Final answer:
The atomic radius decreases from lithium to fluorine due to the stronger nuclear charge which attracts electrons more closely, reducing the atomic size.
Step-by-step explanation:
The trend in atomic radius from lithium (Li) to fluorine (F) across a period in the periodic table decreases as the atomic number (Z) increases. This observed decrease is due to the added protons in the nucleus which create a stronger positive charge, attracting the electrons more closely towards the nucleus. Concurrently, electrons are being added to the same principal energy level, which does not significantly expand the atom's electron cloud.
As a result of the increasing nuclear charge, the effective force of attraction between the nucleus and the electrons in the same energy level strengthens. Consequently, this increased attraction decreases the atomic size. Therefore, lithium, which has the fewest protons in its nucleus among these elements, has the largest atomic radius, while fluorine, with the most protons, has the smallest atomic radius in this group.
It should be noted that while the radii decrease along each row of the periodic table, they increase down each group. The largest atoms are in the lower left corner, while the smallest are in the upper right corner of the periodic table.