context updates

When using an early bound data context,  does the code below send the whole record back to crm, or just the updated pieces? Is there a more efficient way to do this from a console app?

var accts = db.accounts.Where(x => x.accountnumber == null);

                    foreach (var a in accts)

                    {

                       a.accountnumber = <someValue>

cx.Update(a);

cx.SaveChanges();

                    }