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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.

Occurrence

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

Question

Why do noble gases have very low boiling points?

Answer

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

Reason:

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

Example:

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).

Properties

XeO3 is a colourless explosive solid.

XeOF4 is a colourless volatile liquid.

Structure

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)

Occurrence

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 > 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

Occurrence

Oxygen 

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

Sulphur

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

Atomic Properties 

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 group,

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