If you only want to draw them by hand, as you were on a blackboard you can use any simple paint program, there are several free ones.
Take a look ...
Word and Powerpoint or their free rivals such as LibreOffice or OpenOffice have editors for quite rich formulas. They are very easy, but typographic control in LaTeX style is zero.
Structure diagrams need a drawing program. Office products have something quite clunky but easy to learn. Inkscape is the complex but capable one freebie.
Check this molecule ...
I know there's an open-source extension to LATEX specifically for chemical formulae - I think it's called chemfig - here's what I just found on quick lookup:
Overleaf - Chemfig
Also, for simple diagrammatic stuff, if you don't feel you need a chemistry formula specific tool, and general graphics / vector drawing works for you, then bear in mind that ...
Stardew Valley developer concernedApe has said that he drew that game's pixel art in Paint.net. It works on Win10 and is free to use too (you can pay extra for getting it on the windows store, but you don't have to).
It doesn't have any animation-specific features, but seeing what Stardew has achieved, it's defintely possible to do great graphics in it.