Which Elements Have Complete Outer Shells

The quest to understand which elements boast complete outer shells is a journey into the heart of chemistry and the fundamental principles governing the behavior of matter. Elements with complete outer shells, also known as noble gases or inert gases, hold a unique position in the periodic table and exhibit remarkable stability and lack of reactivity. This article explores these elements in detail, delving into their electronic configurations, properties, and the reasons behind their inertness.

The Significance of Outer Shells

The outermost electron shell of an atom, often referred to as the valence shell, plays a pivotal role in determining the chemical behavior of an element. Atoms strive to achieve a stable electron configuration, typically resembling that of the noble gases, which have completely filled valence shells. This drive for stability is the driving force behind chemical bonding.

• Atoms with incomplete outer shells tend to gain, lose, or share electrons to achieve a full valence shell, leading to the formation of chemical bonds and compounds.
• Elements with complete outer shells, on the other hand, are already in a stable state and have little to no tendency to participate in chemical reactions.

Identifying Elements with Complete Outer Shells

The elements with complete outer shells are located in Group 18 (formerly Group VIIIA) of the periodic table. These elements are:

1. Helium (He): Atomic number 2
2. Neon (Ne): Atomic number 10
3. Argon (Ar): Atomic number 18
4. Krypton (Kr): Atomic number 36
5. Xenon (Xe): Atomic number 54
6. Radon (Rn): Atomic number 86
7. Oganesson (Og): Atomic number 118

These elements are collectively known as the noble gases.

Electronic Configurations of Noble Gases

The stability of noble gases stems from their unique electronic configurations. Let's examine the electron configuration of each noble gas to understand why they possess complete outer shells:

1. Helium (He): Helium has an atomic number of 2, meaning it has two electrons. Its electron configuration is 1s². The first electron shell (n=1) can hold a maximum of two electrons, so helium's outer shell is completely filled.
2. Neon (Ne): Neon has an atomic number of 10. Its electron configuration is 1s² 2s² 2p⁶. The second electron shell (n=2) can hold a maximum of eight electrons (2s² 2p⁶), so neon's outer shell is completely filled.
3. Argon (Ar): Argon has an atomic number of 18. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶. The third electron shell (n=3) can hold a maximum of 18 electrons, but only eight electrons are needed in the outermost shell to achieve stability. Argon's outer shell (3s² 3p⁶) is completely filled with eight electrons.
4. Krypton (Kr): Krypton has an atomic number of 36. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶. The fourth electron shell (n=4) can hold a maximum of 32 electrons, but krypton's outer shell (4s² 4p⁶) is completely filled with eight electrons.
5. Xenon (Xe): Xenon has an atomic number of 54. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶. The fifth electron shell (n=5) can hold a maximum of 50 electrons, but xenon's outer shell (5s² 5p⁶) is completely filled with eight electrons.
6. Radon (Rn): Radon has an atomic number of 86. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶. The sixth electron shell (n=6) can hold a maximum of 72 electrons, but radon's outer shell (6s² 6p⁶) is completely filled with eight electrons.
7. Oganesson (Og): Oganesson has an atomic number of 118. Its electron configuration is [Rn] 7s² 5f¹⁴ 6d¹⁰ 7p⁶. The seventh electron shell (n=7) can hold a maximum of 98 electrons, but oganesson's outer shell (7s² 7p⁶) is predicted to be completely filled with eight electrons. Due to its radioactivity and extremely short half-life, its properties are mostly theoretical.

The Octet Rule and the Exception of Helium

The observation that noble gases, except for helium, have eight electrons in their outermost shell leads to the octet rule. The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons. This rule is a useful guideline for understanding chemical bonding, although there are exceptions.

Helium is an exception to the octet rule because it only has two electrons, and its first electron shell (n=1) can only hold a maximum of two electrons. Therefore, helium achieves stability with a filled 1s² electron configuration, resembling the stability of other noble gases.

Properties of Noble Gases

The complete outer shells of noble gases give rise to their distinctive properties:

1. Inertness: Noble gases are known for their chemical inertness. They have little to no tendency to form chemical bonds with other atoms because their outer shells are already stable.
2. Monoatomic Nature: Noble gases exist as individual atoms rather than forming molecules. This is because they do not need to share electrons to achieve stability.
3. Low Boiling Points: Noble gases have very low boiling points compared to other elements. This is because the only intermolecular forces between noble gas atoms are weak London dispersion forces. The strength of London dispersion forces increases with the size and number of electrons in the atom.
4. Gaseous State at Room Temperature: Noble gases are all gases at room temperature due to their weak intermolecular forces.
5. Colorless and Odorless: Noble gases are colorless and odorless in their elemental state.
6. Low Density: Noble gases have low densities compared to other elements and compounds.

