set a id for a div class in drupal
You would be better off using jQuery:

(function($) {
  $(function() {
    // Grab the element with both group_left and article_left class and set an ID attribute.
    $('.group_left.article_left').attr('id', 'pad_id');
  });
})(jQuery);


It's worth checking that the element with both of those classes is unique on the page, if the same ID is added to more than one element then the document will become invalid and you might have problems with other javascript.