Example: The electric dipole moment (p) in the water molecule

% The electric dipole moment (p) in the water molecule 
% Author: Jimi Oke
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[>=latex,scale=1.3]
  \shade[ball color=gray!10!] (0,0) coordinate(Hp) circle (.9) ;
  \shade[ball color=gray!10!] (2,-1.53) coordinate(O) circle (1.62) ;
  \shade[ball color=gray!10!] (4,0) coordinate(Hm) circle (.9) ;
  \draw[thick,dashed] (0,0) -- (2,-1.53) -- (4,0) ;
  \draw[thick] (2,.2) -- (2,1.5) node[right]{$\mathbf{p}$} ;
  \draw (2.48,-1.2) arc (33:142:.6)  ;
  \draw (2,-.95) node[above]{$105^{\circ}$} ;
  \draw (0,.2) node[left]{H$^+$} ;
  \draw (4,.2) node[right]{H$^-$} ;
  \draw (2,-1.63) node[below]{O$^{2-}$} ;
  \foreach \point in {O,Hp,Hm}
    \fill [black] (\point) circle (2pt) ;
\end{tikzpicture}
\end{document}