Calculate the standard enthalpy of Si(s) + 2F2(g) > SiF4(g)

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

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

Density and pressure

C. definite shape, definite volume. that is a solid.

Your question is not clear but if you can explain yourself i would be glad to .  just trying to : ))