First write the exact formula of the complex compound.For example, yellow blood salt - potassium ferrocyanide.Its formula: K4 [Fe (CN) 6].
determine the composition of the complex ion .In this case, the ion [Fe (CN) 6], the magnitude and sign of the charge which you have to determine.A four potassium ion up outside the scope of this compound.
Now you can help one of the basic rules of chemistry, stating: any molecule is electrically neutral.That is, in this particular case, the net charge of the molecule potassium hexacyanoferrate also zero.And this can only be when the charge of the
complex ion [Fe (CN) 6] is balanced by a net charge of four potassium ions in an external field.That is the charge of the complex ion same in dimension, but with the opposite sign.
Look in the periodic table.Potassium - one of the most active metals, it is second only to his fellow by the first main group of the table - rubidium, cesium and France.Therefore, as compared with other elements that are part of this compound - iron (Fe), carbon (C) and nitrogen (N), potassium, certainly the most electropositive.That is, it will not attract the total electron density of the molecule, and to push her away.This is understandable, because the potassium level in the outer electron is only one electron, and it is much easier to give it (to the previous level in which eight electrons become stable) than pull another seven.
Thus, each atom of potassium, forming a chemical bond in the molecule, put one electron and become, respectively, an ion with a positive charge 1.There are four ions, therefore, the total charge is equal to the outer sphere 4.And that was a neutral molecule, it must balance the -4 charge.Here is the answer to this question.