Mutually Exclusive CSS Apply to buttons

var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  $scope.name = 'World';
});

app.directive('selectable', function(){
  
  var selected;
  
  var unselect = function(element) {
    element.removeClass('selected');
  }
  
  var select = function(element) {
    if (selected){
      unselect(selected);
    }
    selected = element;
    element.addClass('selected');
  }
  
  return {
    
    link : function(scope, element, attrs){
      
       element.on('click', function(){
         select(element);
       });
       
    }
  }
});

<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
    <script data-require="angular.js@1.2.x" src="http://code.angularjs.org/1.2.5/angular.js" data-semver="1.2.5"></script>
    <script src="app.js"></script>
  </head>

  <body>
  
  
  <div  ng-controller="MainCtrl">
    <b>different scope:</b>
    <div selectable>1</div>
    <div selectable>2</div>
  </div>
  
  <b>$rootScope</b>
  <div selectable>3</div>
  <div selectable>4</div>
 
</html>

/* Put your css in here */

.selected{
 background-color:green; 
}