"Whether any "padding" bytes are added between elements by the compiler."

Union elements are overlaid in memory so there won't be any padding between them. A union may, however, be padded out to an address boundary at the high address end should the alignment of a particular element require it. The alignment of the lowest address will be chosen to suit all elements.

If any type used as a union element itself contains padding, for instance a structure, then that padding will have to be taken into account if that structure element is accessed by reference to any other element of the union.

As Michael pointed out it is in fact illegal to store data in a union through one element then retrieve it through another. In reality, things work as expected on most(?) implementations.

A possibly less non-portable method of accessing one type as another, particularly where structure elements are involved, is to scrap the union altogether and use a pointer to the object of interest cast to a pointer to unsigned char. This avoids the structure padding issue and may even be within the bounds of what is allowed by the 'C' standard.