Chemistry Group 17 Elements

Topics Covered :

● Group 17 Elements
● Occurrence
● Electronic Configuration
● Ionisation Enthalpy
● Electron Gain Enthalpy
● Electronegativity
● Physical Properties

Group 17 Elements :

=> Fluorine, chlorine, bromine, iodine and astatine are members of Group 17.

=> These are collectively known as the halogens (Greek halo means salt and genes means born i.e., salt producers).

=> The halogens are highly reactive non-metallic elements.

=> Like Groups 1 and 2, the elements of Group 17 show great similarity amongst themselves. That much similarity is not found in the elements of other groups of the periodic table.

=> There is a regular gradation in their physical and chemical properties.

=> Astatine is a radioactive element.

Occurrence :

=> Fluorine and chlorine are fairly abundant while bromine and iodine less so.

=> Fluorine is present mainly as insoluble fluorides (fluorspar color{red}(CaF_2), cryolite color{red}(Na_3AlF_6) and fluoroapatite color{red}(3Ca_3(PO_4)_2.CaF_2)) and small quantities are present in soil, river water plants and bones and teeth of animals.

=> Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium, but is mainly sodium chloride solution (2.5% by mass).

=> The deposits of dried up seas contain these compounds. e.g., sodium chloride and carnallite, color{red}(KCl.MgCl_2 .6H_2O).

=> Certain forms of marine life contain iodine in their systems; various seaweeds, for example, contain upto 0.5% of iodine and Chile saltpetre contains upto 0.2% of sodium iodate.

=> The important atomic and physical properties of Group 17 elements along with their electronic configurations are given in Table 7.8.

Electronic Configuration :

All these elements have seven electrons in their outermost shell (color{red}(ns^2 np^5)) which is one electron short of the next noble gas.

=> 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.

Electron Gain Enthalpy :

=> Halogens have maximum negative electron gain enthalpy in the corresponding periods.

=> This is due to the fact that the atoms of these elements have only one electron less than stable noble gas configurations.

=> Electron gain enthalpy of the elements of the group becomes less negative down the group.

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

=> As a result, there are strong interelectronic repulsions in the relatively small color{red}(2p)-orbitals of fluorine and thus, the incoming electron does not experience much attraction.
Q 3010691510

Halogens have maximum negative electron gain enthalpy in the respective periods of the periodic table. Why?

Solution:

Halogens have the smallest size in their respective periods and therefore high effective nuclear charge. As a consequence, they readily accept one electron to acquire noble gas electronic configuration.

Electronegativity :

=> They have very high electronegativity.

=> The electronegativity decreases down the group.

=> Fluorine is the most electronegative element in the periodic table.

Physical Properties :

=> Halogens display smooth variations in their 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.

=> All halogens are coloured.

● This is due to absorption of radiations in visible region which results in the excitation of outer electrons to higher energy level.

● By absorbing different quanta of radiation, they display different colours.

● For example, color{red}(F_2) has yellow, color{red}(Cl_2) greenish yellow, color{red}(Br_2) red and color{red}(I_2) violet colour.

=> Fluorine and chlorine react with water. Bromine and iodine are only sparingly soluble in water but are soluble in various organic solvents such as chloroform, carbon tetrachloride, carbon disulphide and hydrocarbons to give coloured solutions.

=> One curious anomaly we notice from Table 7.8 is the smaller enthalpy of dissociation of color{red}(F_2) compared to that of color{red}(Cl_2) whereas color{red}(X-X) bond dissociation enthalpies from chlorine onwards show the expected trend : color{red}(Cl – Cl > Br – Br > I – I).

● A reason for this anomaly is the relatively large electron-electron repulsion among the lone pairs in color{red}(F_2) molecule where they are much closer to each other than in case of color{red}(Cl_2).
Q 3030691512

Although electron gain enthalpy of fluorine is less negative as compared to chlorine, fluorine is a stronger oxidising agent than chlorine. Why?

Solution:

It is due to
(i) low enthalpy of dissociation of F-F bond (Table 7.8).
(ii) high hydration enthalpy of F^– (Table 7.8).