A sample of any given pure element is composed only of the atoms characteristic of that element, and the atoms of each element are unique.
When two distinct elements are chemically combined—i.e., chemical bonds form between their atoms—the result is called a chemical compound.
Most elements on Earth bond with other elements to form chemical compounds, such as sodium (Na) and Chloride (Cl), which combine to form table salt (NaCl). Water is another example of a chemical compound. The two or more component elements of a compound can be separated through chemical reactions.
Definitions of Chemical Compounds
There are multiple definitions of chemical compounds
- Any substance consisting of two or more different atoms (chemical elements) in a fixed stoichiometric proportion can be termed a chemical compound; the concept is most readily understood when considering pure chemical substances. It follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, via chemical reaction, into compounds or substances, each having fewer atoms. The ratio of each element in the compound is expressed in a ratio in its chemical formula. A chemical formula expresses information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements and subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O. In the case of non-stoichiometric compounds, the proportions may be reproducible with regard to their preparation and give fixed proportions of their component elements, but proportions that are not integral [e.g., for palladium hydride, PdHx (0.02<x<0.58)].
- Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement 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 not considered chemical compounds, failing the two or more atom requirement. However, they often consist of molecules composed of multiple atoms (such as in the diatomic molecule H2 or the polyatomic molecule S8, etc.). Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number.
- There is varying and sometimes inconsistent nomenclature differentiating substances, including truly non-stoichiometric examples, from chemical compounds requiring fixed ratios. Many solid chemical substances—many silicate minerals—are chemical substances but do not have simple formulae reflecting chemical bonding of elements to one another in fixed ratios. Even so, these crystalline substances are often called “non-stoichiometric compounds”. It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is often due to either the presence of foreign elements trapped within the crystal structure of an otherwise known true chemical compound or due to perturbations in structure relative to the known compound that arise because of an excess of deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of the crust and mantle of the Earth. Other compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which changes the ratio of elements by mass slightly.
Chemical Formula of Compounds
Compounds are represented by their chemical formula. A chemical formula is a symbolic representation of theproportions of atoms that constitute a particular chemical compound.
- Chemical compounds have a unique and defined structure, which consists of a fixed ratio of atoms held together in a defined spatial arrangement 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.
- Complexes held together by coordinate covalent bonds.
Pure chemical elements are not considered chemical compounds, even if they consist of diatomic or polyatomic molecules (molecules that contain only multiple atoms of a single element, such as H2 or S8).
History of Chemical Compounds
Prior to the 1800s, the term compound had relatively little precise meaning. When used, it was often unclear as to whether one was referring to what scientists now call a mixture or to what they now know as a compound.
During the nineteenth century, the debate as to the meaning of the word intensified. And it became one of the key questions in the young science of chemistry. A critical aspect of this debate focused on the issue of constant composition. The issue was whether all compounds always had the same composition, or whether their composition could vary.
The primary spokesman A mixture versus a compound. Illustration by Argosy. The Gale Group. for the latter position was the French chemist Claude Louis Bartholet.
Bartholet pointed to a considerable body of evidence that suggested a variable composition for compounds.
For example, when some metals are heated, they form oxides that appear to have a regularly changing percentage composition. The longer they are heated, the higher the percentage of oxygen found
in the oxide.
Bartholet also mentioned alloys and amalgams as examples of substances with varying composition.
Bartholet’s principal antagonist in this debate was his countryman Joseph Louis Proust. Proust argued that Dalton’s atomic theory required that compounds have a constant composition, a position put forward by Dalton himself. Proust set out to counter each of the arguments set forth by Bartholet.
In the case of metal oxides, for example, Proust was able to show that metals often form more than one oxide.
As copper metal is heated, for example, it first forms copper(I) or cuprous oxide and then, copper(II) or cupric oxide. At any one time, then, an experimenter would be able to detect some mixture of the two oxides varying from pure copper(I) oxide to pure copper(II) oxide. However, each of the two oxides itself, Proust argued, has a set and constant composition.
