Atomic radius is expressed in ______units.
A. angstrom
B. cm
C. m
D. nm

Hint: Try to recall that atomic radius is a measure of the size of an atom. Also, it is known to us that the size of an atom is nearly equal to the order of the wavelength of X-ray. Now by this you can answer this question easily.

Complete step by step solution:
It is known to us that the atomic radius of an atom is defined as the distance from the center of nucleus to the point up to which the probability of finding an electron or electron density is maximum.
The atomic radius of an atom is generally measured by X-ray diffraction technique and is generally expressed in angstrom units $\mathop {\text{A}}\limits^{\text{o}}$.
Picometer(pm) and nanometer(nm) are other units in which you can express atomic radii of elements.
Also, you should know that $1\mathop {\text{A}}\limits^{\text{o}} = {10^{ - 10}}m$
                                             $1nm = {10^{ - 9}}m$
                                             $1pm = {10^{ - 12}}m$.
Hence, from above it can be easily concluded that option A is the correct option to the given question.

Additional information:
Depending upon whether an element is metal or non-metal, three different types of atomic radii namely covalent radii, metallic radii and Vander Waal’s radii are found.
Covalent radius: It is defined as half of internuclear distances between centers of nuclei of two similar atoms bonded by a single covalent bond and is generally found in non-metals or between metal and non-metal.
Vander Waals radius: It is defined as half of the internuclear distance between two adjacent elements of the same element belonging to nearest neighboring molecules of the same element.
Metallic radius: Metallic radius is defined as half of internuclear distance between two adjacent atoms in metal lattice and is found among metals.

Note: You should remember that atomic radii tend to decrease on going from left to right along a period and increase on going down in a group of periodic tables.
Also, it should be remembered that out of three different types of atomic radii, Vander Waal’s radius is largest in size and noble gases tend to possess Vander Waal’s radius.