Explain what happens when an element undergoes radioactive decay ?

When an element goes under radioactive decay the element spontaneously emits radiation or radioactive particles such as alpha, beta and gamma particles.  When this happens, the element usually transform into another atom by changes in the number of protons except in the cause of gamma particles which energy is released from the atom. 

The reaction will shift to the right

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.