All the elements you listed share a common trait: they are synthetic, radioactive elements. These elements do not have a stable isotope, so for a radioactive element, its most stable isotope's atomic mass will be listed in parentheses.
One such example would be plutonium. This element has 6 different isotopes with differing half-lives. The most stable isotope for plutonium is plutonium-244, with a half-life of around 80 million years. Therefore, the atomic mass listed for plutonium will be (244).
Probably C hope this helps
Answer:
The answer to your question is below
Explanation:
Propane = C₃H₈
Process
1.- Write the chemical reaction
C₃H₈ + O₂ ⇒ CO₂ + H₂O
balanced chemical reaction
C₃H₈ + 5O₂ ⇒ 3CO₂ + 4H₂O
Reactants Elements Products
3 C 3
8 H 8
10 O 10
b) Standard enthalpy
Propane -104.7 kJ/mol
Oxygen 0 kJ/mol
Carbon dioxide -393.5 kJ/mol
Water -241.8 kJ/mol
ΔH° = ∑ΔH° products - ∑H° reactants
ΔH° = 3(-393.5) + 4(-241.8) - [(-104.7) + 0]
-Simplification
ΔH° = -1180.5 kJ/mol - 967.2 kJ/mol + 104.7 kJ/mol
ΔH° = - 2042.5 kJ/mol
This reaction is exothermic because ΔH° is negative.
