Learning Objectives General trends of group 17 elements Chemical properties of group 17 elements Oxoacids of group 17 elements

Occurrence The halogens are very reactive, and donot occur in free state. All except At are found in combined form in the earth’s crust. Astatine is radioactive and has a short half life.

General trends of group 17 elements Electronic Configuration All these elements have seven electrons in their outermost shell (ns2np5) which is one electron short of the next noble gas. Atomic and Ionic Radii The halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear charge. The atomic radius of fluorine like the other elements of second period is extremely small. Atomic and ionic radii increase from fluorine to iodine due to increasing number of quantum shells.

Ionisation Enthalpy They have little tendency to lose electron. Thus they have very high ionisation enthalpy. Due to increase in atomic size, ionisation enthalpy decreases down the group. Electronegativity They have very high electronegativity. The electronegativity decreases down the group. Fluorine is the most electronegative element in the periodic table.

Electron affinity Electron affinity values are high in case of halogens. As we move down the group, the electron affinities decrease as the size of the halogen increases. Electron affinity of fluorine is lower than that of chlorine. This is due to small size of fluorine atom. As a result of which strong electron-electron repulsions are present in the relatively compact 2p-orbitals of fluorine. Therefore, the electron affinities decrease in the order: Cl > F > Br > I

Physical Properties Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Their melting and boiling points steadily increase with atomic number. Chlorine Fluorine Iodine

Chemical Properties Oxidation states and trends in chemical reactivity All the halogens exhibit –1 oxidation state. However, chlorine, bromine and iodine exhibit + 1, + 3, + 5 and + 7 oxidation states also as explained below:

Stability of X-X bond The bond energy in the X2 molecules would decrease as the atoms become larger, since increased size results in less effective overlap of orbitals. The F — F bond energy is smaller than that of Cl — Cl. Due to small size of fluorine atoms, there are high interelectronic repulsions between non-bonding electrons in 2p-orbitals of fluorine. As a result, F — F bond is weaker in comparison to Cl — Cl and Br — Br bonds.

Illustrative Problem Solution Explain why F2 is a stronger oxidizing agent than Cl2 while electron affinity of fluorine is less than that of chlorine. Solution F2 is a stronger oxidizing agent than Cl2 because F2 has higher oxidation potential than Cl2. Electron affinity is the energy released when a gaseous atom accepts an electron to form a gaseous anion while oxidation potential is the sum of energy changes taking place in the following steps:

Illustrative Problem Explain why fluorine does not undergo disproportionation reaction but other halogens do. Solution : Fluorine being the most electronegative element undergoes only reduction but not oxidation. As a result, it shows only –1 oxidation state while other halogens show both negative (–1) and positive (+1, +3, +5, +7) oxidation states. Thus, F does not show disproportionation reactions while other halogens do.

Reactivity towards hydrogen Hydrides : HX HF is liquid HCl, HBr and HI are gases. Thermal stability : HF > HCl > HBr > HI Reducing character: HF < HCl < HBr < HI Acid strength : HF < HCl < HBr < HI H — X bond strength : HF > HCl > HBr > HI

Properties of Hydrogen Halides

Reactivity towards oxygen: Halogens form many oxides with oxygen but most of them are unstable. Fluorine forms two oxides OF2 and O2F2. However, only OF2 is thermally stable at 298 K. Fluorine forms two oxides OF2 and O2F2. However, only OF2 is thermally stable at 298 K. Chlorine oxides, Cl2O,ClO2, Cl2O6 and Cl2O7 are highly reactive oxidising agents and tend to explode. The bromine oxides, Br2O, BrO2 , BrO3 are the least stable halogen oxides. The iodine oxides, I2O4 , I2O5, I2O7 are insoluble solids and decompose on heating. I2O5 is a very good oxidising agent and is used in the estimation of carbon monoxide.

Reactivity towards metals Halogens react with metals to form metal halides. For e.g., bromine reacts with magnesium to give magnesium bromide. Mg (s) + Br2(l) → MgBr2 The ionic character of the halides decreases in the order MF > MCl > MBr > MI where M is a monovalent metal. Reactivity of halogens towards other halogens: Halogens combine amongst themselves to form a number of compounds known as interhalogens of the types XX ′ , XX3′, XX5′ and XX7′ where X is a larger size halogen and X′ is smaller size halogen.

Oxo acids Hypohalous acid, HOX Halous acid, HXO2 Halic acid, HXO3 Acid strength: HClO3 < HBrO3 < HIO3 HClO4 > HClO3 > HClO2 > HOCl

Oxoanions of Chlorine

Oxoacids of Halogens

Structures of Oxoacids of chlorine