Find out if radio button checked with JQuery?

I can set a radio button to checked fine, but what I want to do is set up a sort of 'listener' that activates when a certain radio button is checked.

Take, for example, the following code:

$("#element").click(function()
{ 
    $('#radio_button').attr("checked", "checked");
});

it adds a checked attribute and all is well, but how would I go about adding an alert. For example, that pops up when the radio button is checked without the help of the click function?