2. Introduction to the Group 0 Noble Gases

• The "Noble Gases" are the last group in the Periodic Table i.e. they form the last elements at the end of a period, the final vertical column of elements in the periodic table.

  • In descending order, the six noble gases are ...

    • helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).

    • They were called noble 'gases' because they were so unreactive (the same reasons unreactive metals like gold are sometimes referred to as a 'noble metal').

    • They are described as monatomic, because they exist as individual atoms and do not form e.g. diatomic molecules like hydrogen, chlorine or oxygen.

• Noble gases form the last element of a any period, with a full outer electron shell. This outer electron similarity of the noble gases makes them behave in a chemically similar way e.g. the least reactive of any elements and is a modern pre-requisite of a set of elements belonging to the same group. BUT their similarity in physical properties and the chemical reactions now known (e.g. for xenon) fits in well with Mendeleev's original conception of a group classification even though he had no idea they existed !

• The noble gases are all non-metallic elements and all are colourless gases at room temperature and pressure with very low melting points and boiling points.

• The noble gases form 1% of air, and most of this is argon.

• They are not flammable, a property that makes them very useful for certain applications.

• All the noble gases, except radioactive radon, are separated by the fractional distillation of liquefied air.

  • Helium can also be obtained from natural gas wells where it has accumulated from radioactive alpha decay (alpha particles become atoms of helium gas when they gain two electrons).

  • alpha particle decay of the radioisotope uranium-235

  • Many of the unstable nuclei of heavier atoms (high proton/atomic number) decay by alpha emission. The alpha particle (helium nucleus of 2 protons + 2 neutrons) grabs 2 electrons to form a stable atom of helium gas.

• The electronic structure of the noble gases is the basis for explaining why they are so unreactive.

• The noble gases are very unreactive elements because the highest occupied electron level is complete, meaning they have a full shell of outer electrons! (see diagrams below).

  • Helium has a full outer shell of 2 electrons, and the rest of the noble gases - neon, argon, xenon etc. also have a full outer shell of 8 electrons, giving them great electronic stability and therefore chemical stability - very unreactive.

  • The group 0 noble gases are the least reactive elements in the whole of the periodic table.

  • They have no great 'wish' electronically to share electrons to form a covalent bond or to lose or gain electrons to form an ion as part of an ionic bond. In other words, they are electronically very stable and don't wish to change!

  • They are sometimes referred to as inert gases, because of their lack of chemical reactivity.

  • In fact their lack of reactivity was one reason why they were some of the last naturally occurring elements of the periodic table to be discovered.

    • They were discovered because the density of chemically made nitrogen was different from the nitrogen obtained from air after removal of oxygen.

    • Therefore it was reasoned that there must be some 'unknown' gases in air.

    • With improved technology in the early 20th century, careful low temperature fractional distillation of liquefied air revealed fractions containing noble gases, and this is how they are usually obtained in industry.

• The noble gases exist as single atoms, that is they are 'monatomic molecules' He Ne Ar etc. (NOT diatomic molecules as with many other gases like oxygen O₂ and nitrogen N₂ - reasons given above). This is because of their electronic stability - see point above and diagrams below.

• The very inertness of the noble gases and their non-flammability, are important features of their practical uses. 

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3. Group 0 noble gas trends in physical properties and explanations (data table)

• Important group 0 noble gas trends to know down the group  with increase in atomic number and relative atomic mass...

  • ... down the group the noble gas atoms get larger, the increase in radius is due to an extra shell of electrons added from one atom to the next down the group - the volume of the atom increases with more electrons in it.

  • ... the melting point and boiling point of noble gases steadily increase down the group (see data) because the weak intermolecular bonding forces increase with the size of the atom, from one noble gas down to the next, there is an extra shell of electrons.

    • This is because of the increase in the number of electrons, due to an extra electron shell from one noble gas, down the group to the noble gas on the next period.

    • This trend produces an increasingly larger atom with more, which results in an increase in the weak electrical attractive forces between the noble gas atoms (increasing intermolecular forces/intermolecular bonding).

