I read this story over thirty years ago in my 7th algebra textbook (of all places, right?). That textbook used to put in all sort of short stories to get kids into math and this was one of the stories.
It was about radio astronomer who is trying to decode a mysterious alien signal. Everyone keeps trying to crack the code using math, but in the end, they learn the signal was, in fact, a representation of the periodic table.
Any idea who wrote it and the title?