Isotopes include two or more versions of an atom of a given element that have the same number of protons but a different number of neutrons.  The isotope in this question is a charged atom, also known as an ion. They have unequal numbers of protons and electrons. An isotope symbol communicates information about the element (specifically, the symbol), the atomic number of that element, the mass number of the specific isotope of the atom, and the electrical charge on the ion.
 

According to the Collision Model, one of the requirements for a collision to result in a reaction is that the two colliding particles must collide with the proper orientation. To picture this, consider kicking a soccer ball with the toe of your foot. The collision is between the foot and the ball. But more specifically, it is between the toe on your foot and the location on the ball that actually made contact. 

When two reactant particles collide, it is not the entire particle that undergoes the contact during the collision. Only certain atoms in reactant particle #1 will contact certain atoms in reactant particle #2. For a reacant particle with many atoms, you can imagine that there would be many atom-to-atom collision possibilities. But not all those possibilities meet the proper orientation requirement. 

The way to think about what orientation is "proper" is to think about which bond(s) in reactant particle #1 has to break. You can do this by looking at the reactant and then glancing across to the product side of the equation to see what that reactant turns into. For instance, CO turns into CO₂. So if you examine the particle picture, a C-O bond must break and the O atom must be released from the CO₂ particle. Now you ask, where does this released O atom end up? Once more, by examining the particle picture of the reaction, you realize that the O atom must bond to the N atom of reactant particle #2. And so for all this reaction magic to happen, it would be most "proper" for the N atom of NO₂ to collide with the O atom of CO₂.  This would release the O atom from CO₂ and attach it to the N atom to form NO₃.