      • Generally speaking, these weak attractive electrical forces, the intermolecular forces (NOT chemical bonds) increase with increasing numbers of electrons in the molecule.

      • Although noble gases exist as single atoms, the argument is the same for a series of hydrocarbon compounds like alkanes where the boiling points increase with increase in size of molecules.

    • So, because the strength of the intermolecular forces or intermolecular bonding forces between the atoms increases, so more energy (higher particle kinetic energy of vibration or free movement) at a higher temperature) is needed to melt or boil the condensed states of Noble Gas atoms and overcome these attractive forces.

    • Note this intermolecular force argument is the same for molecules of at least two atoms - think of the trend with hydrocarbon molecules like alkanes - its often just about the number of electrons in the 'particle'.

  • ... the density of noble gases steadily increases down the group (see data)

  • ... down the group, theoretically, the monatomic noble gases are more likely to react and form a compound with very reactive elements like fluorine, this is because the outer electrons are increasingly less strongly held giving a greater chance of electron sharing in forming a covalent bond with other very reactive non-metals like fluorine, chlorine and oxygen e.g.

    • Stable compounds of xenon, like xenon tetrafluoride, are now known and synthesised BUT not before 1961!

    • The most reactive Noble Gas would be radon BUT this is a highly dangerous radioactive gas!

    • Some VERY unstable compounds have been made from argon and krypton.

    • The reason for the lack of reactivity by noble gases is further explained in the box below.

The electron arrangement of the first 4 Noble Gases with very stable full outer shells (are shown) making the noble gas elements VERY unreactive.

Because the noble gas atoms have a full, and very stable, outer electron shell, they are very reluctant to share electrons to form a covalent bond OR lose or gain electrons to form an ion and ionic bond.

As the atom gets bigger, the outer electrons are less strongly held and the lower member noble gases e.g. xenon can be tempted into chemically reacting with VERY reactive chemical reagents like fluorine (the most reactive non-metal).

The electronic situation is quite different from:

(i) the Alkali Metals, whose atoms readily lose an electron to form a positive ion with a noble gas arrangement, in an ionic compound like sodium chloride, NaCl,

(ii) the Halogens, non-metal atoms, which readily gain an electron from a metal atom to form a negative ion with a noble gas arrangement, as in sodium chloride, AND, they also readily share an electron to form a covalent bond with other non-metals like hydrogen (HCl) or with themselves like the chlorine molecule, Cl₂.

5. Uses of the Group 0/8 Noble Gases

The noble gases are non-flammable and chemically very unreactive and this chemical inertness is an important characteristic when understanding the uses and applications of noble gases and sometimes the low density is important too.

Another advantage is that because of its inertness, helium doesn't burn in air UNLIKE hydrogen which used to be used in large balloons with  'flammable' consequences e.g. like the R101 airship disaster!

Helium is also used in gas mixtures for deep-sea divers respiration.

Liquid helium is used to achieve very low temperatures in cryogenics technology.

Helium is used to protect metals that are being welded. The inert atmosphere stops the hot metal being oxidised giving a brittle coating of the metal oxide.

Argon is also used to produce an inert atmosphere in high temperature metallurgical processes, e.g. in welding where it reduces brittle oxide formation reducing the weld quality.

Argon bubbles are used to stir mixtures in steel production.

Argon is used in photographic flash lamps to stop the filament burning up in the high temperature flash.

Argon is the cheapest noble gas to produce, it has the biggest % of any noble gas in air.

Krypton stops the filament burning up in the high temperature flash lamps.

Xenon stops the filament burning up in the high temperature flash lamps.

Revision notes on the physical and chemical properties of the non-metal group 0 noble gases inert gases, explaining lack of chemical reactivity of noble gases, boiling/melting point trends of noble gases, uses of noble gases, help when revising for AQA GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry, OCR GCSE 21st century science chemistry GCSE 9-1 chemistry examination questions on inert gases.

6. Extra 'bits and bobs' on the Noble Gases

Xe + 2F₂ ==> XeF₄

Using Ni catalyst 60^oC, easy if you know how! and another catalytic example of a transition metal.

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