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Group 18 elements, also known as noble gases, are inert in nature because they have completely filled ns2 np6 electronic configuration in their valence shells. These gases also have high ionization enthalpy and more positive electron gain enthalpy. All the given gases except radon occur naturally in the atmosphere. Though Helium is considered a noble gas as per Ramsay’s Periodic Table, it has completely different properties. Helium and neon are found in minerals of radioactive origin like pitchblende, monazite, and cleveite. Xenon and radon are the rarest elements of Group 18. Group 18 Elements consist of six elements. They are as mentioned below: Helium, Neon, Argon, Krypton, Xenon, and Radon. The chapter of Group 18 Elements is a sub-topic of P Block Elements.
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Keyterms: Helium, Neon, Argon, Krypton, Xenon, Radon, Noble Gas, P Block Elements, Atomic Radii, Hydrogen Chloride
Electronic Configuration of Group 18 Elements
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The electronic configuration of Group 18 Elements along with their symbol and atomic number is given in the tabulated form below;
| Element | Symbol | Atomic number | Electronic Configuration |
|---|---|---|---|
| Helium | He | 2 | 1s2 |
| Neon | Ne | 10 | [He] 2s2 2p6 |
| Argon | Ar | 18 | [Ne] 3s2 3p6 |
| Kyrpton | Kr | 36 | [Ar] 3d10 4s2 4p6 |
| Xenon | Xe | 54 | [Kr] 4d10 5s2 5p6 |
| Radon | Rn | 86 | [Xe] 4f14 5d10 6s2 6p6 |
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General Characteristics of Group 18 Elements
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- Electronic configuration of noble gases: Most properties of noble gases including their inactive nature are ascribed to their closed-shell structures. The members of Group 18 except for helium have eight valence electrons in their outermost orbit. Thus, these elements exhibit a stable octet configuration while helium exhibits a duplet configuration. The general configuration of noble gases is given as ns2np6. However, helium has a general configuration of 1s2.
- Atomic radii of noble gases: The members of Group 18 have very small atomic radii. The atomic radii of the noble gases increase with the increase in atomic number due to the addition of new shells.
- Ionization enthalpy of noble gases: Since noble gases except helium have eight valence electrons, they exhibit stable octet or duplet configuration. Therefore, Group 18 elements exhibit high ionization enthalpies. Ionization enthalpy of noble gases decreases with an increase in their atomic size.
- Electrons gain enthalpy of noble gases: The noble gases have stable electronic configurations. Therefore, they have no tendency to accept electrons and thus have large positive values of electron gain enthalpy.
Characteristics of Group 18 Elements
Physical Properties of Group 18 Elements
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- All the noble gases are monatomic ie. They have a stable nature and are in a free state.
- These gases are colorless, odorless, and tasteless.
- They are sparingly soluble in water i.e. slightly soluble in water. From Helium to Radon, the solubility increases.
- These gases have low melting and boiling points due to the weak dispersion forces. Amongst other gases, Helium has the lowest boiling point.
- The Group 18 elements are condensed at very low temperatures. Ease of liquefaction increases as the size of the atom increases.
- Ionization enthalpies of these gases are highest in their respective periods as their electronic configuration is stable.
- Atomic and ionic radius increase with the increase in size as we move down the group.
- Except for helium, all the noble gases are adsorbed by charcoal, and the ease of adsorption increases as we move down the group.
Chemical Properties of Group 18 Elements
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- In general, all the Noble gases are least reactive.
- The main reason for the noble gases inertness to chemical reactivity is as follows; Noble gases except Helium have completely filled ns2 np6 electronic configuration in their valence shell. These gases have high ionization enthalpy and more positive electrons gain enthalpy.
The preparation and properties of compounds are as follows:
Preparation and properties of compounds
- The partial hydrolysis of XeF6 gives oxyfluorides XeOF4 and XeO2F2
- XeF2, XeF4 and XeF6 are crystalline solids and are colorless. These gases are powerful fluorinating agents and are readily hydrolyzed by traces of water.
- XeF6 cannot be stored in glass or quartz vessels as it can readily react with SiO2 that is present in the glass.
- XeO3 is a colorless, explosive solid and has a pyramidal shape. XeOF4 is a colorless, volatile liquid and has a square pyramidal shape.
- XeF2 is linear and XeF4 is a square planar. XeF6 has a distorted octahedral shape.
- The hydrolysis of XeF6 doesn’t lead to a redox reaction; The products of hydrolysis are XeOF4 and XeO2F2 where oxidation states of all elements remain the same as they were in the reacting state.
