However, the problem is that not all molecules are similar. To simplify, we can say that some vibrate a lot and some vibrate a little, while others vibrate according to which molecules are adjacent to them. The latter case applies to butter, in which ice crystals are close to fat molecules. This is a rather an unusual sort of structure: it is somewhat stable and does not vibrate a lot, but if heated for any length of time, tends to collapse all of a sudden.
This is an explanation that would horrify the experts, but it does explain why butter, when placed in a microwave oven, is transformed from an attractively solid slab to a greasy mess. In brief, if you just wish to soften butter and not actually melt it, the microwave oven is not the best solution. Unless you know a couple of tricks, of course.
How long in the microwave to soften butter?
Here is the first way to soften your piece of butter: place it in the microwave for just a few seconds, ten at the most, then turn it and place it on the other side, continuing to repeat this operation. After having done it for three or four times, the butter should be soft enough without liquefying.
Another method consists in placing a small dish of water close to the slab of butter and switching on the microwave oven for 35-40 seconds. The oven will become a sort of 'steam chamber', which will soften the butter.
Finally, do bear in mind that the result will vary considerably according to the butter quality (and consequently its fat content), the weight of the piece of butter and the type of microwave oven being used.