def to_symbol(atom):
  """Converts a Chem.Atom to a string like "C", "O-", or "Fe+2"."""
  symbol = atom.GetSymbol()
  charge = atom.GetFormalCharge()
  if charge == 0:
    return symbol
  suffix = '+' if charge > 0 else '-'
  if abs(charge) > 1:
    suffix += str(abs(charge))
  return symbol + suffix