Applications of Noble Gases

Despite their inertness, noble gases have a wide range of applications in various fields:

1. Helium (He):
   • Cryogenics: Liquid helium is used as a coolant in cryogenics due to its extremely low boiling point (-269 °C or -452 °F).
   • Balloons and Airships: Helium is used to fill balloons and airships because it is lighter than air and non-flammable.
   • Magnetic Resonance Imaging (MRI): Helium is used to cool superconducting magnets in MRI machines.
   • Welding: Helium is used as a shielding gas in welding to prevent oxidation.
2. Neon (Ne):
   • Neon Signs: Neon is used in neon signs because it emits a bright reddish-orange light when an electric current is passed through it.
   • High-Voltage Indicators: Neon is used in high-voltage indicators and switching gear.
   • Plasma Research: Neon is used in plasma research and experiments.
3. Argon (Ar):
   • Welding: Argon is used as a shielding gas in welding, similar to helium.
   • Incandescent Light Bulbs: Argon is used to fill incandescent light bulbs to prevent the filament from oxidizing.
   • Preservation: Argon is used to preserve documents and artifacts by displacing oxygen and preventing degradation.
   • Medical Procedures: Argon is used in some medical procedures, such as argon plasma coagulation.
4. Krypton (Kr):
   • Lighting: Krypton is used in some high-intensity lamps, such as those used in airport runway lighting.
   • Lasers: Krypton is used in certain types of lasers.
   • Photography: Krypton is used in some photographic flashes.
5. Xenon (Xe):
   • Lighting: Xenon is used in high-intensity lamps, such as those used in car headlights and projectors.
   • Anesthesia: Xenon has anesthetic properties and is used as an anesthetic in some medical procedures.
   • Ion Propulsion: Xenon is used as a propellant in ion propulsion systems for spacecraft.
   • Medical Imaging: Xenon is used in some medical imaging techniques.
6. Radon (Rn):
   • Radiotherapy: Radon was historically used in radiotherapy for the treatment of certain cancers, but its use has declined due to its radioactivity and the availability of safer alternatives.
   • Geological Tracing: Radon is used as a tracer in geological studies to understand groundwater movement.
7. Oganesson (Og):
   • Scientific Research: Oganesson is primarily used for scientific research due to its radioactivity and extremely short half-life.

Chemical Reactivity of Noble Gases: A Closer Look

While noble gases are generally considered inert, it is not entirely accurate to say they are completely unreactive. Under certain extreme conditions, some noble gases can form compounds with highly electronegative elements like fluorine and oxygen.

• Xenon Compounds: Xenon is the most reactive of the noble gases and forms several compounds, including xenon difluoride (XeF₂), xenon tetrafluoride (XeF₄), and xenon hexafluoride (XeF₆). These compounds are formed by reacting xenon with fluorine at high temperatures and pressures. Xenon can also form compounds with oxygen, such as xenon trioxide (XeO₃).
• Krypton Compounds: Krypton can form a few compounds, primarily with fluorine. Krypton difluoride (KrF₂) is the most well-known krypton compound.
• Radon Compounds: Radon is predicted to be more reactive than xenon due to its lower ionization energy. However, due to its radioactivity and short half-life, only a few radon compounds have been synthesized and characterized. Radon difluoride (RnF₂) is the most well-known radon compound.
• Argon, Neon, and Helium: Argon, neon, and helium are generally considered to be truly inert and do not form stable chemical compounds under normal conditions. However, there have been reports of the formation of some exotic compounds under extreme conditions, such as in matrices at very low temperatures.
• Oganesson: Oganesson's chemical properties are largely unknown due to its extremely short half-life and the difficulty of producing it in sufficient quantities for experimentation. Theoretical calculations suggest that oganesson might be more reactive than the lighter noble gases due to relativistic effects, which can affect the energies and shapes of its electron orbitals.

Relativistic Effects and Their Influence

Relativistic effects become significant for heavy elements like gold, mercury and the noble gas Oganesson. These effects arise from the fact that electrons in heavy atoms move at a significant fraction of the speed of light, which alters their mass and energy. In the case of oganesson, relativistic effects are predicted to:

• Stabilize the 7s electrons: This makes it more difficult to remove these electrons, which could reduce oganesson's reactivity.
• Destabilize the 7p electrons: This makes it easier to remove these electrons, which could increase oganesson's reactivity.
• Cause the 7p orbitals to split: This can affect the bonding properties of oganesson.

