Chapter 7: Elements of Groups 16, 17, and 18
Chapter 7: Elements of Groups 16, 17, and 18
7.1 Introduction: The p-block elements have their differentiating electron entering the p-orbital of the outermost shell. Since a maximum of six electrons can be accommodated in a p-subshell, it gives rise to groups 13 to 18. In this chapter, we will study the properties of elements of groups 16, 17, and 18 in detail.
7.2 Occurrence:
Group 16 (Oxygen family): Also known as chalcogens, this group includes oxygen, sulfur, selenium, tellurium, and polonium. Oxygen is the most abundant element on Earth, forming 20.95% of air by volume and 46.6% of the Earth’s crust by mass. Sulfur is found in sulfates (gypsum, Epsom salt) and sulfides (galena, zinc blende, copper pyrites). Selenium and tellurium occur in metal selenides and tellurides, while polonium is a radioactive decay product of thorium and uranium.
Group 17 (Halogens): Fluorine, chlorine, bromine, iodine, and astatine belong to this group. These elements are highly reactive and mostly occur in compounds. Fluorine is found in fluorspar, cryolite, and fluorapatite, while chlorine is mainly found in sea water as NaCl. Astatine is radioactive and has a half-life of approximately 8.1 hours.
Group 18 (Noble gases): Helium, neon, argon, krypton, xenon, and radon make up this group. Noble gases are present in the atmosphere (~1% by volume), with argon as the major constituent. Helium is obtained from natural gas, while xenon and radon are rare. Radon is a decay product of radium.
7.3 Electronic Configuration:
Group 16: ns²np⁴
Group 17: ns²np⁵
Group 18: ns²np⁶
7.4 Atomic and Physical Properties:
Atomic and Ionic Radii: Increase down the group due to additional quantum shells.
Ionization Enthalpy: High for all three groups, but decreases down the group.
Electronegativity: Decreases down the group. Oxygen is the most electronegative element after fluorine.
Electron Gain Enthalpy: Negative for groups 16 and 17, but noble gases have large positive values due to their stable configuration.
Melting and Boiling Points: Increase down the group for groups 16 and 17. Noble gases have very low melting and boiling points.
Density: Increases down the group for all three groups.
7.5 Chemical Properties:
Oxidation States:
Group 16: Typically -2, but also +2, +4, and +6.
Group 17: Primarily -1, but some elements show +1, +3, +5, and +7 oxidation states.
Group 18: Mostly inert, but xenon forms compounds like XeF₂, XeF₄, and XeF₆.
Reactivity with Hydrogen:
Group 16 forms hydrides (H₂O, H₂S, H₂Se, H₂Te, H₂Po) with decreasing thermal stability down the group.
Group 17 forms hydrogen halides (HF, HCl, HBr, HI), where acidic strength increases down the group.
Group 18 does not react with hydrogen.
Reactivity with Oxygen:
Group 16 forms oxides like SO₂, SeO₂, TeO₂ (acidic in nature).
Group 17 forms unstable oxides (ClO₂, BrO₃, I₂O₅).
Group 18 does not form stable oxides.
Reactivity with Halogens:
Group 16 forms halides like SF₆, SeF₆, and TeF₆.
Group 17 forms interhalogen compounds (ClF, BrCl, IBr, ClF₃, BrF₅).
Group 18 elements like xenon react with fluorine to form XeF₂, XeF₄, and XeF₆.
7.6 Important Compounds:
Group 16: SO₂, SO₃, H₂SO₄, SeO₃, TeCl₄
Group 17: HF, HCl, Cl₂, Br₂, I₂O₅
Group 18: XeF₂, XeF₄, XeF₆, XeO₃, XeOF₄
7.7 Uses:
Group 16: Oxygen is essential for respiration, sulfur is used in vulcanization of rubber, selenium in photocells, and tellurium in metallurgy.
Group 17: Chlorine is used for water purification, fluorine in toothpaste, iodine in medicine, and bromine in fire retardants.
Group 18: Helium is used in balloons and cryogenics, neon in lighting, argon in welding, krypton in high-performance lighting, and xenon in anesthesia.
This chapter provides a detailed insight into the occurrence, properties, reactions, and applications of these essential elements in chemistry and industry.