What is a chain reaction?

i'm looking for 2-3 sentences.

20 points are available.

answer:

1. a chain reaction is a sequence of reactions where a reactive by product or product causes another or additional reactions.  

2. chain reaction also define as self-sustaining fission reaction scattered by neutrons which happen in nuclear reactors and bombs.  

explanation:

i hope, it .

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.