F7 S-block Physical Properties

Variation in Physical Properties of s-Block elements

Variation in atomic / ionic radius.ionization enthalpyhydration enthalpymelting point.

s-Block elementsThey 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.

Group I

[X]ns1

Li

Na

K

Rb

Cs

Group I

Group II

[X]ns1

[X]ns2

Li

Be

Na

Mg

K

Ca

Rb

Sr

Cs

Ba

Variation in atomic and ionic radii

Trends in atomic/ionic radius

Li

Be

Na

Mg

K

Ca

Rb

Sr

Cs

Ba

increases

( increase of e- shells

decreases

( increase of effective nuclear charge

Trends in atomic/ionic radius1) 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 periodbecause of their small effective nuclear charge

Variation in Ionization Enthalpy

Variation in Ionization Enthalpy

Trends in ionization enthalpy

Li

Be

Na

Mg

K

Ca

Rb

Sr

Cs

Ba

decreases

( atomic radius increases

outer e- are further out

and better shielded from the nucleus

increases

( nuclear charge / ENC increases

1) Both Gp I and II metals have low first and second ionization enthalpy respectivelyReson: Their outer s-electrons are well shielded from the nucleus by inner shells of electrons.

2) I.E. decrease as both groups are descendedReason: As the atomic radius increases down both groups, the outer electrons are further out and better shielded from the nucleus.Trends in I.E.

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

Variation in Hydration Enthalpy of Gp I metal ions (always exothermic)

Variation in Hydration Enthalpy of Gp II metal ions (always exothermic)

Trends in hydration enthalpy

Li+

Be2+

Na+

Mg2+

K+

Ca2+

Rb+

Sr2+

Cs+

Ba2+

decreases

( ion gets larger,

charge density falls

increases

( ion gets higher charge but smaller size,

charge density increases

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.

Variation in Melting Point

Trends in Melting Point

Li

Be

Na

Mg

K

Ca

Rb

Sr

Cs

Ba

decreases

( ionic radius increases

e- cloud becomes more diffuse

weaker attraction between e-

and the ion

increases

( 2 e- per atom participate

in metallic bond for Group II,

1 e- for Group I metals

Trends1) 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.

Reason: It is partly due to differences in crystal structure of the metalsChanges in Group II are irregular

Group I element

Structure

Group II element

Structure

Li

Na

K

Rb

Cs

b.c.c.

b.c.c.

b.c.c.

b.c.c.

b.c.c.

Be

Mg

Ca

Sr

Ba

h.c.p.

h.c.p.

f.c.c.

f.c.c.

b.c.c.

Flame Tests for Group I and II compoundsWhen 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 compoundsThe energy is released in the form of lightE = hIt appears as visible light giving the characteristic flame colourations.

Characteristic flame colours ofGroup I and II compounds

Group I

Flame colour

Group II

Flame colour

Li

crimson

Be

no characteristic colour

Na

yellow

Mg

no characteristic colour

K

lilac

Ca

brick red

Rb

red

Sr

crimson

Cs

blue

Ba

apple green

How to carry out flame tests? Pt wire is first cleaned of any impurities by dipping it into conc. HCl solutionheating it in a non-luminous Bunsen flames till no flame colour is detectedthe 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.

Lilac flame of potassium

Golden yellow flame of sodium

Brick-red flame of calcium

Apple green flame of barium

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