Atomic radius — পারমাণবিক ব্যাসার্ধ
Atomic radius is the measured size of an atom, half the distance between the nuclei of two bonded identical atoms.
Definition
Atomic radius is the usual measure of an atom's size, worked out as half the distance between the nuclei of two identical bonded atoms, in picometres. Since the electron cloud has no hard edge, this is a practical convention, not a border. Two trends rule it. Down a group, each new shell swells the atom: lithium measures about 150 pm, caesium about 260. Across a period, more protons pull the same shells inward, so sodium (about 180 pm) shrinks steadily to chlorine (about 100 pm).
For example
Compare the two neighbours sodium and magnesium: magnesium's extra proton tightens its cloud, so it is the smaller atom despite sitting to sodium's right in the same period.