2. 1. Metallic Radii
2. Vanderwaal’s Radii
3. Variation of Atomic Radii Along a Period and
Down the Group, s- & p- block elements
4. Variation of Atomic Radii Along a Period and
Down the Group, d-block elements
3. Metallic Radii
o Metallic radii is defined as one half of the distance
between the nuclei of two adjacent metal atoms in the
metallic close packed crystal lattice in which metal
exhibit a coordination number of 12
o For Na metallic radii = 3.80 / 2 = 1.90 Å
o Metallic radii are 10% to 15% above the covalent
radii.
o Metals in the solid state have closed packed
structures and atoms touch each other.
o Metals have large number of neighbouring atoms
usually 12
o Metallic bond is not a localised bond like covalent
bond but this bond is stronger than Vander Waals
force.
4. VanderWaals Radii
Vander Waals radii is defined as one half of the internuclear
distance between the adjacent atoms belonging to nearest
neighbouring molecule of the same substance in the solid state.
There are some weak forces of attraction
between the neutral noble gas atoms or
in between various non polar
molecules which is called Vander Waals Force.
rVDW is the shortest distance to which two non bonded atoms can
approach before repulsion between their electron clouds come into
play.
Internuclear distance between two non bonded nearest neighboring
atoms for Cl = 3.6 Å, rvdw (Cl) = 3.6/2 Å = 1.80 Å
5. The Vander Waals radii of some atoms are given below along with their covalent
radii.
The Vander Waal radii are larger than the covalent radii because in the
formation of covalent bond the atoms come close to each other due to the
overlapping of orbital.
On the other hand Vander Waal forces existing between atoms or molecules
non bonded weaker and atoms are at large distances.
Hence the internuclear distances in case of atoms held by Van der Waals forces
is greater than between covalently bonded atoms consequently
rVander Waal > rcovalent
Elements N O F Ne P S Cl Ar
rcov (Å) 0.77 0.74 0.72 ----- 1.10 1. 04 0.99 -----
rvdw (Å) 1.50 1.40 1.35 1.31 1.80 1.90 1.80 1.74
7. I. Variation of Atomic Radii Along a Period and Down the Group (s
& p block elements)
(i) atomic radius decreases from left to right in a period.
Alkali metals have highest value and halogens have smallest radius.
(ii) Atomic radius increases on descending a group.
For noble gases the atomic radii is not the covalent radii it is the Bragg-Slater/Van
der Waals radii.
Noble gases do not form covalent bonds. So no chemical forces operate between
the atoms
Only Van der Waals forces at the attractive forces among the atoms of noble
gases so they have Van der Waals radii as the atomic radii.
Reason:
Along a period:
(i) Nuclear charge increases, atomic number increases
(ii) Electrons are added to the same Principal quantum shell
(iii) electrons are more strongly pooled towards the nucleus as nuclear charges
increases. So atomic size decreases
8. Along a group:
(i) Size increases or descending the group new line electrons
are added to a new quantum shell at each state
(ii) Addition of extra shells of electron outweighs the effect of
increased nuclear charges
1s2,2s2,2p6,3s2,3p6,4s2,
3d10,4p6,5s1
37Rb
1s2,2s2,2p6,3s2,3p6,4s119K
1s2,2s2,2p6,3s111Na
1s2,2s13Li
Electronic
configuration
Atomic no.Element
9.
10.
11. II. Variation of Atomic Radii Along a Period and Down the
Group for d- block elements i.e. for transition elements:
The relation of atomic structure and atomic radii along a period
or group is a generalized statement which is true in s and p
block elements.
Along a Period:
For transition series atomic radius decreases from left to right
as the nuclear charge increases.
The decrease is very small within some range in the period .
In the first transition series decrease is very small from Cr to Ni.
For 3d, 4d, 5d all series the same trend is observed.
12. Along a Group:
Increase in size is prominent from 3d to 4d i.e. 1st to 2nd
transition series.
From 4d to 5d series, the radii is virtually same.
4f orbital must be filled before the 5d orbital starts filling.
So, a regular decrease in size occurs due to “Lanthanoid
Contraction”. f-orbitals are poor shielder of nuclear charge.
4d and 5d series have similar physical & chemical properties.
Another remarkable observation is that the atomic radius
increases after that e.g. from Ni to Zn.
13. Two opposing factors:
Poor shielding effect of “d” increase in nuclear charge
decrease the atomic size.
Repulsion takes place among the electron population, size
increases.
From Cr to Ni these two effects opposes each other and
combined effect is not much increase in radius value.
After Ni repulsion forces out weigh and an increase in radius
observed.
If we look at the radii of transition elements that is d- block
elements there will be some exceptions.
14.
15. If we look at the radii of transition elements that is d- block elements there
will be some exceptions.
If we look at the electronic configuration of Magnesium (Mg12), Calcium
(Ca20) and Aluminium (Al13), Gallium (Ga31)
Element Period
No.
Valance shell
electronic
configuration
Atomic
Radius
(Å)
SET-I
Normal
Trend
Mg12 3 1s22s22p63s2 1.60 At. radii
increas
es
Ca20 4 1s22s22p63s23p64s2 1.97
SET-II
Exception
Al13 3 1s22s22p63s23p1 1.26 No
change
in
atomic
radii
Ga31 4 1s22s22p63s23p63d10
4s24p1
1.26
16. Atomic radii value for the pairs remained same but for
Calcium to Gallium (1.97 Å →1.26 Å) the decrease is more
than the decrease in radii seen in Magnesium to Aluminium
(1.60 Å →1.26 Å).
After Ca the d level electrons continue to be added. Radial
distribution function shows that 3d electron do not
extend beyond the 4s electrons. So although the quantum
number increases and new quantum shells are added on
going from Al to Ga poor shielding effect of 3d10 electrons
comes into play. Increase in nuclear charge and poor
shielding power of d10 electron effectively comes into play
and atomic radius remain unchanged.
