Final answer:
The atomic radius is a measure of the size of an atom. It depends on the distance between the nuclei of two identical atoms bonded together. The atomic radius decreases across a period and increases down a group on the periodic table.
Step-by-step explanation:
The atomic radius is a measure of the size of an atom. It can be determined as half the distance between the nuclei of two identical atoms bonded together. The units for atomic radius are picometers (pm), which is equal to 10-12 meters. For example, the internuclear distance between the two hydrogen atoms in an H2 molecule is measured to be 74 pm, so the atomic radius of a hydrogen atom is 74 pm divided by 2, which is 37 pm.
The atomic radius of different elements changes across a period. It generally decreases from left to right across a period. This is because, as you move across a period, the number of protons in the nucleus increases, leading to a stronger attractive force on the electrons in the outer shell and a smaller atomic radius.
The atomic radius also changes from top to bottom within a group. It generally increases from top to bottom within a group. This is because, as you move down a group, additional electron shells are added, leading to an increase in the distance between the nucleus and the outer electron shell and a larger atomic radius.