<span>According to octet rule, atoms with an atomic number less than 20 tend to combine with other atom such that both of these atoms have eight electrons in their valence shells, which gives them the same electronic configuration as that of noble gas.
However, there are few compound that donot obey octel rule. Among the elements mentioned above i.e. oxygen and helium obeys octet rule.
In case of nitrogen, oxide of nitrogen (like NO and NO2) have incomplete octet.
While there are few compounds of Br wherein Br has expanded octet. For example, in BrF5, Br has 12 electrons in valence shell. </span>
Groundwater adds water to lakes, rivers, and oceans.
Hydrogen-
Hydrogen is the answer because the number of protons is the atomic number. All the elements have different atomic numbers and hydrogen has the least with an atomic number of 1.
Answer : The change in enthalpy for the reaction is, 175 kJ
Explanation :
According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.
According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.
The main reaction is:

The intermediate balanced chemical reaction will be,
(1)

(2)

(3)

Now we are dividing reaction 1 by 2, dividing reverse reaction 2 by 2 and then adding all the equations, we get :
(1)

(2)

(3)

The expression for change in enthalpy of the given reaction is:



Therefore, the change in enthalpy for the reaction is, 175 kJ