17 Group elements

GROUP 17

Group 17 of periodic table consists of Fluorine , Chlorine ,Bromine , Iodine and Astatine. These are also named as halogens because the are salt producers.All halogens are coloured, F2 is yellow colored,Cl2 is greenish yellow,Br2 is red and I2 is violet in color. Fluorine and chlorine are gases,Bromine is a liquid and Iodine is a solid. 

OCCURANCE: Fluorine is mainly present in mineral CaF2, Na3AlF2 and 3Ca3(Po4)2CaF2.

Chlorine is found in nature in the combined state in the form of chlorides of various metals ex NaCl .

Bromine is found in sea water NaBr

Iodine occurs only in low concentration in sea water ads iodides.ex NaI .

Astatine is radioactive element and produced by the decay of Actinium and Thorium.

ELECTRONIC CONFIGURATION: All these elements have seven electrons in their outermost shell and show general electronic configuration ns2np5.

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 from Fluorine to iodine due to increasing number of shells.

IONIZATION ENTHALPY: They have little tendency to loose electrons. Thus they have very high ionisation enthalpy. Down the group ionization enthalpy decreases.

ELECTRON GAIN ENTHAPY : Halogens have maximum negative electron gain enthalpy in the corresponding period due to the presence of only one electron less than stable noble gas configuration,

The negative electron gain enthalpy of fluorine is less than that of chlorine due to small size of Fluorine atom.

ELECTRONEGATIVITY: They have very high electronegativity. It decreases down the group,i.e. :F>Cl>Br>I

MELTING POINT AND BOILING POINT: Melting and boiling point increases with atomic number.

BOND DISSOCIATION ENTHALPY: Fluorine has low bond dissociation enthalpy due to large electron-electron repulsion among loan pairs in f2 molecule where they are much closer to r=each other then in case of Cl2, X-X bond dissociation enthalpy from Cl toI show the bond :Cl-Cl >Br-Br>I-I . 

CHEMICAL PROPERTIES

1. Oxidation states and trends in chemical reactivity:

All the halogens exhibit -1 oxidation state. However Cl,Br, and Iodine exhibit +1,+3,+5 and +7 oxidation state also. 

Fluorine atom has no D orbital in its valence shell and cannot show any of higher oxidation state.
2.Reactivity toward hydrogen: all halogens react with hydrogen to give hydrogen halides . the reactivity of halogens decrease from Fluorine to Iodine .

H2(g)+X2(g)⇌2HX(g)

where X2 can be Cl2, Br2, or I2.

• Bond dissociation enthalpy  of halides decrease down the group ie H--F > H--Cl >H--I.
• Stability also decrease down the group .
• Acidic strength is inversely proportional to bond dissociation enthalpy 
• Melting point :HI>HF>HBr>HCl  , Boiling point :HF>HI>HBr>HCl . 

3.Reactivity toward oxygen: Halogens react with oxygen to form compounds most of which are unstable , Florine forms two oxides OF2 and O2F2. These are known as Fluorides rather than oxides because fluorine is more electronegative than oxygen . Compounds with Cl,Br,I are known as oxides because oxygen is more electronegative than Cl,Br, and I . Chlorine forms Cl2O,ClO2,Cl2O6,Cl2O7.

Bromine forms Br2o,BrO2,BrO3

Iodine forms I2O4,I2O5,I2O7.

all oxides are powerful oxidising agents and decomposes explosively when subjected to mechanical shock of heat.

the stability of oxids frmed by halogen in decreasing order I>Cl>Br.

4.Reactivity toward metals: Halogens react with metals to form metal halides, The ionic character of halides decreases in the order MF>MCl>MBr>MI   (M= monovalent metals )

if a metal exhibits more than one oxidation state will be more covalent than the one in lower oxidation state. ex, SnCl4>SnCl2, PbCl4> PbCl2, SbCl4> SbCl3,  UF6> UF4.  

5.Reactivity toward halogens : Halogens combines amongst themselves to form interhalogen  compounds of type XX" , XX"3  , XX"5  , and XX"7  

{here X = larger size halogen  and X" = smaller size halogens }