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From Wikipedia, the free encyclopedia

A chemical compound is a pure chemical substance consisting of two or more different chemical elements[1][2][3] that can be separated into simpler substances by chemical reactions.[4] Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms[3] that are held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be compound molecules held together by covalent bonds, salts held together by ionic bonds, metallic compounds held together by metallic bonds, or complexes held together by coordinate covalent bonds. Substances such as pure chemical elements and elemental molecules consisting of multiple atoms of a single element (such as H2, S8, etc.) are not considered chemical compounds.[5]

Elements form compounds to become more stable. They become stable when they have the maximum number of possible electrons in their outermost energy level, which is normally two or eight valence electrons. This is the reason that noble gases do not frequently react: they already possess eight valence electrons (the exception being helium, which requires only two valence electrons to achieve stability).


Wider definitions

There are a few exceptions to the definition above. Certain crystalline compounds are called "non-stoichiometric" because they vary in composition due to either the presence of foreign elements trapped within the crystal structure or a deficit or excess of the constituent elements. Some compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which will make the ratio of elements by mass vary slightly. A compound therefore may not be completely homogeneous, but for most chemical purposes it can be regarded as such.

Elementary concepts

Characteristic properties of compounds:

1. Elements in a compound are present in a definite proportion
Example- 2 atoms of hydrogen + 1 atom of oxygen becomes 1 molecule of compound-water.
2. Compounds have a definite set of properties
Elements of the compound do not retain their original properties.
Example- Hydrogen(element{which is combustible and non-supporter of combustion}) + Oxygen(element{which is non-combustible and supporter of combustion}) becomes Water(compound{which is non-combustible and non-supporter of combustion})
3. Elements in a compound cannot be separated by physical methods.

Valency is the number of hydrogen atoms which can combine with one atom of the element forming a compound.

Compounds compared to mixtures

The physical and chemical properties of compounds are different from those of their constituent elements. This is one of the main criteria for distinguishing a compound from a mixture of elements or other substances because a mixture's properties are generally closely related to and dependent on the properties of its constituents. Another criterion for distinguishing a compound from a mixture is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, evaporation, or use of a magnetic force, but the components of a compound can only be separated by a chemical reaction. Conversely, mixtures can be created by mechanical means alone, but a compound can only be created (either from elements or from other compounds, or a combination of the two) by a chemical reaction.

Some mixtures are so intimately combined that they have some properties similar to compounds and may easily be mistaken for compounds. One example is alloys. Alloys are made mechanically, most commonly by heating the constituent metals to a liquid state, mixing them thoroughly, and then cooling the mixture quickly so that the constituents are trapped in the base metal. Other examples of compound-like mixtures include intermetallic compounds and solutions of alkali metals in a liquid form of ammonia.


Chemists describe compounds using formulas in various formats. For compounds that exist as molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metal oxides, the empirical formula is normally given, e.g. NaCl for table salt.

The elements in a chemical formula are normally listed in a specific order, called the Hill system. In this system, the carbon atoms (if there are any) are usually listed first, any hydrogen atoms are listed next, and all other elements follow in alphabetical order. If the formula contains no carbon, then all of the elements, including hydrogen, are listed alphabetically. There are, however, several important exceptions to the normal rules. For ionic compounds, the positive ion is almost always listed first and the negative ion is listed second. For oxides, oxygen is usually listed last.

Organic acids generally follow the normal rules with C and H coming first in the formula. For example, the formula for trifluoroacetic acid is usually written as C2HF3O2. More descriptive formulas can convey structural information, such as writing the formula for trifluoroacetic acid as CF3CO2H. On the other hand, the chemical formulas for most inorganic acids and bases are exceptions to the normal rules. They are written according to the rules for ionic compounds (positive first, negative second), but they also follow rules that emphasize their Arrhenius definitions. Specifically, the formula for most inorganic acids begins with hydrogen and the formula for most bases ends with the hydroxide ion (OH-). Formulas for inorganic compounds do not often convey structural information, as illustrated by the common use of the formula H2SO4 for a molecule (sulfuric acid) that contains no H-S bonds. A more descriptive presentation would be O2S(OH)2, but it is almost never written this way.

Phases and thermal properties

Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat. The temperature at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on the rate of heating.

CAS number

Every chemical substance, including chemical compounds, that has been described in the literature carries a unique numerical identifier, its CAS number.

See also


  1. ^ Brown, Theodore L.; LeMay, H. Eugene; Bursten, Bruce E.; Murphy, Catherine J.; Woodward, Patrick (2009), Chemistry: The Central Science, AP Edition (11th ed.), Upper Saddle River, NJ: Pearson/Prentice Hall, pp. 5–6, ISBN 0132364891, 
  2. ^ Hill, John W.; Petrucci, Ralph H.; McCreary, Terry W.; Perry, Scott S. (2005), General Chemistry (4th ed.), Upper Saddle River, NJ: Pearson/Prentice Hall, p. 6, ISBN 9780131402836,,3110,0131402838,00.html 
  3. ^ a b Whitten, Kenneth W.; Davis, Raymond E.; Peck, M. Larry (2000), General Chemistry (6th ed.), Fort Worth, TX: Saunders College Publishing/Harcourt College Publishers, p. 15, ISBN 9780030723735 
  4. ^ Wilbraham, Antony; Matta, Michael; Staley, Dennis; Waterman, Edward (2002), Chemistry (1st ed.), Upper Saddle River, NJ: Pearson/Prentice Hall, p. 36, ISBN 0132512106 
  5. ^ Halal, John (2008). "Chapter 8: General Chemistry". Milady's Hair Structure and Chemistry Simplified (5 ed.). Milady Publishing. pp. 96–98. ISBN 1428335587. 

Simple English

A chemical compound is a chemical made by joining together atoms of different chemical elements. The different atoms stick together so strongly that the compound behaves like one substance. What it is made of depends on how the atoms are joined together.

Chemical structure of water

[[File:|thumb|Chemical structure of sodium chloride]] Chemical compounds can be liquids, like water which is made from atoms of hydrogen and oxygen joining together. They can also be solids, like sodium chloride that contains sodium and chlorine atoms. Some chemical compounds are natural, which means that they are not made by people. Some chemical compounds, often called just chemicals, are synthetic, which means they made by people using machines.

File:Lead (II) iodide precipitating out of
Two colorless liquids react to make a yellow solid

Sometimes when a person mixes together two different liquids, they can turn into a different liquid that is not anything like the ones he or she started with.

Chemists are people who find new ways to mix liquids and other things together in order to make new liquids that do new things. They make chemicals in order to find new ways to do things, like to make a better glue, or to make new medicines, or to make things clean, for example.

Most of the time the word 'chemical' is only used to talk about liquids that can hurt people, or things that people have to be careful when they use them. A lot of chemicals are used by people every day, even though a lot of people are afraid of chemicals. A lot of chemicals are safe, when used the right way. Some chemicals are not safe at all, but those are not used by normal people every day. Chemicals that are not safe are called 'hazardous chemicals', and most of them can only be used by people who have gone to a special school, and even then they require special handling. These chemicals might be poisonous, corrosive, explosive, they may catch fire easily, or they may react with other substances.[1]

Some people who work in factories are near chemicals when they work, and some of them can be dangerous. Those people should do special things to make sure they are safe, and sometimes they have to wear special clothes that stop the chemicals from hurting their body.


  1. Devlin, Jacinta; Cochrane, Helen (2005). "1" (in English). Science Links 2. Coffey, Rhonda (VELS Edition ed.). Heinemann. pp. 22. ISBN I740815203. 

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