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From Wikipedia

Diatomic molecule

Diatomic molecules are molecules composed only of two atoms, of either the same or different chemical elements. The prefix di- means two in Greek. Common diatomic molecules are hydrogen (H2), nitrogen (N2), oxygen (O2), and carbon monoxide (CO). Seven elements exist in the diatomic state in the liquid and solid forms: H2, N2, O2, F2, Cl2, Br2, and I2. Most elements (and many chemical compounds) aside from the form diatomic molecules when evaporated, although at very high temperatures, all materials disintegrate into atoms. The noble gases do not form diatomic molecules.


Hundreds of diatomic molecules have been characterized in the terrestrial environment, laboratory, and interstellar medium. About 99% of the Earth's atmosphere is composed of diatomic molecules, specifically oxygen and nitrogen at 21 and 78%, respectively. The natural abundance of hydrogen (H2) in the Earth's atmosphere is only on the order of parts per million, but H2 is, in fact, the most abundant diatomic molecule seen in nature. The interstellar medium is, indeed, dominated by hydrogen atoms.

Elements that consist of diatomic molecules, under typical laboratory conditions of 1 bar and 25 °C, include hydrogen (H2), nitrogen (N2), oxygen (O2), and the halogens. Again, many other diatomics are possible and form when elements are evaporated, but these diatomic species repolymerize at lower temperatures. For example, heating ("cracking") elemental phosphorus gives diphosphorus.

If a diatomic molecule consists of two atoms of the same element, such as H2 and O2, then it is said to be homonuclear, but otherwise it is heteronuclear, such as with CO or NO. The bond in a homonuclear diatomic molecule is non-polar and covalent. In most diatomic molecules, the elements are nonidentical. Prominent examples include carbon monoxide, nitric oxide, and hydrogen chloride, but other important examples include MgO and NaCl.

Molecular geometry

Diatomic molecules cannot have any geometry but linear, as any two points always lie in a line. This is the simplest spatial arrangement of atoms after the sphericity of single atoms.

Historical significance

Diatomic elements played an important role in the elucidation of the concepts of element, atom, and molecule in the 19th century, because some of the most common elements, such as hydrogen, oxygen, and nitrogen, occur as diatomic molecules. John Dalton's original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, giving the atomic weight of oxygen as 8 times that of hydrogen, instead of the modern value of about 16. As a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century.

As early as 1805, Gay-Lussac and von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen, and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules. However, these results were mostly ignored until 1860. Part of this rejection was due to the belief that atoms of one element would have no chemical affinity towards atoms of the same element, and part was due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules.

At the 1860 Karlsruhe Congress on atomic weights, Cannizzaro resurrected Avogadro's ideas and used them to produce a consistent table of atomic weights, which mostly agree with modern values. These weights were an important pre-requisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer.

Energy levels

It is convenient, and common, to represent a diatomic molecule as two point masses (the two atoms) connected by a massless spring. The energies involved in the various motions of the molecule can then be broken down into three categories.

  • The translational energies
  • The rotational energies
  • The vibrational energies

Translational energies

The translational energy of the molecule is simply given by the kinetic energy expression:


where m is the mass of the molecule and v is its velocity.

Rotational energies

Classically, the kinetic energy of rotation is

E_{rot} = \frac{L^2}{2 I} \,
L \, is the angular momentum
I \, is the moment of inertia of the molecule

For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by

L^2 = l(l+1) \hbar^2 \,
where l is a non-negative integer and \hbar is Planck's reduced constant.

Also, for a diatomic molecule the moment of inertia is

I = \mu r_{0}^2 \,
\mu \, is the reduced mass of the molecule and

From Yahoo Answers

Question:Which one of the following exists as a diatomic element at room temperature? a) Na b) N c) B d) Ar e)P

Answers:Na exists as Na metal, N exists as N2 gas, I'm not sure what boron's natural state is.... Argon exists as a unimolecular gas, and phosphorous exists as P6 I think........ The answer is in there.

Question:So: N2 CL2 I2 H2 BR2 ???

Answers:H2, O2, N2, F2, Cl2 - gases Br2 - liquid I2 - solid

Question:Does this have to do with the bonding and structure?

Answers:Yes it does. NaF, or Sodium Fluoride is an Ionic substance. Ionic substances are characterised by higher melting point, non-conductivity, and brittleness. Often when substances combine chemically, the properties of the compound differ from the elements it was formed from. Think of water (H2O) - both H (Hydrogen) and O (Oxygen) are gases at room temperature, but H2O is a liquid. I hope this helps you :-)

Question:AND WHY?

Answers:Ethanol is a liquid at RT Propane is a gas at RT NaCl is a solid at RT These properties are all due to intermoleccular attractions between the molecules in the samples. In gases, molecules are all floating around seperate from one an other. They hardly interact with each other at all. In liquids, molecules are quite close to each other and are held together in a smaller volume then gases. The molecules do not have as much energy as gas molecules. For a molecules to go from liquid to gas phase you have to provide it with enough energy to break away from all the other molecules. In solid phase molecules are very close to each other and hardly move at all. To go from solid to liquid you have to provide enough energy for the molecules to break their attractive forces with each other enough to be able to move around more freely. Propane is a hydrocarbon and is a non-polar compound. There are only very weak attractive forces between 1 propane molecule and another propane molecule. Because of this there are only very small forces holding the molecules in a sample together, so it does not take very much energy for the molecules to be in the gas phase. RT is enough. Ethanol has a polar group on it (OH). Polar groups have a slightly positive and a slightly negative end. The positive and of one molecule is attracted to the negative end of another molecule. So they are held close to each other by these "intermolecular" forces. These forces are moderate in stranght. Because ethanol molecules are attracted to each other like this it takes quite a bit of energy from an individual molecule to break away from these attractions. So ethanol is a liquid at RT. NaCl is an ionic compound. It consists of a cation Na+ and anion Cl- Cations and anions are extremely attracted to each other. In solid NaCl the cations and anions are arranged in a crystal lattice structure with alternating cations and anions that are all very strongly attracted to each other. It takes a large amount of energy to provide enough energy for these bonds to break. For this reason NaCl is solid at RT.

From Youtube

Chemistry Tutorial 7.03b: Monatomic And Diatomic Molecules & Phases Of Elements :Monatomic and diatomic molecules are explained, as are the phases of the elements on the Periodic Table at room temperature.

Effect Of Temperature On Rate Of Reaction :Check us out at www.tutorvista.com As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10 C rise in temperature. You have to be careful not to take this too literally. It doesn't apply to all reactions. Even where it is approximately true, it may be that the rate doubles every 9 C or 11 C or whatever. The number of degrees needed to double the rate will also change gradually as the temperature increases. Particles can only react when they collide. If you heat a substance, the particles move faster and so collide more frequently. That will speed up the rate of reaction. That seems a fairly straightforward explanation until you look at the numbers! It turns out that the frequency of two-particle collisions in gases is proportional to the square root of the kelvin temperature. If you increase the temperature from 293 K to 303 K (20 C to 30°C), you will increase the collision frequency by a factor of: That's an increase of 1.7% for a 10 rise. The rate of reaction will probably have doubled for that increase in temperature - in other words, an increase of about 100%. The effect of increasing collision frequency on the rate of the reaction is very minor Increasing the temperature increases reaction rates because of the disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least ...