When moving across a period from left to right in the periodic table (excluding transition elements) which trend occurs?
1)The energy levels decrease by one.
2) Valence electrons increase by one.
3) Valence electrons decrease by one
4) The energy levels increase by one.
3) Valence electrons decrease by one.
The correct answer is 3) Valence electrons decrease by one.
As you move from left to right across a period in the periodic table (excluding transition elements), the number of valence electrons decreases by one. Valence electrons are the electrons found in the outermost energy level of an atom, and they determine the chemical properties of an element. Each subsequent element in a period has one additional proton in the nucleus, which results in the addition of one more electron to the electron configuration. However, these additional electrons are added to the same energy level or shell, causing an increase in the nuclear charge and therefore a stronger pull on the valence electrons. This results in a smaller atomic radius and a decrease in the number of valence electrons as you move from left to right across a period.