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Showing posts with label PERIODIC TABLE. Show all posts
Showing posts with label PERIODIC TABLE. Show all posts

Monday, August 28, 2023

Dobereiner’s law of traids:

It was first attempt towards classification. He arranged similar elements in a group of three elements called triad and the atomic mass of the middle elements of the traid is approximately the arithmetic mean of the other two.

For example : Ca (40), Sr (87.5) Ba (137)

At. wt. of Sr = (40+137)/2= 88.5

88.5 is nearly similar to 87.5 of atomic wt. of Sr

Such a group of elements is called Dobereiner’s triad.


Dobereiner could arrange only a few elements as triads and there are some such elements present in a 
triad, whose atomic weights are approximately equal, e.g.
Fe Co Ni
Ru Rh Pd
Therefore, this hypothesis was not acceptable for all elements.

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

(2) Name of total metalloids present in periodic table ?

(3) Total numbers of elements which are liquid at normal temperature is ?

(4) What is Mendeleev's periodic table ? give important features and draw back of Mendeleev's table.

(5) What is atomic density ? give the periodicity of atomic density in periods and groups.

(6) What is atomic volume ? and what is periodicity of atomic volume in groups and periods ?

(7) Why there are 2, 8 and 8 elements in first, second and third periodic of periods table respectively ? Explain.

(8) In alkali metal group which is the strongest reducing agent in aqueous solution and why?

(9) The electron affinity of sulphur is greater than oxygen. Why?

(10) The first ionization energy of carbon atom is greater than that of boron atom, whereas reverse is true for the second ionization energy. Explain.

Lother Meyer's Curve:

(1) The graphs plotting the atomic volumes against atomic weights are known as Lothar Mayer volume
curves.

(2) The alkali metals have highest atomic volumes.

(3) Alkaline earth metals (Be, Mg Ca, Sr, Ba, etc.) which are relatively a little less electropositive. Occupy
positions on the descending part of the curve.

(3) Halogens and the noble gases (except helium) occupy positions on the ascending part of the curve.

(4) Transition elements have very small volumes and therefore these are present at the bottoms of the curve.
Example: Lother Meyer attempt was based on plotting atomic mass vs
(A) Atomic size 
(B) Atomic volume 
(C) Density 
(D) Melting point
Ans. (B)

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

Sunday, August 15, 2021

Why radii of Ar is greater than the radii of chlorine?

In chlorine, the radii mean the atomic or covalent radii which is actually half the intermolecular distance between 2 atoms whereas in Argon the radii means the Vander waals radii as Argon is not a diatomic molecule. Vander Waal’s radii are actually half the distance between adjacent molecule. So Vander Waal’s radii being larger than atomic radii, Argon has a larger radii than chlorine.

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

Atomic Radius:

(1)     The radius of an atom may be taken as the distance between atomic nucleus and the outermost shell of electrons of the atom.

(2)     According to the Heisenberg’s uncertainty principle the position of a moving electron cannot be accurately determined. So the distance between the nucleus and the outermost electron is uncertain.

(3)     Atomic radius can be determined indirectly from inter nuclear distance between the two atoms in a gaseous diatomic molecule. This internuclear distance between the two atoms is called bond length.

(4)     The internuclear distance between the two atoms can be measured by X– ray diffraction or spectroscopic studies and also nuclear magnetic resonance (NMR) spectrum.

(5)     Atomic radius depends on the type of chemical bond between atoms in a molecule. These are:

(i)   Covalent radius

(ii) Metallic radius or crystal radius

(iii) Vander Waal’s radius

(iv) Ionic radius or Collision radius

(6) Periodicity in Atomic Radius and Ionic Radius: 

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

(2) Name of total metalloids present in periodic table ?

(3) Total numbers of elements which are liquid at normal temperature is ?

(4) What is Mendeleev's periodic table ? give important features and draw back of Mendeleev's table.

(5) What is atomic density ? give the periodicity of atomic density in periods and groups.

(6) What is atomic volume ? and what is periodicity of atomic volume in groups and periods ?

(7) Why there are 2, 8 and 8 elements in first, second and third periodic of periods table respectively ? Explain.

(8) In alkali metal group which is the strongest reducing agent in aqueous solution and why?

