Q.Which of the following gas is mono-atomic?
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Group 18: The Noble Gases
Imagine you are at a party. Most people are mingling, exchanging handshakes, forming groups, and reacting with each other. Now picture a person standing calmly in a corner, perfectly content, not looking for a handshake or a conversation. That person is completely satisfied with their own company.
That is the noble gas.
The Intuition: Why They Are "Noble"
Atoms want to be stable. For most elements, stability means having eight electrons in their outermost shell — the octet rule. Atoms that are short of this octet will grab, share, or give away electrons to get there. That is what drives chemical reactions.
But the noble gases are born with a full outer shell. Helium has 2 electrons (a duplet, which is its full shell), and the rest — neon, argon, krypton, xenon, radon — have 8. They have nothing to gain and nothing to lose. They are chemically satisfied from the start.
This is why they were once called the "inert gases." They simply do not need to react.
The Precise Statement
Group 18 of the periodic table consists of six elements: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Their defining feature is a completely filled valence shell — ns2np6 for all except helium (1s2).
This electronic configuration gives them three key properties:
- Very high ionization enthalpy — pulling an electron away from a full shell requires a huge amount of energy.
- Almost zero electron gain enthalpy — there is no room in the valence shell for an extra electron, and adding one would have to go into a much higher energy level.
- Extremely low reactivity — for a long time, chemists believed they formed no compounds at all.
The term "inert gas" is outdated. We now call them noble gases because, like a nobleman who does not mingle with commoners, they can react — but only under extreme conditions and with the most reactive partners.
The Exception: Xenon Chemistry
For decades, the inertness of noble gases was a dogma. Then in 1962, Neil Bartlett made a breakthrough. He noticed that platinum hexafluoride (PtF6) was powerful enough to pull an electron from oxygen — and reasoned that xenon, which has a similar ionization enthalpy to oxygen, might also react.
He was right.
Xenon reacts directly with fluorine under the right conditions:
Xe+F2heat, pressureXeF2,XeF4,XeF6
These are real, stable compounds. Xenon difluoride (XeF2) is a white crystalline solid. Xenon also forms oxides like XeO3 and oxyfluorides like XeOF4.
| Noble Gas | Known Compounds | Reason |
|-----------|----------------|--------|
| He, Ne, Ar | None | Ionization enthalpy too high |
| Kr | KrF2 (unstable) | Borderline reactivity |
| Xe | XeF2,XeF4,XeF6,XeO3,XeOF4 | Lowest ionization enthalpy among noble gases | …
Helium (a noble gas) exists as single atoms, so it is monoatomic; Cl2, N2 and O2 are diatomic. …
Helium is monoatomic; the others are diatomic.
Noble gases have a stable, completely filled valence shell and therefore exist as single, unbonded atoms — they are monoatomic. Helium is a noble gas, so it exists as He atoms …
- CBSE 2023Set ANNUAL1 markQ.______ inert gas is used as beacon light.
›Reveal solutionSolution
Neon is the inert (noble) gas used in beacon lights and neon signs, since its discharge glow is highly visible and penetrates fog well.
When an electric discharge is passed through neon gas at low pressure, it emits a characteristic reddish-orange glow. Because this glow can be seen through fog and mist more easily than most other light sources, neon-filled tubes …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following noble gases is radioactive and used in the treatment of cancer?(i) Helium(ii) Neon(iii) Argon(iv) Radon
›Reveal solutionSolution
Radon is the only naturally radioactive noble gas and its emissions are used in radiotherapy.
Among the noble gases, Helium, Neon and Argon are stable, non-radioactive elements. Radon (Rn) is radioactive — it is produced in the radioactive decay series of uranium and thorium and itself decays by alpha emission. Its radioactivity has been historically exploited in radiotherapy (radon 'seeds'/needles) for trea …
- CBSE 2022Set M1 markQ.Which noble gas has lowest boiling point?
›Reveal solutionSolution
Helium has the lowest boiling point of all noble gases.
Among the noble gases, boiling point increases down the group (He < Ne < Ar < Kr < Xe < Rn) because the strength of the London dispersion forces increases with atomic size and polarisability. Helium, being the smallest and least polarisabl …
- CBSE 2022Set HE2181 markQ.Answer in one word/sentence: Write the name of noble gas which is used in theraphy of cancer.
›Reveal solutionSolution
Radon, a radioactive noble gas, is used in cancer radiotherapy because the alpha/gamma radiation it emits can destroy malignant tissue.
Among the noble gases, radon is the heaviest and the only one that is radioactive in all its known isotopes (it arises from the radioactive decay of radium in the uranium/thorium decay series). Its radioactivity, particularly of the isotope Rn-222, has been historically used in sealed capsules ( …
- CBSE 2020Set ANNUAL1 markQ.Which noble gas mixed with oxygen is used by sea-divers for their respiration under water?
›Reveal solutionSolution
Sea-divers breathe a mixture of helium and oxygen (Heliox) instead of compressed air, to avoid the dangers of nitrogen dissolving in the blood under high pressure.
At the high pressures encountered underwater, ordinary compressed air (which is ~78% nitrogen) causes nitrogen to dissolve in significant amounts in the diver's blood and tissues. This leads to nitrogen narcosis at depth and, more seriously, decompression sickness ('the bends') as bubbles form when ascending. Helium is used to replace nitrogen because it is far less soluble in blood/fatty tissue and is chemically inert, so a helium-oxygen mixture avoids these hazards while still suppl …
- CBSE 2019Set HE1 markQ.Answer in one word: Which gas is filled in the balloons used for meteorological experiments?
›Reveal solutionSolution
Helium is used to fill meteorological (weather) balloons because it is much lighter than air and, unlike hydrogen, is completely non-inflammable/inert, making it safe.
…
- CBSE 2019Set ANNUAL1 markMCQQ.The symbol of Helium is -(a) He(b) Hi(c) Hm(d) All
›Reveal solutionSolution
Helium's symbol is He.
Helium is the first noble gas (Group 18, atomic number 2). Its internationally accepted chemical symbol is He (first two letters of the name …
- CBSE 2019Set ANNUAL1 markMCQQ.Which of the following gas is mono-atomic?(a) Chlorine(b) Helium(c) Nitrogen(d) Oxygen
›Reveal solutionSolution
Helium is monoatomic; the others are diatomic.
Noble gases have a stable, completely filled valence shell and therefore exist as single, unbonded atoms — they are monoatomic. Helium is a noble gas, so it exists as He atoms …
- CBSE 2018Set ANNUAL1 markMCQQ.Match the following. Group A item: Radon. Choose its correct match from Group B: (A) Lanthanides and Actinides (B) Used in treatment of cancer (C) Purple of Cassius (D) Barbituric acid (E) Hydrated ferric oxide
›Reveal solutionSolution
Radioactive radon gas (Rn-222) is used in radiotherapy for treating cancer.
Radon is a radioactive noble gas. Its radioactive emissions are used in radiotherapy — small sealed radon sources (radon seeds) have historically been implanted near tumours to deliver …
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