Question

what is the difference between valency and charge? why do we include charge of magnesium instead of valency (while writing the chemical formulae)​

Answer 1

Answer:

Valency and predicting formulas in ionic compounds

In ionic compounds, valency gives an indication of the charge an ion formed from a particular element will carry.

Take for example, magnesium, Mg, this has a valency of two and tends to form ions carrying two positive charges, Mg2+. Note that the valency does not indicate whether an ion will carry a positive or negative charge: oxygen, O, also has a valency of two, and forms negative ions, O2−.

However, you can predict whether an atom will form negative or positive ions from its position in the Periodic Table: elements to the left (metals) will tend to form positive ions; those on the right (non-metals) will tend to form negative ions. And recall when oppositely charged ions combine to form compounds, there must be the same number of positive and negative charges, so that the compound carries no overall charge.

Thus, when Mg2+and O2− combine to form the ionic compound magnesium oxide, there have to be equal numbers of magnesium cations and oxide anions.

Magnesium oxide has the formula MgO.

This reveals another corner chemists cut when writing formulas. Note that charges do not appear in the chemical formulas of compounds: you don’t write Mg2+O2−).

Similarly, when magnesium cations, Mg2+, combine with chloride anions, Cl −, there must be twice as many chloride anions as magnesium cations in order for the charges to balance each other. The chemical formula of magnesium chloride is MgCl2.

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