Scriptstyle has no effect - Chemfig

Question

Just noticed that adding \scriptstyle{\oplus} or \scriptsize{\oplus} doesn't actually change anything. I'm not sure if I have been using the command wrong (I'm trying to decrease the size of the \oplus), since I've included it in a lot of my other mechanisms. Thanks in advance for any suggestions on this.

Edit: Clarified the effect I expected to see Edit 2: Changed incorrect charge on N from - to +

\documentclass{article}
\usepackage{graphicx}
\usepackage{chemfig}
\usepackage{geometry}
\usepackage{tabu}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{siunitx}
\usepackage{chemmacros}
\usetikzlibrary{arrows}

\title{}
\author{}
\date{}

\begin{document}

\maketitle

\chemfig{-[:30]-[:-30]N^{\scriptstyle{\oplus}}H_3}
\chemfig{-[:30]-[:-30]N^{\scriptsize{\oplus}}H_3}

\chemfig{-[:30]-[:-30]N^{\oplus}H_3}

\end{document}

Answer 1

\scriptsize is invalid in math mode (and the syntax is \scriptsize abc not \scriptsize{a}bc) You get the warning

LaTeX Font Warning: Command \scriptsize invalid in math mode on input line 21.

\scriptstyle also does not take an argument but selects the size used in superscripts, so does nothing here. You can select scriptscriptstyle which is the smallest of the three math styles.

\documentclass{article}
\usepackage{graphicx}
\usepackage{chemfig}
\usepackage{geometry}
\usepackage{tabu}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{siunitx}
\usepackage{chemmacros}
\usetikzlibrary{arrows}

\title{}
\author{}
\date{}

\begin{document}

\maketitle

\chemfig{-[:30]-[:-30]N^{\scriptscriptstyle \ominus}H_3}


\chemfig{-[:30]-[:-30]N^{\ominus}H_3}

\end{document}

Answer 2

No problem for me (TexLive 2021 and chemfig 1.6a). I suggest using the \chemabove command for a better result.

\chemfig{-[:30]-[:-30]\chemabove{N}{\scriptstyle{\oplus}}H_3}
    
    \chemfig{-[:30]-[:-30]\chemabove{N}{\scriptsize{\oplus}}H_3}
    
    \chemfig{-[:30]-[:-30]\chemabove{N}{\oplus}H_3}