Why we study periodic properties
Here we will continue periodicity of a periodic table . In the last post we have seen the important concept of effective nuclear charge . Periodic properties is the main reason why we study periodic table. As we say elements are classified into different categories in periodic table because of its periodic properties.Some of the elements have metallic properties so they are classified into generally in 's' and 'd block' and others which are following non-metallic character ,generally classified in 'p block'. And because of many other such properties the elements are categories into period, group and block ,etc.
So we will study all the periodic properties of elements one by one and we will also see the effect due to effective nuclear charge .
1. ATOMIC RADIUS
Atomic radius of the elements is the distance from the center ( i.e central nucleus ) to the outermost shell where the probability of finding electron become zero . Here you might be confuse that what is probability of finding electron so don't worry we will learn in upcoming post . I will provide You link below after some time.
Types Of Atomic Radius :
- Covalent radius
- Metallic radius
- Vander wall radius
I. Covalent radius
Hope you would know what is covalent bond otherwise we will see in Chemical Bonding . As we know in the formation of covalent bond is due to overlapping of orbitals . So half of the inter-nuclear distance between the two bonded atom is defined as covalent radius.
δ
Bond length is the inter nuclear distance as shown in figure. So covalent radius is the half of the bond length.
Covalent radius is generally applicable for non-metal or p-block element (except inert gas).
Ex. -- H2 O2 N2 F2 Cl2
II. Metallic radius
Metallic bond is the bonding formed between metals due to intermittent presence of valance electron. So metallic radius is half of the inter-molecular distance of two touching atom of metal . Metallic radius is always larger than covalent radius. Since metallic bond is property of metal so it is applicable for s-block , d-block, and f-block elements.
III. Vander Waal radius
Vander-Waal radius is mainly property of inert gas due to formation of Vander-Waal bond. Due to large size of atom there will Vander-Waal force of attraction and that's why this type of bond formation is possible. It is a weak electrostatic force between molecules or atom / noble gas .
Note: 1.Wander-Waal radius is the largest radius .
2.Generally Vander-Waal radius is two times of covalent radii.
VanderWall Radius > metallic Radius > Covalent Radius
General Trend of Atomic radius
- Left to right in the period atomic radius decreases because Zeff (effective nuclear charge) increases in period as we seen earlier .
Ex. Li > Be > B > C > N > O > F
- Inert gas have largest atomic radius in the period due to Wanderwaal radius .
- When we move top to bottom in the group atomic radius increases because the number of shell increases. Here point to be noted that size of atom increases not because of effective nuclear charge ,if you remember we have studied that in group there will not change in effective nuclear charge. So here size increases due to number of shell is increases .
Ex. B 2s2 2p1
Al 3s2 3p1
Ga 4s2 4p1
In 5s2 5p1
Th 6s2 6p1
Irregular Trend in Periodic Table
- As we move from top to bottom in the Boron family the atomic radius from Boron to Aluminium increases abnormally while from Al to Ga it remain almost constant .
Why is this happen ...
As we move from 'B' to 'Al' the atomic size increases abnormally due to strong screening effect of 'S' and 'P' orbital .
Zeff = | Z - σ |
As we move from 'Al' to 'Ga' atomic size is almost constant due to poor screening effect of d-orbital .
Zeff ↑ = Z- σ ↑
Note : This is applicable on B- Family , C- Family and N- Family .
Lanthanide Contraction
It is one of the most important concept in periodic properties . when we move from 4d series (5th peiod ,d-block) to 5d series (6th period , d-block) the atomic size remain almost constant due to poor screening effect of 5d and 4f orbital this phenomena is defined as "lanthanide contraction".
4d series : Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
5d Series : Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Effect due to lanthanide contraction
Effect 1: IIIB IVB VB VIB
Sc Ti V Cr
Y Zr Nb Mo
La Hf Ta W
Atomic radius of 4d series and 5d series are approximately equal
i.e Zr = Hf , Nb = Ta , Mo = W
Effect 2: Inert pair effect
P block elements are generally form co-valent bond so for this excitation of e- will happen .
But in the case of 6th period excitation will not possible due to lanthanide contraction ( more Z* ) and so that nucleus will attract e- with high magnitude . This is inert pair effect.
It is the non-participation of 's' orbital electron in bond formation due to more closer to nucleus and lanthanide contraction . Nucleus have more effective nuclear charge.
IIIA IVA VA
B C N
Al Si P
Ga Ge As
In Sn Sb
Tl Tl+ is stable Pb Pb2+ is stable Bi Bi3+ is stable
IV. Ionic radius
The size of cation is always smaller smaller than its neutral form. In cation, some of the electron is lesser than neutral form.
Ex . Na > Na+ , K > Na+
As the number of +ve charges increases atomic size decreases because due to less electron.
Ex . Fe2+ > Fe3+
As One of the most important concept related to isoelectronic species and is frequently asked in all competitive examinations.
Higher the atomic number lower the atomic radius.
Ex . F- > Na+ > Na+ > Mg2+ > Al3+
Note : H+ is the smallest cation while Cs+ is largest cation and H- is the smallest anion while I- is the largest anion.
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