Atoms unite to form molecules by means of available electrons. Bonds can form by electron transfer or by electron sharing. Many chemical bonds are intermediate between those two. Elements combine in definite proportions peculiar to the atoms involved. Formulas can be predicted, but there are pitfalls.
Instructions
Periodic Table of the Elements
1. Examine a periodic table of the elements. Unless the valences are listed on it, a copy of the periodic table of the elements won't be of much use in predicting formulas. There are simply too many exceptions to make it a useful predictor.
2. Find the valences of all the elements in the compound. All elements have valences, based on the number of electrons they seek to gain or to lose. Atoms gain or lose electrons to increase stability and lose energy. Valences can be determined from a textbook or online source.
3. Determine the formula from the valences. If element A has a plus 3 charge and combines with an element B of minus 2 charge, simply write the positive element down and the negative element next to it, and swap the numbers:
A₂B₃
This is the result because two atoms at plus 3 give a total charge of plus 6. Three atoms at minus 2 gives a total charge of minus 6. Plus 6 plus minus 6 equals zero. When the charge adds up to zero, the correct formula has probably been found.
4. Reduce the numbers, if necessary. If one number is divisible into the other number, you will have to do so. For instance, magnesium sulfide consists of plus 2 magnesium and minus 2 sulfur. Switching the numbers gives:
Mg₂S₂.
The formula is actually found by dividing the one number by the other or:
MgS.
Problems That May Be Encountered
5. Realize that some compounds are hydrates, which means they contain "water of crystallization." For instance, gypsum ordinarily contains two molecules of water in its structure. Its formula is:
CaSO₄·2H₂O.
6. Learn about dimers. Some molecules are more stable if they form dimers. Hydrogen fluoride, by valence alone, should be:
HF.
In fact, this is considered acceptable for most purposes. However, there is evidence for the existence of a dimer because of hydrogen bonding. Thus,
H₂F₂.
Hydrogen bonding is one of the leading causes of dimers.
7. Recognize that not all possible structures that can be drawn actually exist in nature.
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