How to save default values into an option and delete those upon actvation and deactivation in wordpress?

The following code worked. According to the comment by Milo, I have been doing it wrong by using the option name. So instead I used each keys from the option and used the array to pass the defaults. Thanks milo! :_

register_activation_hook( __FILE__, 'twmm_activate'); //activate plugin options open activation of plugin
register_deactivation_hook(__FILE__, 'twmm_deactivate'); //deactivate plugin options open deactivation of plugin
function twmm_activate() {
  $defaults_twmm = array(
    'twmm_on_off' => 'disabled',
    'twmm_middle_layout_selector' => 'email',
    'twmm_permission_lvl' => 'admin',
    'twmm_page_title' => 'We will be back',
    'twmm_custom_message' => 'Please check back after some time. <br />Thank you!',
    'twmm_facebook' => 'http://facebook.com/techtuft',
    'twmm_twitter' => 'http://twitter.com/techtuft',
  );
  $options = wp_parse_args(update_option('twmm_options', $defaults_twmm));
}
function twmm_deactivate() {
  delete_option('twmm_on_off'); //delete plugin specific options upon deactivation
  delete_option('twmm_middle_layout_selector');
  delete_option('twmm_permission_lvl');
  delete_option('twmm_page_title');
  delete_option('twmm_custom_message');
  delete_option('twmm_facebook');
  delete_option('twmm_twitter');
}