Validate a checkbox

[Scottdg]

I have created a form that requires checking a box certifying that you agree to terms.

<input name="certify" type="checkbox" value="Y" ID="CertChkbox"/>

This is the last item on the form which contains mostly address info. I then added the following javascript:

<script>
function validateForm()
{
	
var x=document.forms["PledgeForm"]["FirstName"].value;
if (x==null || x=="")
  {
  alert("First name is a required field and must be completed");
return false; 
  }
var x=document.forms["PledgeForm"]["LastName"].value;
if (x==null || x=="")
  {
  alert("Last name is a required field and must be completed");
return false; 
  }


this goes on for a few more fields and then I tried adding the following code for  the checkbox:

if (PledgeForm.Certify.value == null)
{
	alert("error message");
}

I am not sure what I am doing wrong. How can I validate the checkbox?

Thanks in advance.