CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.3Elements in Group 18

Group 18 of the Periodic Table consists of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og). These elements are known as noble gases or inert gases. They are chemically unreactive, exist as monoatomic gases, and exhibit distinct physical trends and practical applications across various industries.

Learning Objectives

Relate the inert nature of Group 18 elements to their electron arrangement stability.
Generalise the changes in physical properties of elements when going down Group 18.
Describe briefly the uses of Group 18 elements in daily life.

Electron Arrangements and Trends Down Group 18

Why noble gases are inert, and how their physical properties change down the group.

Helium — Duplet

Helium has a stable duplet electron arrangement with 2 valence electrons.

He 2

Neon — Octet

Neon has a stable octet electron arrangement with 8 valence electrons.

Ne 2.8

Argon — Octet

Argon has a stable octet electron arrangement with 8 valence electrons.

Ar 2.8.8

Trends Down Group 18

↑ Atomic radius increases

↑ Melting point increases

↑ Boiling point increases

↑ Density increases

Brief Explanation

Understand why noble gases are inert and how their properties change down the group.

Noble gases are located on the far right of the Periodic Table in Group 18. Their outermost electron shells are completely filled, forming a stable duplet configuration in helium and an octet configuration in the other members. Because their valence shells are full, they do not gain, lose, or share electrons, making them unreactive and causing them to exist as monoatomic gases.

Chemical Inertness

Noble gas atoms have achieved a stable duplet or octet electron arrangement, so they do not donate, accept, nor share electrons with atoms of other elements.

Physical Trends

Going down Group 18, atomic radius, melting point, boiling point and density all increase as the number of electron-filled shells increases.

Element	Atomic radius (nm)	Melting point (°C)	Boiling point (°C)	Density (g cm⁻³)
Helium	0.050	-270	-269	0.00017
Neon	0.070	-248	-246	0.00080
Argon	0.094	-189	-186	0.00170
Krypton	0.109	-156	-152	0.00350
Xenon	0.130	-122	-108	0.00550
Radon	—	-71	-62	—

As the atomic size of elements increases going down the group, the attraction force between atoms becomes stronger. Therefore, more heat energy is required to overcome these forces during melting and boiling.

Try These Questions First

Answer mentally, then check the explanation.

1

Why are noble gases chemically unreactive?

Answer: Noble gases are chemically unreactive because their valence shells are fully filled with electrons, having achieved a stable duplet or octet electron arrangement, meaning they do not donate, accept, or share electrons with other elements.

2

Explain why the boiling points of elements increase when going down Group 18.

Answer: Going down Group 18, the atomic size increases. This causes the attraction force between atoms to become stronger, requiring more heat energy to overcome the force.

3

State two uses of argon in daily life.

Answer: Argon is used to fill electric bulbs and to provide an inert atmosphere for high-temperature welding.

Common Mistakes

Avoid these errors when learning about Group 18 elements.

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Describing helium as having an octet electron arrangement instead of a duplet electron arrangement.

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Writing chemical formulas for noble gases as diatomic molecules, such as He₂ or Ar₂, instead of monoatomic entities like He and Ar.

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Confusing the specific applications of different noble gases, such as mixing up the gas used in advertising lights Neon with the gas used in electric bulbs Argon.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Noble gases have high melting and boiling points because their atoms have strong chemical bonds.

Correct Fact

Noble gases exist as individual monoatoms held together only by weak attraction forces between atoms, which is why they have very low melting and boiling points (helium to radon are gases at room temperature).

Misconception

Atoms of noble gases form compounds easily because their valence shells are full.

Correct Fact

A fully filled valence shell (duplet or octet) confers chemical stability; therefore, noble gas atoms do not donate, accept, or share electrons under standard conditions.

Summary

Key facts to remember.

Group 18 consists of helium, neon, argon, krypton, xenon, radon, and oganesson.
Noble gases have fully filled valence shells (duplet for helium, octet for others) and exist as monoatoms.
They are chemically inert because they do not donate, accept, or share electrons.
Going down Group 18, atomic radius, melting point, boiling point and density increase.
Noble gases have diverse uses: weather balloons and divers' tanks (He), advertising lights (Ne), electric bulbs and welding atmosphere (Ar), flashlights and eye retina laser treatment (Kr), lighthouse lamps and anesthesia (Xe), and cancer treatment (Rn).

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Why do Group 18 elements exist as monoatoms?

They have empty valence shells. They form strong covalent bonds between identical atoms. They have achieved a stable duplet or octet electron arrangement. They readily donate electrons to form stable cations.

2

Which of the following explains why the melting points and boiling points increase down Group 18?

The attraction force between atoms becomes stronger as atomic size increases. The valence shell loses electrons more readily as atomic radius increases. The number of covalent bonds between monoatoms increases down the group. The atomic mass decreases, allowing atoms to absorb more heat.

B. Fill in the Blanks

3

Helium achieves a stable __________ electron arrangement with two valence electrons in its filled shell.

4

Argon is used in electric bulbs and to provide an __________ atmosphere for high-temperature welding.

C. Match the Noble Gas with Its Use

Drag each noble gas on desktop. On mobile, tap a noble gas and then tap the matching box.

Answer Choices

Helium Neon Krypton Radon

Application in Daily Life

1Used to fill weather balloons and oxygen tanks of divers

2Used in advertising board lights

3Used in camera flashlights and lasers for eye retina treatment

4Used to treat cancer

Complete every answer before checking.

Keywords

Tap or hover over a term to read its meaning.

Noble gases (Inert gases) Duplet electron arrangement Octet electron arrangement Monoatom Atomic radius Attraction force between atoms

Quick Notes