The Smallest Things: Unseen Mono-Aromatic Molecules

The term monatomic is a combination of the words mono and atomic, meaning single atom. Monatomic molecules are atoms that exist in their ground state without any orbital angular momentum or spin. In physics and chemistry, this term is usually applied to gases, where atoms are not bound to each other. At standard temperature and pressure, the only chemical elements that are stable single atoms are the noble gases: helium, neon, argon, krypton, xenon, and radon.

Monatomic molecules are single atoms that exist in their ground state without any angular momentum or spin

At standard temperature and pressure, the noble gases (helium, neon, argon, krypton, xenon, and radon) are the only chemical elements that exist as stable single atoms. These gases have a full outer valence shell, making them non-reactive and relatively stable.

Monatomic molecules have no vibrational or rotational states, and therefore only contribute to properties like chemical potential and entropy in a gas. The thermodynamic behaviour of monatomic gases is much simpler when compared to polyatomic gases because they are free from rotational or vibrational energy.

The term "monatomic" is also used to describe ions made up of only one element. However, each such ion can contain several atoms of that single element. For example, silver and zinc ions are Type I monatomic ions, always having the same charge in ionic form.

It is important to distinguish monatomic elements from pure elements, which may consist of multiple atoms bonded into diatomic elements (e.g. H2, O2) or other molecules with a single type of atom (e.g. ozone, O3). Pure elements like silver are homonuclear but not monatomic.

While monatomic gold has been marketed as a hoax, with various miraculous properties, there is no scientific evidence to support its existence or that of any other monatomic element besides the noble gases.

Monatomic molecules are stable as single atoms

Noble gases, such as helium, neon, and argon, are examples of monatomic elements that exist as single, unattached particles. They have full outer energy levels, which means they satisfy the octet rule and do not usually form molecules with other atoms. Their stability also means they don't readily form chemical bonds and compounds with other elements.

At high temperatures, other elements can exist in monatomic form. For instance, atomic hydrogen, which is electrically neutral, accounts for about 75% of the mass of the universe. Monatomic oxygen, which makes up 96% of the oxygen in low Earth orbit, is another example.

The thermodynamic behaviour of monatomic gases is much simpler than polyatomic gases because they are free of any rotational or vibrational energy. They only have translational energy, which makes their behaviour easier to predict.

Monatomic molecules are commonly confused with pure elements

Monatomic molecules, or elements, are those that are stable as single atoms. The prefix 'mono-' means 'one', and in order for an element to be stable by itself, it needs to have a stable octet of valence electrons.

Monatomic elements are often confused with pure elements. Pure elements may consist of multiple atoms bonded into diatomic elements, such as H2 and O2, or other molecules consisting of a single type of atom, like ozone or O3. These molecules are homonuclear, meaning they only consist of one type of atomic nucleus, but they are not monatomic. Metals, for example, are typically connected via metallic bonds, so a sample of pure silver would be considered homonuclear but not monatomic.

The noble gases are the only chemical elements that are stable single atoms at standard temperature and pressure (STP). These include helium, neon, argon, krypton, xenon, and radon. Noble gases have a full outer valence shell, making them rather non-reactive. While these elements were historically described as completely inert, chemical compounds have been synthesized with all but neon and helium.

It is worth noting that the term "monatomic" is usually applied to gases. A monatomic gas is one in which atoms are not bound to each other. At sufficiently high temperatures or very low pressures, all chemical elements will be monatomic in the gas phase.

Noble gases are monatomic elements

In physics and chemistry, monatomic gases are gases in which atoms are not bound to each other. Monatomic means "single atom", with "mono-" meaning "one". The noble gases—helium, neon, argon, krypton, xenon, and radon—are monatomic elements. They are the only chemical elements that are stable single atoms at standard temperature and pressure (STP).

Noble gases have a full outer valence shell, making them non-reactive. They have been described as inert, but chemical compounds have been synthesized with all but neon and helium. The full outer energy levels of these stable atoms mean they do not usually form molecules with other atoms. They have low melting and boiling points, as the weak forces of attraction between the atoms are easily overcome.

The thermodynamic behaviour of a monatomic gas is simpler compared to polyatomic gases because it is free of any rotational or vibrational energy. The only possible motion of an atom in a monatomic gas is translation (electronic excitation is not important at room temperature). Thus, by the equipartition theorem, the kinetic energy of a single atom of a monatomic gas at thermodynamic temperature T is given by:

{\displaystyle {\frac {3}{2}}k_{\text{B}}T}

Where kB is the Boltzmann constant.

Noble gases are called "elemental gases" when grouped with homonuclear diatomic gases such as nitrogen (N2). This distinguishes them from molecules that are also chemical compounds.

Monatomic molecules cannot have any vibrational or rotational states

Monatomic molecules, such as He and Ne, are atoms that exist in their ground state without any orbital angular momentum or spin. They are often treated as "point particles", with zero moments of inertia, and thus carry no internal rotational energy.

In the case of a monatomic molecule, the moment of inertia about the symmetry axis is zero. This means there is no kinetic energy for rotation about this axis.

A molecule with more than one atom, such as a diatomic molecule, has additional degrees of freedom associated with its rotational components. This is because the molecule has spatial extension, and its nuclei are separated by a bond length from the origin.

Therefore, a monatomic molecule cannot have any vibrational or rotational states, and its internal motions only contribute to properties like chemical potential and entropy in a gas.

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