Return the number of electrons.

        Args:
            charge (int): Charge of the molecule.

        Returns:
            int:

def get_electron_number(self, charge=0):
        """Return the number of electrons.

        Args:
            charge (int): Charge of the molecule.

        Returns:
            int:
        """
        atomic_number = constants.elements['atomic_number'].to_dict()
        return sum([atomic_number[atom] for atom in self['atom']]) - charge