the left side of the equation, record substance reacts.They are called "precursors".On the right side, respectively, formed substance ("reaction products").
When writing formulas of the molecules using conventional chemical symbols of atoms.The index of each atom is determined by the formula compounds and valence.
Remember that in contrast to the mathematical equations, the equations of chemical reactions in any case can not swap the right and left side!Because of this completely change the meaning of the record.In addition, this reaction often becomes simply impossible.
number of atoms of all elements on the left and right part of the reaction must be the same.If necessary, the "balancing" amount is
produced by selecting the coefficients.
When writing the equation of chemical reaction, first make sure that it is at all possible.That is to say that it is not contrary to the course of known physical and chemical properties of substances and rules.For example, the reaction:

NaI + AgNO3 = NaNO3 + AgI
It flows quickly to the end, the reaction forms an insoluble light yellow precipitate of silver iodide.A backlash:

AgI + NaNO3 = AgNO3 + NaI - impossible, even though the correct character is written, and the number of atoms of all elements on the left and right sides equally.
Record equation in the "complete" form, ie, using their molecular formulas.For example, the reaction of precipitation of barium sulphate:

BaCl2 + Na2SO4 = 2NaCl + BaSO4
And the same reaction can be written in ionic form:

Ba 2+ + 2Cl- + 2Na + + SO4 2-= 2Na + + 2Cl- + BaSO4
You can see that on the left and on the right side of the equation contains exactly the same chlorine ions and sodium.Cross out and get their final abbreviated equation for the reaction in ionic form:

Ba 2+ + SO4 2- = BaSO4
Similarly, we can write the equation in ionic form another reaction.Remember that each molecule soluble (dissociating) the substance is recorded in an ionic form, the same ions in the left and right sides of the equation are excluded.