Why does the number of valence electrons increase as you move across a period in the periodic table?

• A. Because elements across a period gain energy levels as you move from left to right.
• B. Because the atomic radius increases, allowing more electrons to fit in the outer shell.
• C. Because the number of neutrons in the nucleus increases, which attracts more electrons to the valence shell.
• D. Because each successive element has an additional proton and electron, with the added electron entering the same outermost shell.

Answer: (D)

Verified step by step guidance

1. Understand that a period in the periodic table corresponds to elements with electrons filling the same principal energy level or shell.
2. Recognize that as you move from left to right across a period, each successive element has one more proton in the nucleus and one more electron than the previous element.
3. Note that the added electron enters the same outermost energy level (valence shell) because the principal quantum number remains constant across a period.
4. Realize that the increase in the number of protons (atomic number) increases the nuclear charge, which pulls electrons closer but does not add new energy levels in the same period.
5. Conclude that the number of valence electrons increases across a period because electrons are being added to the same outer shell, not because of changes in energy levels, atomic radius, or neutron number.