Click on the diagram to choose which atom is more reactive. Hint: These are nonmetals. Which is going to gain a full valence shell of 8 electrons the easiest?

The first one has 5 valence electrons, and the second has 7. (This can be counted by counting the amount of black dots on the biggest circle). Since 7 > 5, it would be easier for the second one to gain one (so that it completes it's valence shell) so the answer is the second one.

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.