Final answer:
In a row on the periodic table, alkali metals exhibit the largest atomic radius due to their additional electron shells compared to noble gases, halogens, and transition metals within the same period.
Step-by-step explanation:
In a row on the periodic table, the elements that contain the largest atomic radius are alkali metals (Group 1 elements). The atomic radius generally increases as you move down a group in the periodic table because each successive element has an additional electron shell. Alkali metals, therefore, have a larger atomic radius compared to noble gases, halogens, and transition metals within the same period. Conversely, noble gases possess the smallest atomic radii in a period due to their full energy orbitals, making their atomic size compact.
Furthermore, the trend across a period from left to right is that the atomic radius decreases as the effective nuclear charge increases, which pulls the electron shells closer to the nucleus resulting in smaller atomic sizes. Transition metals have a variable but generally smaller radius compared to alkali metals in the same period, while halogens have larger radii than noble gases but smaller than alkali metals.