How does the number of electrons in the outer energy level change moving horizontally from left to right across a period?

As you move horizontally from left to right across a period in the Periodic Table, the number of electrons in the outer energy level, or valence electrons, increases sequentially by one with each successive element. For example, the first element in a period has one valence electron, the next has two, and so on, until the last element in the period, which has eight valence electrons (in the case of the noble gases). This increase in valence electrons influences the chemical properties and reactivity of the elements.