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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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.