The p-Block Elements - II
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.
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
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.
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
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
Colourless crystalline solids
Sublime at 298 K
Powerful fluorinating agents
Readily hydrolysed even by traces of water
XeF4 Square planar
XeF6 Distorted octahedral
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
In filling balloons for meteorological observations as it is a non-inflammable and light gas
In gas-cooled nuclear reactors
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
In discharge tubes and fluorescent bulbs
Neon bulbs − Used in botanical gardens and in green houses
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)
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
PhosphorusOccurs 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.
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
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)
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
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 > SbH3 ≥ BiH3
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 metals to form their binary compounds, exhibiting −3 oxidation state
Example: Ca3N2, Ca3P2, Na3As, Zn3Sb2 and Mg3Bi2
Anomalous Behaviour of Nitrogen
Nitrogen differs from the rest of the elements of this group
Reason: Smaller size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals
Unique ability to form pπ − pπ multiple bond with itself and some other elements
Exists as diatomic molecules with a triple bond; hence, its bond enthalpy is very high (941.4 kJ mol−1). On the other hand, P, As and Sb form single bonds as P−P, As−As and Sb−Sb; bismuth forms metallic bonds
The single N−N bond is weaker than P−P bond.
Reason: Due to small bond length, the interelectronic repulsion of the non-bonding electrons in nitrogen is high
Group 16 Elements
Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po)
Also known as chalcogens
Most abundant of all the elements on earth
Forms about 46.6% by mass of earth’s crust
Dry air contains 20.946% oxygen by volume
Abundance in earth’s crust is 0.03 − 0.1%
Exists as sulphates such as gypsum (CaSO4.2H2O), epsom salt (MgSO4.7 H2O), baryte (BaSO4); as sulphides such as galena (PbS), zinc blende (ZnS), copper pyrite (CuFeS2)
Also occurs as hydrogen sulphide in volcanoes
Present in organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool
Selenium and tellurium − Found as metal selenides and tellurides in sulphide ores
Polonium − Decay product of thorium and uranium minerals
General valence shell electronic configuration is ns2np4.
Atomic and ionic radii increase down the group.
Ionisation enthalpy decreases down the group.
Oxygen has lesser negative electron-gain enthalpy than sulphur because due to small size of oxygen atom, the incoming electron faces a high inter-electronic repulsion due to which its addition becomes difficult and less energy is released. While in the case of sulphur atom, due to its large size, inter-electronic repulsion is less and incoming electron added with an ease, hence it has high negative electron gain enthalpy than oxygen. But from sulphur onwards, the value becomes less negative up to polonium.
Within the g
To view the complete topic, please