Uses of Group 18 Elements
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Here are the important uses of Group 18 Elements-
- Helium: It is used in filling balloons for meteorological observation as it is non-inflammable and light gas. Liquid Helium is used as a cryogenic agent to carry out experiments at low temperatures. It is used to produce and sustain superconducting magnets that form a part of modern NMR spectrometers and Magnetic Resonance Imaging systems for clinical diagnosis. Due to its low solubility in blood, Helium is used as a diluent for oxygen in the diving apparatus.
- Neon: It is used in fluorescent bulbs and discharge tubes for advertisement display purposes. They are also used in botanical gardens and greenhouses.
- Argon: It is used to provide an inert atmosphere in high-temperature metallurgical processes and to fill electric bulbs. It is also used in laboratories to handle substances that are air sensitive.
- Xenon and Krypton: There are not many significant uses of these gases. They are used in light bulbs designed for special purposes.
| Noble Gas | Application |
|---|---|
| Helium; He | To full airship and weather baloons. To cool metals into superconductor |
| Neon; Ne | Used in advertising lights and television tubes. |
| Argon; Ar | To fill light bulbs. To provide inert atmosphere for weilding at high tempertaure. |
| Krypton; Kr | Used in lasers to repair the retina of the eye. To fill photographic flash lamp. |
| Xenon; Xe | For making electron tubes and stroboscopic lamps. |
| Radon; Ra | Used in the treatment of cancer. |
Things to Remember
- Group 18 Elements are also known as the Noble Gases, and it is a sub-category of the p block elements.
- According to the Merrium-Webster Dictionary, the noble gases can be described as- “any of a group of rare gases that include helium, neon, argon, krypton, xenon, and usually radon and that exhibit great stability and extremely low reaction rates”
- These gases are colorless, odorless, and mostly unreactive
- Group 18 has six elements- Helium, Neon, Argon, Krypton, Xenon, and Radon.
- Helium is used in filling balloons for meteorological observation as it is very light, whereas Neon is normally used in fluorescent bulbs on the display boards.
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Sample Questions
Ques. Why are the noble gases inert in nature? (1 marks)
Ans. The noble gases except for Helium are inert in nature because they have completely filled ns2 np6 electronic configuration in their valence shells. These gases also have high ionization enthalpy and more positive electron gain enthalpy.
Ques. What are the binary fluorides of Xenon? (1 marks)
Ans. The binary fluorides of Xenon are as follows: XeF2, XeF4, and XeF6
Ques. Why do noble gases have very low boiling points? (1 marks)
Ans. The noble gases are in general monoatomic because they have no interatomic forces except weak dispersion forces and thus, they are liquefied at low temperatures, and therefore, these gases have very low boiling points.
Ques. (a) Account for the following :
(i) Reducing character decreases from SO2 to TeO2.
(ii) HClO3 is a stronger acid than HClO.
(iii) Xenon forms compounds with fluorine and oxygen only.
(b) Complete the following equations :
(i) 4NaCl + MnO2 + 4H2SO4 →
(ii) 6XeF4 + 12H2O → (2017)
Ans. (a) (i) Reducing character decreases from SO2 to TeO2 because the pπ – pπ bonds in them become weaker with increase in size and bond length along the group.
(ii) HClO3 is a stronger acid than HClO because with increase in oxidation state and oxidation number the acidic character increases i.e. HClO3 (+5) and HClO (+1)
(iii) Xenon forms compounds with fluorine and oxygen only due to their high electronegativity and reactivity. The first ionisation energy of it is fairly close to that of O2 and F2.
(b) (i) 4NaCl + MnO2 + 4H2SO4 → 4NaHSO4 + MnCl2 + 2H2O + Cl2
(ii) 6XeF4 + 12H2O → 4Xe + 2XeO3 + 24HF + 3O2
Ques. (a) When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved. This gas intensified when copper turnings were added to this test tube. On cooling the gas (A) changed into a colourless solid (B).
Identify (A) and (B). Write chemical reactions involved.
(b) Draw structure of XeOF4. (2018)
Ans. (a) The salt is sodium nitrate which is heated with a cone. H2SO4 evolves a brown gas i.e. NO2 which gets intensified when Cu turnings are added.
On cooling, NO2 condenses as a brown liquid which turns pale on cooling and eventually becomes a colourless solid due to formation of dimerized NO2 i.e. N2O4 (Dinitrogen tetroxide).
A: Nitrogen dioxide (NO2)
B : Dinitrogen tetroxide (Na2O4)
(b) Structure of XeOF4:
Structure of XeOF4
Ques. (a) Account for the following:
(i) Helium is used in diving apparatus.
(ii) Fluorine does not exhibit positive oxidation state.
(iii) Oxygen shows catenation behaviour less than sulphur.
