This question uses weighted averages.
Since the average mass of element A is 63.6 amu and the only two isotopes are 63 and 65,
63.6 = x% * 63 + (1-x%) * 65
If you don't want to calculate this (like me), you can look at the answer choices.
If it was 100% A-63, the weight would be 63. Cross off D.
If it was 50/50, the weight would be 64. Cross off C.
So since the weighted average is 63.6, you would expect there to be more A-63 than A-65, so B is the only answer that makes sense.
A reaction is exothermic if Δ<em>H</em> (or 
in some textbooks) is negative:
- H₂ + Br → 2 HBr, ΔH < 0.
- CH₄ + 2 O₂ → CO₂ + 2 H₂O, ΔH < 0.
A reaction is endothermic if Δ<em>H</em> is positive:
- 2 NH₃ → N₂ + 3 H₂, ΔH > 0.
- 2 HCl → H₂ + Cl₂ ΔH > 0.
<h3>Explanation</h3>
The enthalpy of a system is the sum of its internal energy. ΔH < 0 indicates that the reactants lose internal energy in the reaction. Energy conserves, and those internal energies must have converted to some other form of energy. They typically end up as thermal energy. The reaction will release heat since it is exothermic.
Similarly, ΔH > 0 indicates that the reactants gains internal energy in the reaction. Energy conserves. As a result, the reaction must have gained energy from its surroundings. The reaction will be endothermic since it absorbs heat.
Answer:
I was traveling on a train where I fell asleep. someone stole my luggage in which I had money and other essentials. when the ticket checker arrived a lady paid my fine. she helped me a lot. when I told her about being robbed she helped me and took me to the police station and I found my bag. I insisted she take the money she paid for my fine but she said that in return you too help someone."I have never found such a kind person ever since.".....
Explanation: I think this is what you are looking for. Hope this helps.
Its 100% B. my dude because the atomic theory doesn't state anything else
