Make alert window when clicking a disabled button

I have a button which by default is disable, but when a checkbox is checked the button is then enabled. I would like to edit the script to make an alert window when the button is clicked when disabled, to make sure the user knows he/she has to check the checkbox.

My script so far:

<script type="text/javascript">
$('#terms').on('change', function(){
          if ($(this).is(':checked')){
             $('#customButton').removeProp('disabled');
          }
          else{
             $('#customButton').attr('disabled', 'disabled');
          }
       });
</script>

<form id="payment-form" action="chargeCard.php" method="POST" name="payment-form">
<input type="checkbox" id="terms" />
<input type="image"  src="button3.png" id="customButton" value="submit" alt="button" disabled="disabled"/>
</form>