Working in Proust’s favor was an argument that nearly everyone was willing to acknowledge, namely that quantitative techniques had not yet been developed very highly in chemistry.
Thus, it could be argued that what appeared to be variations in chemical composition were really nothing other than natural variability in results coming about as a result of imprecise techniques.
Proust remained puzzled by some of Bartholet’s evidence, the problem of alloys and amalgams as an example.
At the time, he had no way of knowing that such materials are not compounds but are in fact mixtures. These remaining problems notwithstanding, Proust’s arguments eventually won the day and by the end of the
century, the constant composition of compounds was universally accepted in chemistry.
Types of Chemical Compounds
- Chemical compounds may be classified according to several different criteria. One common method is based on the specific elements present. For example, oxides contain one or more oxygen atoms, hydrides contain one or more hydrogen atoms, and halides contain one or more halogen (Group17) atoms. Organic compounds are characterized as those compounds with a backbone of carbon atoms. And all the remaining compounds are classified as inorganic. As the name suggests, organometallic compounds are organic compounds bonded to metal atoms.
- Another classification scheme for chemical compounds is based on the types of bonds that the compound contains. Ionic compounds contain ions and are held together by the attractive forces among the oppositely charged ions. Common salt (sodium chloride) is one of the best-known ionic compounds. Molecular compounds contain discrete molecules, which are held together by sharing electrons (covalent bonding). Examples are water, which contains H2O molecules; methane, which contains CH4 molecules; and hydrogen fluoride, which contains HF molecules.
Based on these classifications there are four types of chemical compounds
1. Molecular Compound
First of all, a molecule refers to a neutral group of two or more atoms that are seized together by chemical bonds. Moreover, molecules certainly bind together by covalent bonds. Furthermore, the separation of the molecules from the ions takes place due to their shortage of electrical charge.
When it comes to biochemistry, quantum organic chemistry, and physics, experts use the word molecule in most cases less severely.
When we talk of the kinetic theory of gases, many use the word molecule for some gaseous particle regardless of the constituent it involves. Under this definition, many consider noble gas molecules as monatomic molecules.
A molecule can certainly be homonuclear in nature. Furthermore, homonuclear means containing atoms of one chemical element only. A good example of a homonuclear molecule can be oxygen (O2). A molecule can also be heteronuclear which means a compound comprising of more than one element.
2. Ionic Compound
Ionic compound refers to a chemical compound that consists of ions held together due to ionic bonding. Moreover, ionic bonding refers to certain electrostatic forces. Furthermore, the ionic compound is completely neutral but consists of positively charged ions called cations and negatively charged ions known as anions.
The ionic compound can be merely ions like in sodium chloride or the polyatomic class like in ammonium carbonate. Experts refer to ionic compounds which comprise of hydrogen ions as acids and those with basic ions hydroxide or oxide as bases.
3. Intermetallic Compounds
Intermetallic compounds are those compounds which are together by metallic bonds. An intermetallic compound refers to a particular type of metallic alloy which forms a solid-state compound and displays distinct stoichiometry and crystal structure.
4. Coordinate Compounds
Coordinate compounds refer to certain complexes that are held together by the coordinate covalent bonds. Furthermore, a coordination complex happens to be commonly metallic.
Moreover, a coordination complex involves a near array of bound molecules or ions and these molecules or ions are the complexing agents or ligands. Also, many compounds that contain metal are coordination complexes.
- A third classification scheme is based on reactivity—specifically, the types of chemical reactions that the compounds are likely to undergo.
For example, acids are compounds that produce H+ ions (protons) when dissolved in water to produce aqueous solutions. Thus, acids are defined as proton donors.
The most common acids are aqueous solutions of HCl (hydrochloric acid), H2SO4 (sulfuric acid), HNO3 (nitric acid), and H3PO4 (phosphoric acid).
Bases, on the other hand, are proton acceptors. The most common base is the hydroxide ion (OH–), which reacts with an H+ ion to form a water molecule.
