, C, and D are each unique atoms; and AB, AD, CD, and CB are each unique compounds.

Further reading

• Robert Siegfried (2002), From elements to atoms: a history of chemical composition, American Philosophical Society, ISBN 978-0-87169-924-4