Periodic properties

Atomic radii:

“The distance from the center of the nucleus to the outermost shell containing electrons”.  Since absolute value of atomic size cannot be determined, instead, it is determined in terms of operational definition such as covalent radius, van der Waals radius etc.

Along the period; Normally, covalent radius and van der Waals radius decreases from left to right because left to right shells remain constants and the relative or effective nuclear charge is increased.
 *Alkali metals have the largest size in a period.                                                                                             

*Atomic size order in; Inert gas > Alkali metals > other group elements > halogen > Nitrogen.

*After nitrogen the atomic size for oxygen increases for oxygen and then decreases for F (why?) because in nitrogen no inter electronic repulsion is observed due to three unpaired electrons in different orbitals but in case of oxygen two paired electron repel each other hence due to this inter electronic repulsion the atomic size increases from N to O. in case of F, the enhanced nuclear charge predominates over inter electronic repulsions.

*The size of the atoms of inert gases are larger than others, (why?) because here van der Waals radius is considered since they do not form covalent bond and van der Wall Radius  is greater than  covalent radius always.

*Periodic order of atomic radii; s block > d block > p block.

*The size of the atoms of inert gases are, however, larger than those of the preceding halogens because here van der Wall Radius (> metallic radius > covalent radius) is considered not covalent radius.

*d and f – blocks elements show exceptional properties.

*d – block; the covalent radii of the elements decrease from left to half midway (to Cr) and the radii of the elements, from Cr to Cu are very close to one another until near the end where the size increase slightly, across a period; (Why?) because on passing from left to right, extra protons are placed in the nucleus and extra orbital electrons are added. The orbital electrons shield the nuclear charge incompletely (because shielding effect order s > p > d > f). Because of this poor shielding effect by d - electrons, the nuclear charge attracts all of the electrons more strongly: hence a contraction in size occurs.¹

            The radii of the elements, from Cr to Cu are very close to one another; this is due to the fact that the successive addition of d – electrons screens the outer electron (4s) from the inward pull of the nucleus. As a result of this, the size of the atom does not alter much in moving from Cr to Cu.²

            Near the end of the period, there is a slight increase in the atomic radii; this is due to the fact that near the end of the series, the electron - electron repulsions between added electrons in the same orbitals are greater than the attractive forces due to the increased nuclear charge. This result in the expansion of the electron cloud and thus the atomic radius increases.

Sc 1.44

Ti 1.32

V 1.22

Cr 1.17

Mn 1.17

Fe 1.17

Co 1.16

Ni 1.15

Cu 1.16

Zn 1.25

*The elements in the 3^rd group of the d-block show the expected increase in size Sc ----Y-----La.

*The elements in the 4^th to 12^th group of d – block the atomic radii of the first and second transition elements show expected increases but  radii of the second  and third transition series are almost same;  (why?) this is due to the fact in the atoms of the second transition series, the number of shells increases, their atomic radii are larger than those of the elements of the first transition series but in second and third transition metals between  lanthanum and hafnium there are  14 lanthanide elements are present, in which the antepenultimate 4f shell of electrons is filled. There is a gradual decrease in size of the 14- lanthanide elements from Ce to Lu. This is called lanthanide contraction. Thus the lanthanide contraction cancels almost exactly the normal size increases on descending a group of transition elements.

*Along the group; Normally, size is increased from top to bottom (why?) because top to bottom number of shells is increased and the relative or effective nuclear charge is decreased.

Ionic Radii:-

“The effective distance from the center of the nucleus of the ion upto which it exerts its influence on its electronic cloud is called ionic radii.”  Cation always has smaller radius and an anion is always larger than it parent atom.

*In case of isoelectronic ions, i.e., ions having same number of electrons but different nuclear charge, ionic radius decreases with increase in nuclear charge.

*Ionic radii follow the same trend as the atomic radii.

*KU 2012; How does the atomic size of elements vary in the periodic table and why?

Answer; Normally, decreases from left to right because left to right shells remain constants, the relative or effective nuclear charge is increased.

*Periodic order of atomic radii; s block > d block > p block.

*d and f – blocks elements show exceptional properties.

*d – block; the covalent radii of the elements decrease from left to half midway (to Cr) and the radii of the elements, from Cr to Cu are very close to one another until near the end where the size increase slightly, across a period; ), because of poor shielding effect by d – electrons.

KU 2005; Why does the size of an anion is greater than its parent atoms?

KU 2011; Which has the smallest size?

(a)    Cl

(b)   Cl^-

(c)    Br

(d)   Br^-

Answer; Cl < Br < Cl^- < Br^-

KU 2011; Why is the size of Na⁺ ion less than of Na atom?

KU 2010; Which following has largest size?

(a)    Al

(b)   Al⁺

(c)    Al⁺²

(d)   Al³⁺

Answer; Al > Al⁺ > Al²⁺ > Al³⁺

KU 2009, 2006; Which following has largest size?

(a)    Li²⁺

(b)   Be²⁺

(c)    O^2-

(d)   N^3-

Answer; N^3- > O^2- > Be²⁺ > Li²⁺

Hint; O and N are isoelectronic, Li and Be follows normal order

KU 2009; Why is the size of Na⁺ ion less than of Na atom and that of Cl^- is bigger than Cl atom?

*Arrange the following ions in order of their increasing ionic radii:

Li⁺, Mg²⁺, K⁺, Al³⁺.

Answer;    Al³⁺ < Li⁺ < Mg²⁺ < K⁺

Hint;   Al³⁺and Mg² are isoelectronic.  Li⁺ and K⁺ follow normal trend.

*Arrange the following ions in the order of increasing size:

Be²⁺, Cl^-, S^2-, Na⁺, Mg²⁺, Br^-.

Answer;  Be²⁺ < Mg² < Na⁺ < Cl^- < S^2- < Br^-.

*Arrange the following in decreasing order of their van der Waals radii:

Cl, H, O, N.

Answer; Cl > N > O > H

Hint; The van der Waals radii increases s the number of energy shells increases and decreases as the nuclear charges increases.

*Atomic size of N < O but van der Waals radii is N > O. (why?)  Because both N and O have two energy shells but the nuclear charge on O (+8) is higher than that on N (+7).

*Among the elements B, Al, C and Si which has the largest radii?

Answer;  Al > B > Si > C.

 Some useful information for you!!!

1.      For Rajya sabha how many members are nominated by president? 12

2.      Chankya was the chief advisor of which ruler? Chandragupta Maurya.

3.      What is the direction of rotation of earth on its axis? West to East

4.      In Computer, One byte equals to how many bits? 8

5.      Which is the highest civilian award in India? Bharat Ratna.