H+ + OH– → HOH (usually written H2O)
Oxidation-reduction reactions constitute another important class of chemical reactions. Oxidation involves a loss of electrons, whereas reduction involves a gain of electrons. For example, in the reaction between sodium metal and chlorine gas to form sodium chloride
2Na + Cl2 → 2NaCl
In this process, each sodium atom loses an electron and is thus oxidized, and each chlorine atom gains an electron and is reduced.
In this reaction, sodium is the reducing agent (it furnishes electrons), and chlorine is the oxidizing agent (it consumes electrons).
The most common reducing agents are metals, for they tend to lose electrons in their reactions with nonmetals.
The most common oxidizing agents are halogens—such as fluorine (F2), chlorine (Cl2), and bromine (Br2)—and certain oxyanions, such as the permanganate ion (MnO4–) and the dichromate ion (Cr2O72-).
Based on this classification there are two types of chemical compounds
1. Inorganic Compounds
Inorganic compounds include compounds that are made up of two or more elements other than carbon. As well as certain carbon-containing compounds that lack carbon-carbon bonds, such as cyanides and carbonates.
The inorganic compounds are most often classified in terms of the elements or groups of elements that they contain.
Oxides, for example, can be either ionic or molecular. Ionic oxides contain O2- (oxide) ions and metal cations, whereas molecular oxides contain molecules in which oxygen (O) is covalently bonded to other nonmetals such as sulfur (S) or nitrogen (N).
When ionic oxides are dissolved in water, the O2- ions react with water molecules to form hydroxide ions (OH–), and a basic solution results.
Molecular oxides react with water to produce oxyacid, such as sulfuric acid (H2SO4) and nitric acid (HNO3).
In addition, inorganic compounds include hydrides (containing hydrogen atoms or H– ions), nitrides (containing N3− ions), phosphides (containing P3- ions), and sulfides (containing S2- ions).
Transition metals form a great variety of inorganic compounds. The most important of these are coordination compounds in which the metal atom or ion is surrounded by two to six ligands.
Ligands are ions or neutral molecules with electron pairs that they can donate to the metal atom to form a coordinate-covalent bond.
The resulting covalent bond is given a special name because one entity (the ligand) furnishes both of the electrons that are subsequently shared in the bond.
An example of a coordination compound is [Co(NH3)6]Cl3 , which contains the Co(NH3)63+ ion, a cobalt ion (Co3+) with six ammonia molecules (NH3) attached to it, acting as ligands.
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2. Organic Compounds
In general, organic compounds are substances that contain carbon (C), and carbon atoms provide the key structural framework that generates the vast diversity of organic compounds.
All things on the Earth (and most likely elsewhere in the universe) that can be described as living have a crucial dependence on organic compounds.
Foodstuffs—namely, fats, proteins, and carbohydrates—are organic compounds, as are such vital substances as hemoglobin, chlorophyll, enzymes, hormones, and vitamins.
Other materials that add to the comfort, health, or convenience of humans are composed of organic compounds, including clothing made of cotton, wool, silk, and synthetic fibres; common fuels, such as wood, coal, petroleum, and natural gas; components of protective coatings, such as varnishes, paints, lacquers, and
enamels; antibiotics and synthetic drugs; natural and synthetic rubber; dyes; plastics; and pesticides.
First Organic Compound
The first significant synthesis of an organic compound from inorganic materials was an accidental discovery of Friedrich Wöhler, a German chemist.
Working in Berlin in 1828, Wohler mixed two salts (silver cyanate and ammonium chloride) in an attempt to make the inorganic substance ammonium cyanate.
To his complete surprise, he obtained a product that had the same molecular formula as ammonium cyanate but was instead the well-known organic compound urea.
From this serendipitous result, Wohler correctly concluded that atoms could arrange themselves into molecules in different ways. And the properties of the resulting molecules were critically dependent on the molecular architecture. (The inorganic compound ammonium cyanate is now known to be an isomer of urea; both contain the same type and number of atoms but in different structural arrangements.)
Encouraged by Wohler’s discovery, others succeeded in making simple organic compounds from inorganic ones. And by roughly 1860 it was generally recognized that a vital force was unnecessary for the synthesis and interconversion of organic compounds.
