As you move down Group 1 (the alkali metals) of the periodic table, the size of the atoms increases. This is because each successive element in the group has an additional energy level, or shell, of electrons. As the number of energy levels increases, the electrons are farther from the positively charged nucleus and experience less electrostatic attraction. This weaker attraction between the nucleus and the electrons results in a larger atomic radius and, therefore, a larger atom.

Here is a table showing the atomic radii of the first five alkali metals:

| Element | Atomic Number | Atomic Radius (pm) |

|---|---|---|

| Lithium | 3 | 155 |

| Sodium | 11 | 190 |

| Potassium | 19 | 235 |

| Rubidium | 37 | 248 |

| Cesium | 55 | 265 |

As you can see, the atomic radius increases from lithium to cesium. This trend is consistent throughout Group 1 and is a result of the increasing number of energy levels as you move down the group.