Form gets submitted even if the required fields are left empty in angular JS

I am newbie to angular JS and i have created a form which is

HTML

<div data-ng-controller="ReservationController"
    <form class="form-horizontal" role="form">
            <div class="form-group">
              <div class="col-sm-10">
                <label>Guest Name</label>
                <input type="text" class="form-control" placeholder="" ng-model="res_guest_name" required>
              </div>
            </div>

            <div class="form-group">
              <div class="col-sm-10">
                <label>Phone</label>
                <input type="phone" class="form-control"  ng-model="res_member_phone">
              </div>
            </div>
            <div class="form-group">
              <div class="col-sm-10">
                <label>FAX</label>
                <input type="phone" class="form-control"  ng-model="res_member_fax">
              </div>
            </div>
            <div class="form-group">
              <div class="col-sm-10">
                <label>Email</label>
                <input type="email" class="form-control"  ng-model="res_member_email" required>
              </div>
            </div>
          <div class="form-group">
              <div class="col-sm-offset-8 col-sm-10">
                <button type="submit" class="btn btn-success" ng-click="res_save()">Save Changes</button>
              </div>
            </div> 
          </form>
          </div>

CONTROLLER

  function ReservationController($scope, $http,       $cookieStore,$location,$filter) {
     $scope.res_save = function()
      {
       var save_res ="http://ift.tt/1vFFwiT"+conId+"&token="+token+"&id="+$scope.resId+"&page=edit&guest_name="+$scope.res_guest_name+"&phone="+$scope.res_member_phone+"&fax="+$scope.res_member_fax+"&email="+$scope.res_member_email;
$http.get(save_res).success(function(response) {
    alert('success');
      });       
      }
      }

My form gets submitted even after the required fields are left empty. it shows the error, then it gets submitted.