Setting jQuery attribute for font-weight

use the .css() function instead of .attr()

$("#opt_" + i).css("font-weight", "bold")

It's not an attribute, but CSS:

$("#opt_" + i).css("font-weight", "bold");

example: http://jsfiddle.net/niklasvh/ENwbn/

$("#opt_" + i).style("font-weight", "bold");