Why does a substances density decrease as it heats up

Heating a certain substance causes molecules to speed up and spread slightly further apart, having a larger volume that results in a decrease in the density. Cooling a substance causes molecules to slow down and get closer together, occupying a smaller volume that results in the increase in density.

Explanation:

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer