Final answer:
Atomic size generally decreases from left to right across a period in the periodic table due to increased nuclear charge attracting valence electrons more strongly.
Step-by-step explanation:
As you move from left to right across a period (horizontal row) in the periodic table, atomic size generally decreases. This trend is observed because, even though the valence electrons are added to the same shell, the number of protons in the nucleus increases. The increased nuclear charge results in a stronger pull on the valence electrons, pulling them closer to the nucleus and thereby reducing the atomic radii.
The electrons are added to the same principal energy level, while the number of protons increases for each consecutive element across a period. This causes the electrostatic attraction between the positively charged nucleus and the negatively charged electrons to strengthen, effectively decreasing the atomic size. Though there are a few exceptions to this rule, where the size of an atom may slightly increase (e.g., when moving from nitrogen to oxygen), the overall trend remains consistent.