Notification when visitor is on specific WordPress Article

There’s no is_article. I assume you guessed there would be analogously to is_page, but I don’t think article is a built-in WordPress post type. I think you want is_page instead, which looks like it covers all post types.

You should also move the page check into the action handler, to be sure that WordPress has enough state loaded to know if it’s on that page or not at the point it does the check, which it won’t when it’s loading the plugin or theme you’ve added this code to, e.g.

function email_alert() {
    if ( is_page( 1234 ) ) {
        wp_mail( '[email protected]', 'Alert', 'This Site was Visited!' );
    }
}
add_action( 'wp', 'email_alert' );