Although all theories are made to explain phenomena, the quality and power of these theories differ. The greatness (or, goodness) of a theory depends on the extent that the theory under question can explain the world. Some ways, listed in order of explanatory power, of how theories can help us understand the world are as follows:
(In relation to facts) 1. Prediction 2. Suggestion 3. Explanation 4. Classification 5. Remembering
Such a theory should give a prediction abou how matter would behave.
Even if a theory cannot make 100% reliable predictions, it might make a suggestion for possible experimentation to test the theory or for guiding practice.
It should be noted that 3-5, or explanation, classification, and remembering are also known as postdiction, or vaticinium ex eventu, or prophesy after the event. Such theories can explain what we know but cannot help us understand that which we don't know, and its ability to help us create knowledge is limited because it only comes into play after an experiment.
In addition, good theories should also be realistic and as simple as possible to make them applicable for practical use (note: practical used loosely, not practical as in knowing how to tie your shoes, but practical as in usable for creating knowledge/understanding with relative ease. They might also help you tie your shoe. - jmm). On the other hand, things should not be oversimplified so that they lose meaning and relevance to known facts. In reality, a theory so broad that it can be applied to explain any observation at all (both one observation and its opposite) ends up explaining nothing. To paraphrase Einstein, scientific theories should be made as simple as possible without removing important pieces.
For example, although some may believe the theory of evolution to contain several inconsistencies or disagree with the its explanations of phenomena, it is nevertheless a powerful theory because of its ability to offer predictions and suggestions for us to explore.
Lewis wanted to test the power of his theory by using it for prediction and suggestion.
Using the number of valence electrons, we would like to try extending Lewis theory, which was meant to explain periodicity, to also to predict constitution (valence numbers for different atoms), structure (distances and angles), charge distribution (polarity), energy content, and reactivity. Lewis' theory sought to explain "the nature and sequence of bonds." His theory offered some surprising results, such as in its prediction of a reaction between NH3 and BH3.
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