Although a large number of organic compounds have since been synthesized, the structural complexity of certain compounds continues to pose major problems for the laboratory synthesis of complicated molecules.
But modern spectroscopic techniques allow chemists to determine the specific architecture of complicated organic molecules. And molecular properties can be correlated with carbon bonding patterns and characteristic structural features known as functional groups.
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Properties of Chemical Compounds
Followings are the properties of chemical compounds
The atomic number indicates the number of protons within the core of an atom. The atomic number is an important concept of chemistry and quantum mechanics.
An element and its place within the periodic table are derived from this concept. When an atom is generally electrically neutral, the atomic number will equal the number of electrons in the atom, which can be found around the core.
These electrons mainly determine the chemical behaviour of an atom. Atoms that carry electric charges are called ions.
Ions either have a number of electrons larger (negatively charged) or smaller (positively charged) than the atomic number.
The name indicates the mass of an atom, expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus.
Each proton or neutron weighs about 1 amu, and thus the atomic mass in always very close to the mass (or nucleon) number, which indicates the number of particles within the core of an atom; this means the protons and neutrons.
Each isotope of a chemical element can vary in mass. The atomic mass of an isotope indicates the number of neutrons that are present within the core of the atoms.
The total atomic mass of an element is an equivalent of the mass units of its isotopes. The relative occurrence of the isotopes in nature is an important factor in the determination of the overall atomicmass of an element.
In reference to a certain chemical element, the atomic mass as shown in the periodic table is the average atomic mass of all the chemical element’s stable isotopes. The average is weighted by the relative natural abundances of the element’s isotopes.
Electronegativity According to Pauling
Electronegativity measures the inclination of an atom to pull the electronic cloud in its direction during chemical bonding with another atom. Pauling’s scale is a widely used method to order chemical elements according to their electro negativity. Nobel prize winner Linus Pauling developed this scale in 1932.
The values of electro negativity are not calculated, based on mathematical formula or a measurement. It is more like a pragmatic range.
Pauling gave the element with the highest possible electro negativity, fluorine, a value of 4,0. Francium, the element with the lowest possible electro negativity, was given a value of 0,7. All of the remaining elements are given a value of somewhere between these two extremes.
The density of an element indicates the number of units of mass of the element that are present in a certain volume of a medium.
Traditionally, density is expressed through the Greek letter ro (written as r).
Within the SI system of units density is expressed in kilograms per cubic meter (kg/m3). The density of an element is usually expressed graphically with temperatures and air pressures, because these two properties influence density.
The melting point of an element or compound means the temperatures at which the solid form of the element or compound is at equilibrium with the liquid form. We usually presume the air pressure to be 1 atmosphere.
For example: the melting point of water is 00C, or 273 K.
Boiling Point of Chemical Compounds
The boiling point of an element or compound means the temperature at which the liquid form of an element or compound is at equilibrium with the gaseous form. We usually presume the air pressure to be 1 atmosphere.
For example: the boiling point of water is 1000C, or 373 K.
At the boiling point the vapour pressure of an element or compound is 1 atmosphere.
Vander Waal’s Radius
Even when two atoms that are near one another will not bind, they will still attract one another. This phenomenon is known asthe Vander Waals interaction.
The Vander Waals forces cause a force between the two atoms. This force becomes stronger, as the atoms come closer together.
However, when the two atoms draw too near each other a rejecting force will take action, as a consequence of the exceeding rejection between the negatively charged electrons of both atoms. As a result, a certain distance will develop between the two atoms, which is commonly known as the Vander Waals radius.
Through comparison of Vander Waals radiuses of several different pairs of atoms, we have developed a system of Vander Waals radiuses, through which we can predict the Vanderwaals radius between two atoms, through addition.
Ionic radius is the radius that an ion has in an ionic crystal. Where the ions are packed together to a point where their outermost electronic orbitals are in contact with each other.
An orbital is the area around an atom. Where, according to orbital theory, the probability of finding an electron is the greatest.
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