PERIODICITY IN PROPERTIES
The periodicity is repetition of elements with similar properties after certain regular intervals when the elements are arranged in order of increasing atomic number.
The periodic repetition of properties is due to the recurrence of similar valence shell configurations after regular intervals. Periodicity in periodic table
Factors affecting atomic size
(a) n increase size increases (n = principal of quantum number)
(b) Zeff increase, size decrease [Zeff = Z – σ]
In a period (L → R) : Atomic size decreases from left to right in a period.
Looking to the E.C. of elements, following observations are made,
This results in net contraction of atomic size.
In a group (up to down): Atomic size increases from top to down in a group.
The electronic configuration of elements of the same group indicates that e-s are added in the new shell in the senior atom. i.e. a new shell is added in the next higher group.
An atom does not have any sharp boundary,
The atomic radii of atoms of compounds are classified as :
(i) Covalent radii (in covalent molecules),
(ii) Ionic radii (in ionic crystals)
(iii) Van-der-wall radii (in atomic crystals)
(iv) Metallic radii (in metals)
Variation in atomic radius in a group :
The distance between the nucleus and the outer most shell of an ion is known as ionic radii.
|Cationic Radii (in Å)||Na(1.54) & Na+(.95)||Mg(1.36) & Mg+2(.65)|
|Anionic Radii||F(.72) & F–(1.36)||Cl(.99) & Cl–(1.81)|
Size of isoelectronic ions:
(i) C+4 < B+3 < Be2+ < Li+
(ii) Al3+ < Mg2+ < Na+ < Ne < F– < O2– < N3–
Example: Compare size of the following
(i) S2- , Cl- , K+ , Ca 2+ , Sc 2+
(ii) SO3 , NO3- , CO32- , COCl2
(iii) N2 , CO , CN-
(iv) NH3 , H3O+
(v) H- , He , Li+
(vi) CH4 , NH4+
(vii) NCS- , CS2
Ionisation Energy Amount of energy required to remove the most loosely bounded electron from an isolated gaseous atom.
Ionisation is endothermic (endoergic) i.e. requires energy hence
Unit: lonisation energy is expressed in the units of energy, i.e. (ev) per atom; (kJ mol-1); kcal mol-1
1 eV/atom = 1.602 ´ 10-16 kJ / atom = 1.602 x 10-16 x 6.02 x 1023 kJ / mol = 96.4 x106 kJmol -1
Factors Affecting I.E. :
(i) Size of Atom: Varies inversely
(ii) Nuclear Charge: Varies directly
(iii) Screening Effect: Varies inversely
(iv) Nature of orbitals containing e-s: (Penetrating effect of e-s)
(v) Electronic Configuration (E.C.) : The atoms having stable E.C. have less tendency to lose es. So have high I.E.
(a) Half filled E.C.: N = 1s2, 2s2, 2p3
(b) Full filled E.C. (Noble gas configuration): Ne = 1s2, 2s2, 2p6
(c) E.C. with all e-s paired: E.g. Be = 1s2, 2s2; Zn = 3d104s2; Cd = 4d10 5s2; Hg = 5d106s2
General Trend Across the Period: L → R .
Along period I.E. increases [with some exception] [Zeff]
Reason: (a) Nuclear Charge (b) Atomic Size
Example: Explain that
(i) Li < Be, (ii) Be > B, (iii) B < C < N, (iv) N > O, (v) O < F < Ne
In a Group : Along a group I.E. decrease [Zeff constant, n ]
(a) Atomic size increases
(b) Increase in screening effect of electrons of inner shells also increase which nullify the increased Nuclear charge.
Fig. : First ionisation energies of the elements
1. Maxima and minima occur at the noble gases and alkali metals respectively indicating special stability associated with closed shell configuration of noble gases.
2. The variation in I.E. along a series of transition metal or inner transition element is much less than that along a period of non transition elements.
3. Half filled E.C. have higher I.E. than the next element along a period (N > O) (P > S).
- Determination of I.E. : I.E. is the difference between energy of a gaseous atom and that of the system, gaseous ion + free e–.
(i) Li < B< Be<C <O < N < F < Ne
(ii) Na < Al < Mg < Si < S < P < Cl < Ar
(iii) K < Ga < Ca < Ge < Se < As < Br < Kr
(iv) Rb < Sr < In < Sn < Sb < Te < I < Xe
(v) Cs < Ba < Tl < Bi < Pb < Po < At < Rn
I.E. variation in transition metals with dn E.C.
(1) Along a period, half filled and fully filled have higher I.E.
e.g. Be > B and N > O.
(2) along a group, Ga > Al
Properties affected by Ionisation Energy:
(1) Metallic character : More ionisation energy means less metallic character (in a period).
(2) Tending to stay in which state A+1, A+2 or A+3 is stable.