Variation in Physical Properties of s-Block elements Variation in • atomic / ionic radius. • ionization enthalpy • hydration enthalpy • melting point.
s-Block elements • They are called s-block elements because their outermost electron(s) is /are in the s-orbitals. • They are all metals (alkali metals, alkaline earth metals). • They are very reactive. • Most of them have characteristic flame colour.
Trends in atomic/ionic radius 1) Ionic radii of Gp I and II elements is always smaller than its atomic radii. Reason: The cation formed M+ or M2+ has one electron shell less than the atom, thus size is smaller. 2) Atomic and ionic radii increase down a group. Reason: There is one more shell of inner core e- than its group predecessor.
Comparing atomic radii of Group I and II in the same period Atomic radius of Group I > Group II. Reason: • Group II atoms have higher effective nuclear charges, and the outer s-electrons do not screen each other effectively, Group II atoms have higher effective nuclear charges, the e- are pulled stronger inward.
Conclusion: S-Block elements have relatively large atomic radii amongst the elements of the same period because of their small effective nuclear charge
Trends in I.E. 1) Both Gp I and II metals have low first and second ionization enthalpy respectively Reson: Their outer s-electrons are well shielded from the nucleus by inner shells of electrons. 2) I.E. decrease as both groups are descended Reason: As the atomic radius increases down both groups, the outer electrons are further out and better shielded from the nucleus.
Conclusion: • S-block metals have generally low first I.E. among the elements on the same period.
Hydration enthalpy • Hydration enthalpy is the energy released when one mole of aqueous ions is formed from its ions in gaseous state . • e.g. Mn+(g) + aq Mn+(aq) H = -ve • or Yn-(g) + aq Yn-(aq) H = -ve
Trends1) On going down both Groups I and II, the hydration enthalpies of the cations decrease in magnitude (less –ve). Reason: As the ions get larger down the group, the charge density of cations decrease, thus the electrostatic interaction between the polar water molecules and ions get less. 2) Group II cations have higher in magnitude(more –ve)hydration enthalpies than group I cations. Reason: Gp II ions have higher charge and smaller size, thus their charge density is higher.
Trends 1) The melting points of the elements are found to decrease generally down each group. Reason: As go down each group, the ionic size increases, thus the metallic bond strength decreases. 2) The m.pt. of group II metals are generally higher than group I metals in the same period. Reason:Greater no. of valence electrons and smaller ionic size of gp II metals increase the metallic bond strength.
Changes in Group II are irregular Reason: It is partly due to differences in crystal structure of the metals
Flame Tests for Group I and II compounds • When salts of Group I and II elements are brought into a hot flame, some M+/M2+ gain e- momentarily to form atoms. • The electrons of this atoms in the outermost shell of M may also be excited to a higher energy level in the flame. • When the electron drops back to its original position it gives out the energy it obtained during excitation.
Flame Tests for Group I and II compounds • The energy is released in the form of light E = h • It appears as visible light giving the characteristic flame colourations.
How to carry out flame tests? Pt wire is first cleaned of any impurities by dipping it into conc. HCl solution heating it in a non-luminous Bunsen flames till no flame colour is detected the clean wire is then dipped in to a fresh clean portion of conc HCl again, and then into a small sample of powdered compound whose flame colour is to be determined.