Ba(NO2)2
Since we're dealing with a nitrite, you know the formula will have NO2 in it. Also, since Barium is in the 2nd column of the periodic table, that means that Barium has 2 valance electrons. And you can kind of think of the nitrite ion as a really odd looking atom that really wants to bind with an electron. And since Barium has 2 electrons available for binding, you'll have 2 nitrite ions bind to it. So you get
Ba(NO2)2
