A big operator — \sum, \prod, \int, \bigcup — takes its bounds as a subscript and a superscript. Step 7 of 17 of the LaTeX tutorial, editable as you read it.
Render a LaTeX or mhchem formula as an SVG or PNG image, learn the notation step by step, and practise it with graded exercises. The tool itself needs JavaScript; the rendering API does not.