(b) Draw the structures of the following molecules:
(i) XeF2 (ii) H2S2O8 (Delhi 2013)
Ans. (i) Helium is used in diving apparatus because of its very low solubility in blood and therefore an oxygen-helium mixture is used for artificial respiration. (ii) Because it is the most electronegative element and does not have d-orbitals for octet expansion, therefore it shows only a negative oxidation state of -1.
(ii) The greater catenation tendency of sulphur is due to two reasons :
(a) The lone pair of electrons feels more repulsion in 0-0 bonds than S-S bonds due to its small size and thus S-S forms strong bonds.
(b) As the size of the atom increases down the group from O – PO, the strength of bond increases and therefore catenation tendency also increases.
(b) (i) XeF2:
XeF2
(ii) H2S2O8:
H2S2O8
Ques. Account for the following :
(a) Phosphorus shows high tendency for catenation.
(b) F2 is more reactive than ClF3 but ClF3 is more reactive than Cl2.
(c) Nitrogen is found in gaseous state.
(d) Decomposition of ozone molecules is a spontaneous process.
(e) SF6 is inert towards hydrolysis. (Delhi 2013)
Ans. (a) The bond strength of P-P is more than N-N, therefore, phosphorus shows more tendency for catenation than nitrogen.
(b) Because the bond between Cl-F is weaker than the bond between Cl-Cl due to less effective overlapping of orbitals of Cl-F than Cl-Cl.
The F2 is most reactive due to its high electrode potential.
(c) Due to small size and high electronegativity nitrogen forms pπ – pπ multiple bonds, which are held together by weak Van-der Waals forces of attraction which can be easily broken. Hence N2 exists as a gas at room temperature.
(d) Because O3 is thermodynamically unstable and its decomposition results in liberation of heat (ΔH is -ve) and increase in entropy (ΔS is +ve) so (ΔG becomes -ve and the process becomes spontaneous.
(c) In SF6, S is protected by 6F atoms and does not allow H2O molecules to attach on it. So SF6 inert towards hydrolysis.
Ques. Account for the following :
(i) Chlorine water loses its yellow colour when standing.
(ii) BrCl3 is more stable than BrCl5.
(iii) Fluorine does not form oxoacids.
(iv) PCl5 acts as an oxidising agent.
(v) SO2 is an air pollutant. (Comptt. All India 2012)
Ans. (i) Cl2 water on standing loses its yellow colour due to the formation of HCl and HOCl. HOCl is unstable and decomposes to HCl. As a result, the yellow colour disappears.
Cl2 + H2O → HCl + HOCl
2HOCl → 2HCl + O2
(Colourless)
(ii) In BrCl3, Br can accommodate three chlorine atoms around it and hence it is stable but in BrCl5, five Cl atoms cannot be accommodated around Br and hence it is unstable.
(iii) Due to high electronegativity and small size F does not form oxoacids.
(iv) In PCl5, the oxidation state of phosphorus is +5. Thus, it cannot increase its oxidation state beyond +5. So PCl5 cannot act as a reducing agent. However, it can decrease its oxidation state from +5 to +3 or to some lower value. Thus, it acts as an oxidising agent.
(v) SO2 is a pungent and irritating gas. It acts as an air pollutant due to the following reasons:
- It causes throat and eye irritation as it is absorbed readily by the respiratory tract.
- It combines with moisture forming sulphurous acid. It is then converted into H2SO4. Both these acids cause acid rain and destroy the marble, corrode metals, deteriorate fabrics, paper, leather etc.
Ques. (a) Complete the following chemical equations :
(i) Cu + HNO3 (dilute) →
(ii) XeF4 + O2F2 →
(b) Explain the following observations :
(i) Phosphorus has a greater tendency for catenation than nitrogen.
(ii) Oxygen is a gas but sulphur a solid.
(iii) The halogens are coloured. Why? (All India 2012)
Ans. (a) (i) 3Cu + 8HNO3 (dil) → 3CU(NO3)2 + 2NO + 4H2O
(ii) XeF4 + O2F2 → XeF6 + O2
(b) (i) The self linking property or catenation property of nitrogen is less than that of phosphorus because the N – N bond is weaker than P – P bond.
(ii) Oxygen forms a stable diatomic molecule. In 02 molecules two atoms of oxygen have joined together through double bond 0 = 0. Multiple bonding is possible due to the small size of oxygen atoms. So oxygen is a gas. S has eight atoms arranged in the form of a puckered ring per molecule in which two sulphur atoms are joined by covalent bonds. So sulphur is a solid at room temperature.
(iii) All halogens are coloured. This is due to absorption of radiation in the visible region which results in the excitation of outer electrons to higher energy levels. So they display different colours.
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