Why does it make sense for the valence electrons to increase going across a period?

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Polyethylene is a strong synthetic polymer that can be used to make toys and bottles. polyethylene is a plastic material called thermoplastic. one very useful property of thermoplastic is that when it is heated it turns into liquid, which can be molded into desired shape and size. also the material do not dissolve at room temperature.

Im not sure but if i figure it out i’ll hello you