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CBSE Class 12th Chemistry Notes: The p-Block Elements (Part - III)

Oct 6, 2016 17:00 IST

    In this article you will get revision notes on CBSE Class 12th Chemistry, Chapter 7- The p-Block Elements. This article is a continuation of the revision notes on Class 12 Chemistry : Chapter- The p-Block Elements, Part-I and Part-II. In Part-I and Part-II we have studied about the Group 15 elements (Nitrogen family) and Group 16 elements respectively. In Part-III, you will get acquainted with Group 17 and Group 18 elements. These notes are based on the latest CBSE syllabus for Class 12th Chemistry.

    The main topics covered in these quick notes are:

    •    Halogen family

    •    General properties of halogen family

    •    Anomalous properties of fluorine

    •    Reactivity of halogens towards:

         o    Hydrogen

         o    Metals

         o    Other halogens (Interhalogens)

    •    Preparation, properties and uses of chlorine (Cl2)

    •    Preparation, properties and uses of hydrogen chloride (HCl)

    •    Oxoacids of halogens

    •    Noble gases

    •    General properties of noble gases

    •    Preparation and properties of xenon fluorine compounds

    •    Preparation and properties of xenon oxygen compounds

    •    Uses of noble gases

    The notes are as follow:

    Group 17 elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At) are collectively known as halogens and are having the general electronic configuration of ns2, np5.

    Atomic and ionic radii

    The halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear charge. Atomic and ionic radii increases down the group due to the addition of a new shell at each step.

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    Ionisation enthalpy

    Due to their small size halogens have little tendency to lose electron. Thus they have very high ionisation enthalpy. Ionisation enthalpy decreases down the group due to the increase in atomic size.

    Electron gain enthalpy

    Electron gain enthalpy of halogens is very high as they are short of only one electron to attain noble gas configuration. Electron gain enthalpy becomes less negative as we move down the group. However F has less electron gain enthalpy than Cl due to its small size and high electron density.


    Electronegativity decreases down the group. F is the most electronegative element in the periodic table.

    Melting and boiling point

    The melting and boiling points increases down the group.

    Bond dissociation enthalpy

    Bond dissociation enthalpy decreases as we move down the group. F2 has less ΔHdiss. Then Cl2 due to small size and strong lone pair-lone pair repulsion.


    All halogens exhibit colour due to the absorption of radiations in visible region of light due to which the electrons get excited to higher energy levels.

    For example, F2 has yellow, Cl2  has greenish yellow, Br2 has red and I2 has violet colour.

    Oxidation state

    The most common oxidation state of halogens is −1. Cl, Br, I also shows positive oxidation states of +1, +3, + 5, + 7. F does not show positive oxidation states due to non-availability of d-orbitals.


    All halogens are highly reactive and the reactivity decreases down the group.

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    Anomalous behaviour of fluorine

    Fluorine is anomalous in many properties like, ionisation enthalpy, electronegativity, enthalpy of bond dissociation that are higher than expected from the regular trends among the halogens. Its  ionic and covalent radii, melting and boiling points,  and electron gain enthalpy is quite lower than expected.
    Reasons for the anomalous behaviour of oxygen are:

    •    Small size and highest electronegativity

    •    Low F-F bond dissociation enthalpy

    •    Absence of d-orbitals

    Reactivity towards hydrogen

    All halogens reacts with H2 to form hydrogen halides (HX) and the reactivity towards H2 decreases down the group

    Acidic strength

    As the size of X increases and the strength of H─X bond decreases down the group, acidic strength dectreses down the group

                                                                HF < HCl < HBr < HI


    As the bond dissociation enthalpy decreases down the group so the stability of hydrogen halides also decreases from HF to HI.

                                                                HF > HCl > HBr > HI

    Boiling point

    Due to the increase in size of halogens the van der Waals forces increases down the group resulting in the increase in boiling point from HCl to HI. HF has the highest boiling point due to the presence of strong intermolecular H bonding

                                                                HCl < HBr < HI < HF.

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    Ionic character

    Due to the decrease in electronegativity down the group the ionic character of hydrogen halides also decreases down the group.

                                                                HF > HCl > HBr > HI

    Dipole moment

    Due to the decrease in electronegativity down the group the ionic character of hydrogen halides also decreases down the group.

                                                                HF > HCl > HBr > HI

    Reducing power

    As the bond dissociation enthalpy decreases, so it becomes easier to give out the hydrogen atom causing the reducing power to increase from HF to HI.

                                                                 HF < HCl < HBr < HI

    Reactivity towards metals:

    Halogens react with metals to form metal halides of the form MX, where  M is a monovalent metal.

    Ionic character

    Due to the decrease in electronegativity down the group the ionic character of metal halides also decreases down the group

                                                                MF > MCl > MBr > MI

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    Reactivity of halogens towards other halogens (Interhalogens):

    Binary compounds of two different halogen atoms of general formula XX’n  are called interhalogen compounds where n = 1, 3, 5, or 7. All the interhalogen compounds are covalent in nature.

    Some properties of interhalogen compounds are given in the following table:

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    Chlorine (Cl)


    Chlorine can be prepared by any of the following processes:


    •     It is a greenish yellow gas with pungent and suffocating odour.

    •     It is soluble in H2O

    •     Reaction with metals and non-metals: Chlorine reacts with a number of metals and non-metals to form chlorides.

