The first shell can hold a maximum of two electrons. how does this explain the valence of hydrogen?

The hydrogen atom has only one electron which is present in the first shell.

We know that first shell can hold a maximum of two electrons so due to incomplete valance shell hydrogen atom is reactive and make covalent bond with other reactive atoms. This bonding will complete its valance shell and finish its reactive nature. In the second shell and other shells can hold 8 electrons so we can conclude that hydrogen has one valance electron.

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"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"