Answer : The energy of the photon emitted is, -12.1 eV
Explanation :
First we have to calculate the 
orbit of hydrogen atom.
Formula used :
where,
= energy of 
orbit
n = number of orbit
Z = atomic number of hydrogen atom = 1
Energy of n = 1 in an hydrogen atom:
Energy of n = 2 in an hydrogen atom:
Energy change transition from n = 1 to n = 3 occurs.
Let energy change be E.
The negative sign indicates that energy of the photon emitted.
Thus, the energy of the photon emitted is, -12.1 eV
Se - 78
Selenium - 78
or
78/34 Se2-
I want more points for this answer
It decreases because it is losing an electron
3 - Effective collisions between reacting particles
For a chemical reaction to occur, reacting particles must collide - however, not all collisions result in a reaction. They must have enough energy in the collision to react (that is, be above the activation energy). The rate of reaction is determined by the rate of successful collisions, that is to say, the more successful collisions there are in a unit time, the faster the reaction rate.
