from typing import Tuple


def find_element(atom_symbol: str) -> Tuple[int, int]:
    """Returns the indices of the element component of a SMILES atom symbol.

    That is, if atom_symbol[i:j] is the element substring of the SMILES atom,
    then (i, j) is returned. For example:
        *   _find_element('b') = (0, 1).
        *   _find_element('B') = (0, 1).
        *   _find_element('[13C]') = (3, 4).
        *   _find_element('[nH+]') = (1, 2).

    :param atom_symbol: a SMILES atom.
    :return: a tuple of the indices of the element substring of
        ``atom_symbol``.
    """

    if atom_symbol[0] != '[':
        return 0, len(atom_symbol)

    i = 1
    while atom_symbol[i].isdigit():  # skip isotope number
        i += 1

    if atom_symbol[i + 1].isalpha() and atom_symbol[i + 1] != 'H':
        return i, i + 2
    else:
        return i, i + 1