Killing Session using alter session Command

ALTER SYSTEM KILL SESSION

For Single Instance Database:

SELECT S.SID, S.SERIAL#, S.OSUSER, S.PROGRAM FROM V$SESSION S;

ALTER SYSTEM KILL SESSION 'sid,serial#';

For RAC Instances:

SQL> select inst_id,sid,serial# from gv$session where username='SCOTT';

   INST_ID        SID    SERIAL# 

---------- ---------- ---------- 

         1        252 45632

SQL>  alter system kill session '252,45632,1'; 

 alter system kill session '252,45632,1' 

* 

ERROR at line 1: 

ORA-00026: missing or invalid session ID

Now, it works:

SQL>  alter system kill session '252,45632,@1';

System altered.