Question

why does the atomic radius decrease going from left to right across a period?
increased number of neutrons
increased number of energy levels
valence electrons
increased number of protons

Explanation:

As you move from left - right across a period, the number of protons in the nucleus increases. This leads to a greater nuclear charge. The electrons are added to the same energy level. The increased nuclear charge pulls the electrons closer to the nucleus, resulting in a decrease in atomic radius. Neutrons do not affect the atomic radius in this context. The number of energy levels remains the same across a period. Valence electrons are part of the outer - most shell but it is the increased nuclear pull due to more protons that causes the radius decrease.

Answer:

Increased number of protons