How many isomers of C8H18 are present in the box at right?

How many molecules of C8H18 are present in the box at right?

How many isomers of C4H8 are present in the box at right?

How many molecules of C4H8 are present in the box at right?

Which rule about bonding is violated in the molecule at right?
Which rule about bonding is violated in the molecule at right?