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The p-Block Elements

General Trends of Group 15 Elements-Group B

Group 18 Elements

  • Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn)

  • All are gases.

  • Chemically uncreative − Hence, they are termed as noble gases.


  • Occur in atmosphere (except Rn)

  • Atmospheric abundance in dry air is 1%. (Argon is the major constituent)

  • Xe and Rn are the rarest elements of the group.

  • Rn is the decay product of 226Ra.

  • He and Ne are found in minerals of radioactive origin such as pitchblende, monazite, cleveite, etc.

Atomic Properties

  • General outer electronic configuration is ns2 np6.

Exception − He (1s2)

  • High ionisation enthalpy

    • Reason − Stable electronic configuration

    • However, ionisation enthalpy decreases down the group (that is, with the increase in atomic size).

  • Atomic radii increase down the group.

  • Large positive values of electron gain enthalpy

Reason − Stable electronic configurations

Physical Properties

  • Monoatomic

  • Colourless, odourless, and tasteless

  • Sparingly soluble in water

  • Low melting and boiling points.

He has the lowest boiling point of 4.2 K.

  • Unusual property of diffusing through most commonly used laboratory materials such as rubber, glass, or plastics


Why do noble gases have very low boiling points?


Being monoatomic, they have no interatomic forces except weak dispersion force.

Hence, they are liquefied at very low temperature and they have low boiling points.

Chemical Properties

  • Less reactive


  • Completely filled ns2 2p6 valence shell electronic configuration (Except He − 1s2)

  • High ionisation enthalpy and more positive electron gain enthalpy

  • First noble gas compound − Xe+PtF6

  • Obtained by mixing PtF6 and Xe

Xenon-Fluorine Compounds

  • XeF2, XeF4, and XeF6

    • Obtained by the direct reaction of elements

  • XeF6 can also be obtained by the interaction of XeF4 and O2F2 at 143 K.

XeF4 + O2F2 XeF6 + O2

  • Properties

  • Colourless crystalline solids

  • Sublime at 298 K

  • Powerful fluorinating agents

  • Readily hydrolysed even by traces of water


  • Structure

  • XeF2 Linear

  • XeF4 Square planar

  • XeF6 Distorted octahedral

  • Chemical Properties

Xenon-Oxygen Compounds

  • Hydrolysis of XeF4 and XeF6 with water gives XeO3.

  • Partial hydrolysis of XeF6 gives XeOF4 and XeO2F2 (oxyfluorides).


  • XeO3 is a colourless explosive solid.

  • XeOF4 is a colourless volatile liquid.


  • XeO3 has a pyramidal molecular structure.

  • XeOF4 has a square pyramidal molecular structure.

Uses of Noble Gases

  • Helium

  • In filling balloons for meteorological observations as it is a non-inflammable and light gas

  • In gas-cooled nuclear reactors

  • Liquid Helium

  • As a cryogenic agent

  • As a diluent for oxygen in modern diving apparatus

Reason − Very low solubility in blood

  • To produce and sustain powerful superconducting magnets, which are essential parts of modern NMR spectrometers and MRI instruments

  • Neon

  • In discharge tubes and fluorescent bulbs

  • Neon bulbs − Used in botanical gardens and in green houses

  • Argon

  • To provide an inert atmosphere in high temperature metallurgical processes

  • For filling electric bulbs

  • In the laboratory for handling substances that are air-sensitive

  • Xenon and Krypton

  • In light bulbs designed for special purposes

Group 15 Elements

  • Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi)


  • Nitrogen

  • Comprises 78% of the atmosphere (by volume)

  • Occurs as sodium nitrate (NaNO3) and potassium nitrate (KNO3) in earth’s crust

  • Found in the form of proteins in plants and animals

  • Phosphorus

    • Occurs in minerals of the apatite family Ca9(PO4)6.CaX2 (X = F, Cl or OH), which are the main components of phosphate rocks
    • Essential constituent of animal and plant matter
    • Present in bones as well as in living beings
    • Phosphoproteins are present in milk and eggs.
    • Arsenic, antimony and bismuth are found mainly as sulphide minerals.

Atomic Properties

  • General valence shell electronic configuration is ns2np3

  • Covalent and ionic radii increase down the group

  • Ionisation enthalpy decreases down the group due to gradual increase in atomic size. Order of successive ionisation enthalpies: ΔiH1 < ΔiH2 < ΔiH3

  • Electronegativity decreases down the group, with increase in atomic size

Physical Properties

  • Polyatomic

  • N2 is a diatomic gas; all others are solids

  • Metallic character increases down the group

  • N and P: non-metals; As and Sb: metalloids; Bi: metal

  • Boiling points increase down the group

  • Melting point increases up to arsenic, and then decreases up to bismuth

  • All the elements show allotropy (except nitrogen)

Chemical Properties

  • Common oxidation states are −3, +3 and +5

  • Stability of +5 oxidation state decreases down the group

  • Tendency to exhibit −3 oxidation state decreases down the group

    • Reason: Increase in size and metallic character

  • Nitrogen can exhibit +1, +2, +4 oxidation states

  • Phosphorus exhibits +1 and +4 oxidation states in some oxoacids

  • All oxidation states from +1 to + 4 of nitrogen tend to disproportionate in acid solution

    • Example:

  • Nearly all intermediate oxidation states of phosphorus disproportionate into +5 and −3, both in alkali and acid


  • Reactivity with hydrogen

  • All form hydrides of the type EH3 (E = N, P, As, Sb or Bi)

  • Basicity order: NH3 > PH3 > AsH3 > SbH3BiH3

  • Reactivity with oxygen

  • All form two types of oxides: E2O3 and E2O5

  • Acidic character of oxides decreases down the group

  • Reactivity towards halogens

  • React to form two series of halides: EX3 and EX5

  • Nitrogen does not form pentahalides due to non-availability of the d-orbitals in its valence shell

  • Reactivity towards metals

  • All react with meta‚Ķ

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