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From Wikipedia

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are 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 complexes held together by coordinate covalent bonds. Pure chemical elements are not considered chemical compounds, even if they consist of molecules which contain only multiple atoms of a single element (such as H2, S8, etc.), which are called diatomic molecules or polyatomic molecules.

Wider definitions

There are exceptions to the definition above, and large amounts of the solid chemical matter familiar on Earth do not have simple formulas. 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. Such non-stoichiometric compounds 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 will make the ratio of elements by mass vary slightly.

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.

Formula

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 metaloxides, 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 co


From Yahoo Answers

Question:Ok can you answer these questions? 1.) list 2 properties of covalent compounds. 2.) Explain why an iron ion is attracted to a sulfide ion but not to a zinc ion. 3.) Compare the three types of bonds based on what happens to the valence electrons of the atoms. Thanks a lot!!! seriously!!!

Answers:They share electrons in the bond. They don't conduct electricity well. Ionic bonds form between metals and nonmetals, because they have different charges. Opposites attract. Iron and Zinc are both Cations (positively charged ions), and thus they don't bond naturally. Non polar covalent: the electrons are shared equally. Polar Covalent: The electrons are share unequally. Ionic: The electron is transferred from one to another.

Question:i need 3 examples of compounds that are formed by covalent bonds, i need to show how these compounds are written with single, double or triple bonds. i hope you can help!

Answers:C2H2 (triple) C2H4 (double) C2H6 (single)

Question:SF4 PCI3 N2O Determine whether the chemical formula C5H5N5 is the empirical formula or molecular formula for adenine

Answers:Sulfur Tetrafluoride Phosphorus Trichloride Dinitrogen Oxide That is the molecular formula for adenine...

Question:I need a simple way to write a formula for a covalent compound and an easy example.

Answers:well covalent compounds are formed when two nonmetals bond together. but just take two or more nonmetals and bond them and make sure you write the nonmetal with the lower electronegativity first. ex: N2O3 is a binary covalent compound.

From Youtube

Covalent Compounds Part 1 :The following episode looks at writing formulas for covalent compounds. Download the full episode from iTunes by typing "PapaPodcasts" in the search window.

Ionic and covalent bonding compounds :Oxydation of magnesium metal and sulphur create magnesium oxyde with ionic bond and sulphur oxide with covalent bond