This article is on the P Block Notes Class 12 of Chemistry. The notes on P Block of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the P Block chapter of Class 12 for the students so that they can give a quick glance of the chapter.
This chapter of Class 12 has been divided into four articles. This article (Part 4) includes the Group 18 elements. The second article (Part 1) is on Group 15 elements. The third article (Part 2) is on Group 16 elements. At last, the fourth article (Part 3) is on Group 17 elements.
P block Elements Part 4
Group 18 Elements
Group 18 are also known as Noble gases. It consists of six elements, i.e., helium (2), neon (10), argon (18), krypton (36), xenon (54) and radon (86). All the elements of group 18 are gases and are chemically unreactive. These are termed as noble gases because they form very few compounds.
Except for radon, all the noble gases occur in the atmosphere. Their atmospheric abundance in dry air is nearly 1% by volume of which argon (0.93%) is the major constituent.
Helium is also present in natural gas to the extent of 2 to 7%. He and sometimes Ne are found in minerals of radioactive origin, e.g., pitchblende, monazite, cleveite. Xenon and radon are the rarest elements of this group.
Radon is obtained as a decay product of Ra-226. The elements of this group are known as inert gases due to their chemical inertness.
General properties of group 18 elements are discussed below
- Electronic Configuration:
General valence shell electronic configuration of all noble gases is np2np6 except He which has 1s2 configuration.
- Atomic Radii:
They have the highest atomic radii in their respective periods due to large van der Waals’ radii. Atomic radii increase down the group with the increase in atomic number.
- Ionisation Enthalpy:
These gases exhibit very high ionisation enthalpy due to the stable electronic configuration. It decreases down the group with the increase in atomic size.
- Electron Gain Enthalpy:
These gases have large positive values of electron gain enthalpy because these have no tendency to accept the electron. High ionisation enthalpy and more positive electron gain enthalpy.
Different physical properties of group 18 elements are given below
(i) All gases are monatomic due to unavailability of the unpaired electron.
(ii) Melting and boiling points increase down the group due to the increase in the van der Waals’ forces.
(iii) M. P and B. P of the noble gases are quite low in comparison to other elements of comparable masses as they have no interatomic forces except the weak dispersion forces.
(iv) He has the lowest boiling point of any known substance.
(v) He has an unusual property of diffusion through rubber, glass or plastics.
Discovery of Compounds of Noble Gases:
In 1962, Neil Bartlett prepared the first compound, Xe+ PtF6– of Xe by mixing PtF6 and xenon.
Xe + PtF6 —298K–> Xe+ [PtF6]–
Xenon has least ionisation enthalpy (except radon which is radioactive) among noble gases, therefore, it forms compounds with oxygen and fluorine readily which are the most electronegative elements. Xenon forms three binary fluorides, XeF2, XeF4, and XeF6. Hydrolysis of XeF4 and XeF6 gives XeO3. Partial hydrolysis of XeF6 gives XeOF4 and XeO2F2.
XeF2 -linear, XeF4 -square planar, XeF6 -distorted octahedral, XeOF4 -square pyramidal and XeO3-pyramidal in shape.
Argon is used for filling electric bulbs and to provide an inert atmosphere. As Helium is a non-inflammable and light gas, it is used in filling meteorological balloons and also as a diluent for oxygen in modern diving apparatus. For carrying out various experiments at low temperatures, Liquid helium is used in the form of the cryogenic agent. Neon is used in discharged tubes and fluorescent bulbs for display in an advertisement.
Some Important Reactions:
Xe(g) + F2 (g) —673 K,1 bar–> XeF2 (s)
Xe(g) + 2F2 (g) —873 K, 7 bar–> XeF4 (s)
Xe(g) + 3F2 (g) —573 K, 60-73 bar–> XeF6 (s)
XeF4 + O2F2 —-> XeF6 + O2
Check Part 1 of the Chapter Here: Group 15 Elements – p block elements
Check Part 2 of the Chapter Here: Group 16 Elements – p block elements
Check Part 3 of the Chapter Here: Group 17 Elements – p block elements