Maharashtra Board Class 12 Chemistry Chapter 7 Elements Of Groups 16 17 And 18 PDF Download

7. Elements Of Groups 16, 17 And 18

7.1 Introduction

You have learnt in Std. XI that in the p-block elements the differentiating electron (the last filling electron) enters the p-orbital of the outermost shell. Since maximum of six electrons can be accommodated in a p-subshell it gives rise to groups 13 to 18, in the p-block. In this chapter we shall study the properties of elements of groups 16, 17 and 18.

7.2 Occurrence

The elements oxygen (8O), sulfur (16S), selenium (34Se), tellurium (52Te) and polonium (84Po) constitute Group 16, called the oxygen family. Large number of metal ores are oxides or sulfides. Group 16 elements are also called chalcogens or ore forming elements.

Oxygen is the most abundant of all the elements on earth. Oxygen forms 20.95 % by volume of air and about 46.6 % by mass of earth's crust. Sulfur forms 0.034% by mass of the earths crust. It occurs mainly in combined forms as sulfates such as gypsum (CaSO4.2H2O), epsom salt (MgSO4.7H2O), baryte (BaSO4) and sulfides such as galena (PbS), zinc blende (ZnS), copper pyrites (CuFeS2).

Selenium and tellurium are also found as metal selenides and tellurides in sulfide ores. Polonium which is radioactive is a decay product of thorium and uranium.

Fluorine (9F), chlorine (17Cl), bromine (35Br), iodine (53I) and astatine (85At) constitute Group 17. These are collectively known as halogens (Greek halo means salt, gene means born), that is, salt producing element.

Halogens are very reactive due to high electronegativities and hence they are not found in free sate. They occur in the form of compounds.

Fluorine occurs mainly as insoluble fluorides (fluorspar CaF2, cryolite Na3AlF6, fluorapatite 3Ca3(PO4)2.CaF2) and small quantities are present in soil, fresh water plants, and bones and teeth of animals. Sea water contains chlorides, bromides and iodides of Na, K, Mg and Ca. However it mainly contains NaCl (2.5 % by mass). The deposits of dried up sea beds contain sodium chloride and carnallite, KCl.MgCl2.6H2O. Marine life also contains iodine in their systems. For example, sea weed contains upto 0.5 % iodine and chile saltpetre contains upto 0.2 % of sodium iodate. Astatine, the last member of halogen family is radioactive and has a half life of 8.1 hours.

The elements helium (2He), neon (10Ne), argon (18Ar), krypton (36Kr), xenon (54Xe) and radon (86Rn) constitute the Group 18.

All the noble gases except radon occur in the atmosphere. Their abundance in dry air is a 1% (by volume) with argon as the major constituent. The main commercial source of helium is natural gas. Helium and neon are found in minerals of radioactive origin e.g. pitchblende, monazite, cleveite. Xenon and radon are the rarest elements of the group. Radon is a decay product of 226Ra.

Can you recall?

How does the valence shell electronic configuration of the elements vary in the p-block of periodic table?

Name the first element of groups 16, 17 and 18.

Teacher's Note

Group 16 has oxygen which is very important for life. In India, we can see sulfur deposits in some areas. Sulfur is used to make matches.

Exam Trick

Remember: Group 16 = Oxygen family. Group 17 = Halogen family (salt forming). Group 18 = Noble gases (completely full, very lazy to react).

Points to Remember

Oxygen is the most abundant element on Earth and is in air we breathe.
Sulfur is found as sulfates and sulfides in nature.
Halogens are very reactive and never found free in nature.
Noble gases are very unreactive because their outer shells are completely full.
Fluorine, chlorine, bromine, and iodine are halogens that form salts.

7.3 Electronic Configuration Of Elements Of Group 16, 17 And 18

The general electronic configuration of the group 16 elements is ns2np4 while that of group 17 elements is ns2np5. The group 18 elements are shown by ns2np6 configuration.

The elements of groups 16 and 17 repectively have two and one electrons less than the stable electronic configuration of the nearest noble gas.

Table 7.1 shows the condensed electronic configuration of the elements of group 16, 17 and 18.

Teacher's Note

The electronic configuration tells us how many electrons are in the outer shell. Group 16 has 6 electrons in the outer shell, so they want 2 more electrons. This is like someone needing 2 apples to have 8 apples total.

Exam Trick

Remember: ns2np4 for Group 16 (oxygen family), ns2np5 for Group 17 (halogens), ns2np6 for Group 18 (noble gases with full outer shell).

Points to Remember

Group 16 elements have 6 electrons in their outer shell and need 2 more.
Group 17 elements have 7 electrons in their outer shell and need 1 more.
Group 18 elements have 8 electrons in their outer shell and don't need any more.
The s and p stand for different types of orbitals where electrons sit.
The numbers show how many electrons are in each orbital.

7.4 Atomic And Physical Properties Of Elements Of Group 16, 17 And 18

7.4.1 Atomic Properties Of Group 16, 17 And 18 Elements

These properties are given in Tables 7.2, 7.3 and 7.4.

i. Atomic and Ionic radii: In group 16, 17 and 18 atomic and ionic radii increase down the group, as a result of increase in the number of quantum shells.

Across a period atomic or ionic radii decrease with increasing atomic number, consequent to increase in (Zeff) effective nuclear charge.

Group 17 elements (Halogens) have the smallest atomic radii in their respective periods.

ii. Ionisation enthalpy: The group 16, 17 and 18 elements have high ionisation enthalpy. The ionisation enthalpy decreases down the group due to increase in the atomic size.

Across a period ionisation enthalpy increases with increase of atomic number. This is due to addition of electrons in the same shell. However the elements of group 16 have lower ionisation enthalpy values compared to those of group 15 in the corresponding periods, owing to extra stable half filled electronic configuration of p-orbitals in elements of group 15.

Teacher's Note

Atomic radius means the size of the atom. As we go down the group, atoms get bigger because they have more shells. This is like how a bigger container has more layers.

Exam Trick

Remember: Down the group = atoms get BIGGER and easier to lose electrons. So reactivity of Group 16 and 17 changes as we go down.

Points to Remember

Atomic size increases down a group because more electron shells are added.
Ionization enthalpy decreases down a group because outer electrons are farther from nucleus.
Electronegativity decreases down a group because atoms get larger.
Halogens in Group 17 have smallest size in their period.
These trends help explain why elements behave differently in chemical reactions.

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