Alt Symbol Keys Not Working

I want to type special symbols on my Asus Vivobook S550CM notebook, but it simply doesn't work. I've tried using Number Lock, but it doesn't work. For example if I try it here in Chrome, the Alt key shifts the focus to the menu bar of Chrome and typing 1 will make it switch to the first tab. Alt+2 will switch it to the second tab. I've tried it in gedit. The same thing happens. Alt key shifts the focus to the gedit's menu bar. How can I type the special symbols in such cases? By the way, I'm using Ubuntu 15.10.