          For example:

                                                    2 Al + 3Cl2 → 2AlCl3

                                                    S8 + 4Cl2 → 4S2Cl2

    •     Reaction with ammonia: When treated with excess ammonia, chlorine gives nitrogen and ammonium chloride whereas when excess chlorine reacts with ammonia, nitrogen trichloride is formed.

                             8NH3   + 3Cl2 → 6NH4Cl + N2


                            NH3     +    Cl2  → NCl3 + 3 HCl


    •      Reaction with NaOH: Chlorine reacts differently with cold dilute NaOH and hot concentrated NaOH.

                            Cl2    +   2NaOH        →     NaCl + NaOCl + H2O

                                        Cold dil.

                            Cl2 +      6NaOH        →     NaCl + NaOCl + H2O

                                         Hot conc.

    Reaction with slaked lime: Cl2 when treated With dry slaked lime it gives bleaching powder:

                                          2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O

    Cl2 acts as a powerful bleaching agent and its bleaching action is due to its oxidizing nature.

                                                    Cl2 + H2O → 2HCl + O


    (i) Chlorine is used for bleaching woodpulp.

    (ii) It is used in the extraction of gold and platinum

    (iii) It is used in in sterilising drinking water.

    (iv) It is used in the manufacture of dyes, drugs and organic compounds like CCl4, DDT, refrigerants, etc.

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    Hydrogen chloride (HCl)


    It is prepared by heating sodium chloride with concentrated sulphuric acid.

                                                 2 NaCl + H2SO4 + Heat → Na2SO4 + HCl


    HCl is a colourless gas with pungent odour.

    It is extremely soluble in water, HCl + H2O → H3O+ + Cl

    It decomposes salts of weaker acids, Na2CO3 + 2HCl → 2NaCl + H2O + CO2

    When treated with NH3, it gives white fumes of NH4Cl, NH3 + HCl → NH4Cl

    3HCl : 1HNO3 is called aqua regia, which is used for dissolving noble metals.

                Au + 4 H+ + NO3 + 4Cl → AuCl4 + NO + 2 H2O


    (i) Hydrogen chloride is used in medicine and as a laboratory reagent.

    (ii) It is used in the manufacture of chlorine, NH4Cl and glucose.

    Oxoacids of halogens

    Fluorine due to its small size and high electronegativity forms only one oxoacid HOF (Hypofluorous acid).

    Other halogen form several oxoacids as given in the following table:

    Image Source: NCERT Textbook

    Group 18, Noble gases

    Group 18 elements: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) having the electronic configuration ns2 np6, are named as noble gases. All these are gases and chemically unreactive.

    General properties of noble gases

    Atomic radii:

    Atomic radii of noble gases increases down the group due to the addition of a new shell at each step.

                                                    He < Ne < Ar < Kr < Xe < Rn

    Ionisation enthalpy:

    They have very high ionization enthalpy because of completely filled orbitals. Ionisation enthalpy decreases down the group because of increase in size.

                                                    He < Ne < Ar < Kr < Xe < Rn

    Electron gain enthalpy:

    Because of stable electronic configuration, noble gases have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

    Melting and boiling point:

    Due to the weak dispersion forces they have low melting and boiling point.

    CBSE Syllabus Class 12: Academic Session 2016 – 2017

    Xenon-fluorine compounds

    Xenon forms three binary fluorides, XeF2, XeF4 and XeF6 .



    XeF2 is linear, XeF4 is square planar and XeF6 is distorted octahedral.

    XeF2, XeF4 and XeF6 are colourless crystalline solids

    They are readily hydrolysed

                        2XeF2(s) + 2H2O(l) → 2Xe(g) + HF(aq) + O2(g)

    They react with fluoride ion acceptors to form cationic species and fluoride ion donors to form fluoroanions.

                        XeF2 + PF5 → [XeF] + [PF6]

                        XeF4 + SbF5 → [XeF3]+ [SbF6]

                        XeF6 + MF → M+ [XeF7]

                        [Where, M = Na, K, Rb or Cs]

    Xenon-oxygen compounds

    Xenon forms some important compounds with oxygen like XeO3, XeOF4 XeO2F2.


    Various xenon-oxygen compounds are prepared as follows:

                       6XeF4 + 12H2O →  4Xe + 2XeO3 + 24HF + 3O2

                       XeF6 + 3H2O     →  XeO3 + 6HF

    Partial Hydrolysis XeOF4

                       XeF6 + H2O → XeOF4 + 2 HF

    Partial Hydrolysis also gives XeO2F4

                       XeF6 + 2H2O → XeO2F4 + 4HF


    XeO3 is a colourless explosive solid having a trigonal pyramidal structure.

    XeOF4 is a colourless volatile liquid with a square pyramidal

    Uses of inert gases

    Helium is used:

    •    Gas cooled Nuclear reactors

    •    In filling balloons for meteorological observations.

    •    In the oxygen mixture of deep sea divers

    •    In inflating aeroplane tyres

    •    Used to provide an inert atmosphere in melting and welding of easily oxidizable metals.

    Neon is used:

    •    In discharge tubes and fluorescent bulbs used for advertising purposes

    •    In beacon lights for the safety of air navigators as the light can easily pass through the fog for a clear view.

    Argon is used:

    •    To provide an inert atmosphere in high-temperature metallurgical processes (arc welding of metals or alloys)

    •    For filling electric bulbs.

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

    •    Xenon and Krypton are also used in light bulbs.

    CBSE Class 12th Chemistry Notes: The p-Block Elements (Part - I)

    CBSE Class 12th Chemistry Notes: The p-Block Elements (Part - II)


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