(9) The electron affinity of sulphur is greater than oxygen. Why?

(10) The first ionization energy of carbon atom is greater than that of boron atom, whereas reverse is true for the second ionization energy. Explain.

(11) The electronegativities of B, Al, Ga are 2.0, 1.5, 1.6 respectively. The trend is not regular. Explain.

(12) Li2CO3 decomposes on heating but other alkali metal carbonates don’t. Explain.

(13) Of all noble metals, gold has got a relatively high electron affinity. Explain.

(14) What are the increasing order of ioni radii of first group elements in water ?

(15) What are the increasing order of molar conductivity of first group elements in water ?

Covalent radius:

Covalent radius: It is defined as one half of the distance between the nuclei (inter nuclear distance) of two covalently bonded like atoms in a homo nuclear diatomic molecule is called the covalent radius of that.

(A) For Homo atomic Molecules: The covalent radius (rA) of atom A in a molecule A2 may be given as:

The distance between nuclei of two single covalently bonded atoms in homo diatomic molecules is equal to the sum of covalent radii of both the atoms.

Illustrative example (1): A given compound A2 whose total dA-A is 1.4 A0. The atomic (covalent) radius of an atom is.

Solution:  We known that

(B) For Heteroatomic Molecules: In a hetero diatomic molecules AB where the electro negativity of atoms A and B are different, the experimental values of inter nuclear distance dA-B is less than the theoretical value (rA +rB).

(1) Stevenson & Schoemaker Equation (1941):

Covalent radius of heterogeneous molecule like A-B etc determine by Stevenson & Schromaker Equation, if atoms are formed different type of covalent bond i.e. on atom is more electronegative than the other combined atom. Then the covalent radius is calculated by the relation given by Stevenson & Schoemaker, given as:

For a diatomic Hetero molecule:

Bond Length (lA-B) = r+ rB- 0.09(XA-XB)

Where XA= Electronegativity of more electronegative atom

Where XB= Electronegativity of less electronegative atom


Illustrative examples 

(1) The electronegativity of F and H are 4.0 and 2.1 respectively. The percentage ionic character in H and F bond is.

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

(2) Name of total metalloids present in periodic table ?

(3) Total numbers of elements which are liquid at normal temperature is ?

(4) What is Mendeleev's periodic table ? give important features and draw back of Mendeleev's table.

(5) What is atomic density ? give the periodicity of atomic density in periods and groups.

(6) What is atomic volume ? and what is periodicity of atomic volume in groups and periods ?

(7) Why there are 2, 8 and 8 elements in first, second and third periodic of periods table respectively ? Explain.

(8) In alkali metal group which is the strongest reducing agent in aqueous solution and why?

(9) The electron affinity of sulphur is greater than oxygen. Why?

(10) The first ionization energy of carbon atom is greater than that of boron atom, whereas reverse is true for the second ionization energy. Explain.

(11) The electronegativities of B, Al, Ga are 2.0, 1.5, 1.6 respectively. The trend is not regular. Explain.

(12) Li2CO3 decomposes on heating but other alkali metal carbonates don’t. Explain.

(13) Of all noble metals, gold has got a relatively high electron affinity. Explain.

(14) What are the increasing order of ioni radii of first group elements in water ?

(15) What are the increasing order of molar conductivity of first group elements in water ?

A given compound A2 whose total dA-A is 1.4 A0. The atomic (covalent) radius of an atom is.

 Solution:  We known that


Solved Questions:

If covalent radius of A is 0.99 A0 and bond length is 1.46 A0 Calculate the covalent radius of B. XA and XB are 3.5 and 2.5 respectively.

 (A) 0.35 A0              (B) 0.56 A0              (C) 1.98A0               (D) 10.48A0

The covalent radius of B is calculated by the relation given by Stevenson & Schoemaker, given as:

For a diatomic Hetero molecule:

Bond Length (lA-B) = r+ rB- 0.09(XA-XB)

Where XA= Electronegativity of more electronegative atom

Where XB= Electronegativity of less electronegative atom

Bond Length=1.46 A0 = 0.99 A0 + rB - 0.09(3.5-2.5)

 Hence rB = 0.56 A0 


Solved Questions: