1) Atomic radius is the distance from the nucleus to the outermost shell of electrons or the point where electron density is highest. 2) There are three types of atomic radii: covalent, van der Waals, and metallic. 3) Covalent radius is half the distance between bonded atoms. Van der Waals radius is half the distance between adjacent non-bonded atoms. Metallic radius is half the distance between metal ions in a lattice.

1. By- Sanju Sah
St. Xavier’s, college, Maitighar, Kathmandu
Department of Microbiology
Atomic Radius

2. Atomic radius :
⚫Atomic radius is the distance from the centre of
the nucleus to the outermost shell containing
electrons. In other words, it is the distance from
the centre of the nucleus to the point up to
which the density of the electron cloud is
maximum.
⚫ The units for atomic radii are picometers, equal
to 10^-12 meters. As an example, the
internuclear distance between the two hydrogen
atoms in an H2molecule is measured to
be 74pm. Therefore, the atomic radius of a
hydrogen atom is 37 pm.

4. Types of Atomic Radii
Atomic radii are divided into three types:
⚫ Covalent radius
⚫ Van der Waals radius
⚫ Metallic radius

5. 1) Covalent Radius
⚫ Covalent radius is one half the distance between the
nuclei of two covalently bonded atoms of the same
element in a molecule. Therefore, r covalent = ½
(internuclear distance between two bonded atoms).
The internuclear distance between two bonded atoms
is called the bond length. Therefore,
⚫ r covalent = ½( bond length)

6. 2) Van der Waals Radius
⚫ It is one half the distance between the nuclei of two
identical non-bonded isolated atoms or two adjacent
identical atoms belonging to two neighbouring
molecules of an element in the solid state. The
magnitude of the Van der Waals radius is dependent
on the packing of the atoms when the element is in
the solid state.
⚫ For example, the internuclear distance between two
adjacent chlorine atoms of the two neighbouring
molecules in the solid state is 360 pm. Therefore, the
Van der Waals radius of the chlorine atom is 180 pm.

7. 3) Metallic Radius
⚫ A metal lattice or crystal consists of positive kernels or
metal ions arranged in a definite pattern in a sea of
mobile valence electrons. Each kernel is
simultaneously attracted by a number of mobile
electrons and each mobile electron is attracted by a
number of metal ions.
⚫ Force of attraction between the mobile electrons and
the positive kernels is called the metallic bond. It is
one half the internuclear distance between the two
adjacent metal ions in the metallic lattice. In a metallic
lattice, the valence electrons are mobile, therefore,
they are only weakly attracted by the metal ions or
kernels.

8. ⚫ In a covalent bond, a pair of electrons is strongly
attracted by the nuclei of two atoms. Thus, a metallic
radius is always longer than its covalent radius. For
example, the metallic radius of sodium is 186 pm
whereas its covalent radius as determined by its
vapour which exists as Na2 is 154 pm. The metallic
radius of Potassium is 231 pm while its covalent
radius is 203 pm.

9. Variation of Atomic Radii in the Periodic
Table
⚫ Variation Within a Period
The Covalent and Van der Waals radii decrease with
increase in atomic number as we move from left to right in
a period. The alkali metals at the extreme left of the
periodic table have the largest size in a period.
The halogens at the extreme right of the periodic table
have the smallest size. The atomic size of nitrogen is the
smallest. After nitrogen, atomic size increases for Oxygen
and then decreases for fluorine. The size of atoms of inert
gases is larger than those of the preceding halogens.

10. ⚫ As we move from left to right in a period, nuclear
charge increases by 1 unit in each succeeding
element while the number of shells remains the
same. This enhanced nuclear charge pulls the
electrons of all the shells closer to the nucleus. This
makes each individual shells smaller and smaller.
This result in a decrease of the atomic radius as we
move from left to right in a period.

11. ⚫ The atomic radius abruptly increases as we move
from halogens to the inert gas. This is because inert
gases have completely filled orbitals. Hence, the inter-
electronic are maximum. We express the atomic size
in terms of Van der Waals radius since they do not
form covalent bonds. Van der Waals radius is larger
than the covalent radius. Therefore, the atomic
size of an inert gas in a period is much higher than
that of preceding halogen

12. Variation Within a Group
⚫ The atomic radii of elements increase with an
increase in atomic number from top to bottom in a
group. As we move down the group, the principal
quantum number increases. A new energy shell is
added at each succeeding element. The valence
electrons lie farther and farther away from the
nucleus. As a result, the attraction of the nucleus for
the electron decreases. Hence, the atomic radius
increases.