Did you hear the one about a chemist who was reading a book about helium?

I had that before it's a riddle. Here lemme find it and give you the answer.

Explanation:

1.H

2.E

3.C

4.O

5.U

6.L

7.D

8.U

9.T

10.P

11. N

12.T

13.I

14.T

15.D

16.O

17.N

18.W