With molecular formula, how do I know if it linear or one element surrounded by other elements? If I was given a molecular formula (i.e, PoOBr2), how do I know if the Lewis structure is linear (4 in-a-row, basically a straight line) OR it is an element surrounded by other 3 elements?   

I know the answer for this is Po in the central and surrounded by other elements.
The formula is insufficient. Consider pentane, $\ce{C5H12}$, which occurs in straight and branched isomers, the extreme case being neopentane, in which the central carbon is linked only to four methyl groups.