Welcome to SitePoint!
First, a specification is needed whereby you specify the format of the link you wish visitors to see then the actual link required for the server to display the intended content.
You have that with accesando/ver_articulo-Redes-sociales-2 => accesando/ver_articulo.php?Redes-sociales-2, however, I would recommend that you delete ver_articulo entirely (because it is only confusing you). Your second "new format" is worse because it introduces a wild file extension which must fail!
Second, code the specification:
RewriteRule ^accesando/([-a-zA-Z]+)$ accesando/ver_articulo.php?$1 [L]
Note: This only uses the latin alphabet without accent marks - if you're including those in your links, you'll need to alter the regular expression to include them, too.
Third, you might benefit from reading the mod_rewrite tutorial linked in my signature as it contains explanations and sample code. It's helped may members and should help you, too.