SQL & PL/SQL :: ORA-00933 / SQL Command Not Properly Ended

I created a table named dept2 which is a duplicate of the dept table in scott schema.

create table dept2 as select * from dept;

after that I altered it woth the following statements :

alter table dept2 add ddate date;
alter table dept2 modify deptno unique;

all operations were successful , now when I execute the following ddl statements :

alter table dept2
add constraint xx unique (loc,dname)
add constraint yy check (mod (deptno,10) =0) disable
modify ddate default sysdate
modify deptno number (5)
drop unique (deptno)

I get the error : SQL Error: ORA-00933: SQL command not properly ended

when I remove the statement of drop unique (deptno) it works