Atom - is the smallest particle which cannot be seen with the naked eye. So, it is almost impossible to measure the size of an atom.
Technique to measure the size of an atom - because we cannot measure the size of an atom directly we can measure the distance between the centers of two bonded atoms of any elements. Half of the distance will be size/radius of an atom.
Unit - Radius is measured in Angstrom unit (A ) 1A =
Trend in Groups - Atomic radius increases from top to bottom in the groups - because the increase of the atomic number. In groups shell number also increases from top to bottom so with the increase of shell number atomic size also increase. so, as we move down the group size of an atom increases.
Trend in Periods -Size of an atom decreases from left to right - however atomic number is increases but the positive charge also increases from the left to right - So, positive charge attracts the electron towards itself as the result shrinkage of size occurs. so, size decreases gradually but the difference will be obvious in case of “d” and “f” sub-shells. That is why the decrease in size in Lanthanide known as Lanthanide Contraction
