According to the electron configuration, which atom will be the most reactive? A 1s22s2
B 1s22s22p63s2
C 1s22s22p63s23p6
D 1s22s22p63s23p64s2

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Radon-222 or rn-222 has a half-life of 3.82 days. to determine the fraction of an original sample of rn-222 that remains unchanged after 7.646 days, we calculate the amount of rn-222 remaining after 7.646 days and divide it with the original amount. using 1 gram as a basis and using half-life calculation methods, we can determine that  0.249 or 0.25  grams of rn-222 will remain, and thus the fraction remaining is 0.25 or 1/4.