<script src="https://www.google.com/recaptcha/api.js"></script>
													
grecaptcha.ready(function () {
    if (grecaptcha.getResponse() === "") {
        alert("Please validate the Google reCaptcha.");
    } else {
        alert("Successful validation! Now you can submit this form to your server side processing.");
    }
});
															
<form action="?" method="POST">
  <div class="g-recaptcha" data-sitekey="your_site_key"></div>
  <br/>
  <input type="submit" value="Submit">
  <script src="https://www.google.com/recaptcha/api.js"></script>
</form>