Final answer:
The atomic radius decreases from left to right across a period on the periodic table due to an increase in nuclear charge, which draws valence electrons closer to the nucleus.
Step-by-step explanation:
When considering the periodic trend of atomic sizes in the periodic table, we can observe a specific pattern. Across a period, from left to right, the atomic radius generally decreases. This happens because as we move from left to right, more protons are added to the nucleus, thereby increasing the nuclear charge. This increase in positive charge attracts the valence electrons closer to the nucleus, resulting in a smaller atomic radius.
Even though the valence shell maintains the same principal quantum number across a period, the increased nuclear charge due to the added protons has a greater pull on the valence electrons. Therefore, as one moves from left to right across a period on the periodic table, the atomic radii decrease. Based on this understanding, the correct answer to the question regarding the trend in atomic radius from left to right across a period is Option 1: Decreases.