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Physical Properties Of S-block Elements

Physical Properties of S-Block Elements Assignment / Homework Help
S-block elements are those that have the s-sub shell filled with electrons. Alkali metals are those elements that have one electron in their outermost s-sub shell. In this session let us discuss the properties of alkali metals.

Elements included under Alkali metals:
  • lithium, sodium, potassium, rubidium, caesium and francium.
Physical properties of Alkali Metals:

All alkali metals have one electron in the s-sub shell. Due to the presence of only one electron in the outermost shell, they are highly reactive so as to obtain an octet configuration. Due to the presence of similar electronic configuration, all the elements show similarity in their properties.

  • Physical state: all the elements exist as silvery white, soft and light metals. These are considered as metals due to very low ionisation energies. These elements are malleable i.e. can be compressed into sheets and are also ductile i.e. they can be drawn in to wires. They are lustrous when freshly cut. But, the lustre tarnishes very easily upon exposure to air.
  • Atomic radius:as we know alkali metals are the largest elements of the corresponding periods. Atomic radius increases from lithium to Francium as we move down the period.
  • Density:density increases as we move from Li to Fr. But, an anomaly exists in case of sodium and potassium where K is lighter than Na. Li is the lightest metal among all the metals. Li, Na and K are lighter when compared to water.
  • Melting and Boiling points:due to the presence of large atomic radius, all these elements bond very weakly at the time of forming crystals. Due to weak inter-atomic bonding, all these elements have very low melting and boiling points. The melting and boiling points decrease as we move down from Li to Cs.
  • Ionisation Energy:due to less attraction of the nucleus on the outer most electrons, the outermost electron of all these elements can be very easily removed. Hence, these elements have very low ionisation energy. It decreases as we move down the group from Li to Cs.
  • Electropositive Character:due to low ionisation energy, alkali elements lose the electrons very easily. Hence, they can be said as strongly electropositive. The electropositive character increases as we move down the group from Li to Cs. Due to their strong electropositive nature, they emit electrons when exposed to light. This effect is termed as photo-electric effect. These elements are greatly used in photoelectric cells. As caesium and potassium are highly electropositive when compared to other elements, they are greatly used in the photoelectric cells when compared to other alkali metals.
  • Oxidation State:as alkali metal atoms have only one electron in the valence shell, they exhibit an oxidation state of +1. By losing one electron, these alkali metals form a unipositive ion that has a noble gas configuration. Hence, it is very difficult to remove an electron from these metal ions resulting in very high second ionization energies.
  • Electro negativity:they show very less tendency to attract electrons i.e. very low electronegativity. Electronegativity decreases from Li to Cs.
  • Conductivity:: Due to the presence of a single electron that can be easily removed in the valence shell, alkali metals act as very good conductors of heat and electricity. Conductivity increases from Li to Cs.
  • Flame colourisation:alkali metals have the ability to impart colour to the flame. This property of alkali metals is used in their identification. Unlike other elements, alkali metals do not form insoluble compounds due to poor ionization energies. Hence, they cannot be identified through chemical tests. Characteristic colours given by alkali metals include Li-crimson red, Na- golden yellow, K- pale violet, Rb and Cs- violet.
    The main reason for the colour emission is when the alkali metals are introduced into the flame, the single electron present in the valence shell gets excited and reaches the higher energy levels. Now, these excited electrons emit light while returning to the original energy level. As the amount of liberated energy is low for lithium, the wavelength of the light emitted corresponds to the red region of the spectrum. The liberated energy increases as we move down to Cs. Hence, the wavelength of the light emitted by Cs corresponds to violet region of the spectrum.
  • Hydration of ions:they are easily soluble in water. The extent of hydration of the cations depends on their size. Smaller the size of the ions greater will be the charge density. Hence, greater will be ability to draw electrons from molecules and hence, the solubility. Lithium that forms the smallest cation gets easily dissolved in water.

    The degree of hydration decreases as we move down the period.

    Li+ > Na+ > K+ > Rb+ > Cs+

    Lithium ion that is heavily hydrated cannot move very easily and is hence a poor conductor of heat and electricity. Hence, Caesium has high conductivity compared to other alkali metals.
  • Hydration Energy: it is the energy liberated at the time of hydration. As the degree of hydration decreases as we move down the period, hydration energy decreases from Li to Cs.

Solved problems
  • Test that is used for identification of alkali metals
    • Flame test
    • Chemical test
    • Solubility
    • None
    Answer: a
  • Flame colour given by lithium
    • Violet
    • Green
    • Purple
    • Crimson Red
    Answer: d
  • Energy liberated at the time of hydration of ions is
    • Ionization energy
    • Hydration energy
    • a and b
    • None
    Answer: b
  • Oxidation state exhibited by alkali metals
    • +1
    • +2
    • +3
    • a or b or c
    Answer: a

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