Missing “span” tags

While editing my pages in WordPress and while writing a blog I need to use <span> tags, however when I enter them into the HTML- using the HTML editor- as soon as I click save the span tags are deleted, along with other parts of the code; for example, “100px” is changed to “100”, which doesn’t affect the code essentially, but the missing <span> tags do, drastically.

Does anyone know why this is happening and how I can stop it? Any help is greatly appreciated.