The overall effect of relativistic effects on oganesson's reactivity is still a matter of debate, but it is generally believed that oganesson might be more reactive than expected based on simple extrapolation from the lighter noble gases.

Historical Context and Discovery

The discovery of noble gases spanned several decades and involved contributions from multiple scientists:

• Henry Cavendish (1784): Cavendish noted that when he reacted oxygen and nitrogen in the air to form nitrogen oxides, a small bubble of unreactive gas remained. He did not identify this gas but laid the groundwork for future discoveries.
• Lord Rayleigh and William Ramsay (1890s): Lord Rayleigh observed that nitrogen extracted from the air was denser than pure nitrogen obtained from chemical compounds. William Ramsay hypothesized that this difference was due to the presence of a heavier, unknown gas in the air.
• Discovery of Argon (1894): Rayleigh and Ramsay isolated and identified argon, the first noble gas to be discovered. The name "argon" comes from the Greek word "argos," meaning "lazy" or "inactive," reflecting its chemical inertness.
• Discovery of Helium (1895): Ramsay isolated helium from the mineral cleveite. Helium had previously been detected in the solar spectrum by Pierre Janssen and Norman Lockyer in 1868.
• Discovery of Neon, Krypton, and Xenon (1898): Ramsay and his colleague Morris Travers discovered neon, krypton, and xenon by fractionating liquid air.
• Discovery of Radon (1900): Friedrich Ernst Dorn discovered radon as a radioactive decay product of radium.
• Synthesis of Oganesson (2002): Oganesson was synthesized for the first time in 2002 by a joint team of Russian and American scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia.

Conclusion

Elements with complete outer shells, the noble gases, stand as a testament to the fundamental principles of chemical stability. Their inertness, stemming from their filled valence shells, makes them unique and valuable in various applications. From the cryogenic properties of helium to the bright glow of neon, these elements have found their place in science, technology, and medicine. While their reactivity was once considered non-existent, scientists have discovered that some noble gases can form compounds under specific conditions, expanding our understanding of chemical bonding and the behavior of matter. As we continue to explore the periodic table and the properties of elements, the noble gases will undoubtedly remain a subject of fascination and a cornerstone of chemical knowledge.

FAQ

1. Why are noble gases called "noble"?
   • Noble gases are called "noble" because of their chemical inertness. They were once thought to be completely unreactive, like nobles who did not interact with commoners.
2. Are noble gases really inert?
   • While noble gases are generally considered inert, some of them can form compounds with highly electronegative elements like fluorine and oxygen under certain conditions.
3. What is the octet rule, and how does it relate to noble gases?
   • The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons. Noble gases, except for helium, have eight electrons in their outermost shell, which makes them stable and unreactive.
4. Why is helium an exception to the octet rule?
   • Helium only has two electrons, and its first electron shell (n=1) can only hold a maximum of two electrons. Therefore, helium achieves stability with a filled 1s² electron configuration.
5. What are some common uses of noble gases?
   • Noble gases have various applications, including cryogenics (helium), lighting (neon, krypton, xenon), welding (helium, argon), anesthesia (xenon), and scientific research (oganesson).
6. What are relativistic effects, and how do they affect oganesson?
   • Relativistic effects arise from the fact that electrons in heavy atoms move at a significant fraction of the speed of light, which alters their mass and energy. In the case of oganesson, relativistic effects are predicted to affect the energies and shapes of its electron orbitals, potentially making it more reactive than expected.
7. Which noble gas is the most reactive?
   • Xenon is the most reactive of the noble gases and forms several compounds with fluorine and oxygen.
8. Can argon, neon, and helium form compounds?
   • Argon, neon, and helium are generally considered to be truly inert and do not form stable chemical compounds under normal conditions. However, there have been reports of the formation of some exotic compounds under extreme conditions.
9. How were noble gases discovered?
   • Noble gases were discovered through the work of several scientists, including Henry Cavendish, Lord Rayleigh, William Ramsay, and others, who identified and isolated these elements from air and minerals.
10. Why are noble gases monoatomic?
    • Noble gases are monoatomic because they have complete outer shells and do not need to share electrons to achieve stability. Therefore, they exist as individual atoms rather than forming molecules.