Chemical formulas and equations are used to represent chemical reactions and compounds. A chemical formula uses symbols to show the proportions of atoms in a compound without specifying structure. Empirical formulas show only atomic ratios while molecular formulas show exact atom counts. Structural formulas depict molecular structure and bonding. Skeletal formulas simplify complex organic structures by implying carbon and hydrogen atoms. Empirical formulas give the simplest whole number ratio of elements in a compound without isotopes or structure. Chemical equations balance reactants and products to represent chemical reactions, while ionic and half equations break reactions into component ion exchanges.

1. Chemical formulae
and equations

2. Chemical formulae
 A chemical formula is a way of expressing information about the
proportions of atoms that constitute a particular chemical compound, using
a single line of chemical element symbols, numbers, and sometimes also
other symbols, such as parentheses, dashes, brackets, and plus (+) and
minus (–) signs. These are limited to a single typographic line of symbols,
which may include subscripts and superscripts. A chemical formula is not a
chemical name, and it contains no words. Although a chemical formula may
imply certain simple chemical structures, it is not the same as a full chemical
structural formula. Chemical formulas are more limiting than chemical
names and stuctural formulas.
 The simplest types of chemical formulas are called empirical formulas,
which use only letters and numbers indicating atomic proportional ratios (the
numerical proportions of atoms of one type to those of other types).
Molecular formulas indicate the simple numbers of each type of atom in a
molecule of a molecular substance, and are thus sometimes the same as
empirical formulas (for molecules that only have one atom of a particular
type), and at other times require larger numbers than do empirical formulas.

3. Structural formula
 The structural formula of a chemical compound is a
graphic representation of the molecular structure, showing
how the atoms are arranged. The chemical bonding within
the molecule is also shown, either explicitly or implicitly.
Also several other formats are used, as in chemical
databases, such as SMILES, InChI and CML.
 Unlike chemical formulas or chemical names, structural
formulas provide a representation of the molecular structure.
Chemists nearly always describe a chemical reaction or
synthesis using structural formulas rather than chemical
names, because the structural formulas allow the chemist to
visualize the molecules and the changes that occur.

4. Skeletal formula
 Skeletal formulas are the standard notation
for more complex organic molecules. First
used by the organic chemist Friedrich
August Kekulé von Stradonitz the carbon
atoms in this type of diagram are implied to
be located at the vertices (corners) and
termini of line segments rather than being
indicated with the atomic symbol C.
Hydrogen atoms attached to carbon atoms
are not indicated: each carbon atom is
understood to be associated with enough
hydrogen atoms to give the carbon atom
four bonds. The presence of a positive or
negative charge at a carbon atom takes the
place of one of the implied hydrogen atoms.
Hydrogen atoms attached to atoms other
than carbon must be written explicitly
Skeletal formula of isobutanol

5. Empirical formula
 In chemistry, the empirical formula of a chemical
compound is the simplest positive integer ratio of
atoms present in a compound. A simple example of
this concept is that the empirical formula of
hydrogen peroxide, or H2O2, would simply be HO.
 An empirical formula makes no reference to
isomerism, structure, or absolute number of atoms.
The empirical formula is used as standard for most
ionic compounds, such as CaCl2, and for
macromolecules, such as SiO2.

6. Chemical equation
A chemical equation is the symbolic representation of a
chemical reaction where the reactant entities are given
on the left hand side and the product entities on the
right hand side.[1] The coefficients next to the symbols
and formulae of entities are the absolute values of the
stoichiometric numbers. The first chemical equation was
diagrammed by Jean Beguin in 1615.
A chemical equation consists of the chemical formulas of
the reactants (the starting substances) and the chemical
formula of the products (substances formed in the
chemical reaction).
The two are separated by an arrow symbol (, usually
read as "yields") and each
individual substance's chemical formula is separated
from
others by a plus sign.

7. Ionic equation
 An ionic equation is a chemical
equation in which electrolytes are
written as dissociated ions. Ionic
equation are used for single or double
displacement reaction that occurs in
the aqueous solution. For example in
the following precipitation reaction.
Example of balanced
equation

8. Electron half equation
 When magnesium reduces hot copper (II) oxide to copper, the ionic
equation for the reaction is:
Cu2+ + Mg Cu + Mg2+
You can split the ionic equation into two parts, and look at it from the point of
view of the magnesium and of the copper (II) ions separately. This shows
clearly that the magnesium has lost two electrons, and the copper (II) ions
have gained them.
These two equations are described as "electron-half-equations" or
"half-equations" or "ionic-half-equations" or "half-reactions" - lots of
variations all meaning exactly the same thing!

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