Calculate the standard enthalpy change for the reaction 2A+B⇌2C+2D

where the heats of formation are given in the following table:
Substance ΔH∘f
(kJ/mol)
A -235
B -407
C 209
D -519
Express your answer in kilojoules.

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.