Diatomic elements are examples of homonuclear molecules , where all of the atoms in the molecule are the same. The chemical bonds between the atoms are covalent and nonpolar. The seven diatomic elements are:.
Some sources will say there are five diatomic elements, rather than seven. This is because only five elements form stable diatomic molecules at standard temperature and pressure: the gases hydrogen, nitrogen, oxygen, fluorine, and chlorine. Bromine and iodine form homonuclear diatomic molecules at slightly higher temperatures.
It's possible that an eighth element forms a diatomic molecule. The status of astatine is unknown. Many other diatomic molecules consist of two elements. In fact, most elements form diatomic molecules, particularly at higher temperatures.
Past a certain temperature, however, all molecules break into their constituent atoms. The noble gases do not form diatomic molecules. Diatomic molecules consisting of two different elements are called heteronuclear molecules. Here are some heteronuclear diatomic molecules:. There are many binary compounds consisting of a 1-to-1 ratio of two types of atoms, yet they are not always considered to be diatomic molecules. The reason is that these compounds are only gaseous diatomic molecules when they are evaporated.
When they cool to room temperature, the molecules form polymers. Examples of this type of compound include silicon oxide SiO and magnesium oxide MgO. All diatomic molecules have linear geometry.
There isn't any other possible geometry because connecting a pair of objects necessarily produces a line. Linear geometry is the simplest arrangement of atoms in a molecule.
It's possible for additional elements to form homonuclear diatomic molecules. Also, many diatomic molecules are unstable and highly reactive, such as diphosphorus. A few compounds are made of diatomic molecules, including CO and HBr. If a diatomic molecule consists of two atoms of the same element, such as H 2 and O 2 , then it is said to be homonuclear , but otherwise it is said to heteronuclear , such as with CO or NO. The bond in a homonuclear diatomic molecule is non-polar and fully covalent.
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 As a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century. As early as , Gay-Lussac and von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen, and by 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 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 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. 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 energy of the molecule is simply given by the kinetic energy expression:. For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by.
So, substituting the angular momentum and moment of inertia into E rot , the rotational energy levels of a diatomic molecule are given by:. Another way a diatomic molecule can move is to have each atom oscillate - or vibrate - along a line the bond connecting the two atoms. This information is visually-highlighted in the structure shown below using a blue circle around hydrogen and a green box around iodine. Finally, in order to generate a structure that is more visually-appealing, the shared pair of electrons is replaced with a line that connects the adjacent elemental symbols.
The remaining electrons are redrawn as dots on the resultant structure, as shown below. In the structure that is generated upon the completion of this final step, the line represents a covalent bond, or a shared pair of electrons, and the remaining pairs of dots are called lone pairs. The structure shown above, which is a chemically-correct representation of a covalent compound, is the Lewis structure that represents the molecule that is formed when hydrogen and iodine bond with one another.
Because this molecule only contains one atom of two different elements, it is classified as a heteronuclear diatomic molecule. Draw the Lewis structure that represents the compound that is formed when two chlorine atoms bond with one another. Two chemically-correct electron dot structures for this element are shown below. However, in the current example, each c h l o r i n e atom has the same number of unpaired electrons.
Therefore, in order to satisfy the valences of each of these atoms, the unpaired electrons on each c h l o r i n e atom must be paired, in order to create a shared pair of electrons. This structure contains one shared pair of electrons, which was created by pairing the unpaired electrons on each c h l o r i n e atom. As this electron pair is located in between both c h l o r i n e electron dot structures, these electrons contribute to the overall electron configuration of both atoms.
By correctly executing the pairing process, each c h l o r i n e atom is surrounded by a total of eight, fully-paired dots. This information is visually-highlighted in the structure shown below using a blue circle around one chlorine atom and a green box around the other chlorine atom.
As an octet configuration is the most stable electron arrangement that can be achieved by an atom, this structure represents the most stable bonding arrangement that can be achieved by combining two chlorine atoms.
The structure shown above, which is a chemically-correct representation of a covalent compound, is the Lewis structure that represents the molecule that is formed when two chlorine atoms bond with one another.
Because this molecule only contains two atoms of the same element, it is classified as a homonuclear diatomic molecule. For a covalent molecule, the information represented in its chemical formula must be a direct reflection of its Lewis structure. Elemental symbols are incorporated into a chemical formula by counting the number of times that each symbol appears in the corresponding Lewis structure.
In order to ensure consistent formatting, the elemental symbol that appears fewer times is written first in a covalent chemical formula, and subscripts are used to indicate how many times each elemental symbol appears in the Lewis structure. As indicated previously, values of "1" are usually implicitly-understood in chemistry and, therefore, should not be written in a chemical formula. The subscripts must not be reduced to the lowest-common ratio of whole numbers, even if it is mathematically-possible to do so , as dividing the subscripts would cause their values to be inconsistent with the number of times that each elemental symbol appears in the Lewis structure.
The chemical formula of a heteronuclear diatomic molecule can be determined using a modified version of the rules presented above. For example, consider the Lewis structure shown below, which represents the covalent molecule that is formed when hydrogen and iodine bond with one another.
This Lewis structure contains one hydrogen atom and one iodine atom. As stated above, the elemental symbol that appears fewer times is typically written first in a covalent chemical formula.
However, in the current example, the elements are present in equal quantities. As a result, the order in which the elemental symbols are written in the corresponding chemical formula must be determined using a secondary rule: The elemental symbol for the element that is farther away from fluorine on the periodic table is written first. Therefore, the elemental symbol for hydrogen , " H ," is written before iodine's elemental symbol, " I.
The resultant chemical formula, H I , accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule. The chemical formula of a homonuclear diatomic molecule can be determined using a modified version of the rules presented above. For example, consider the Lewis structure shown below, which represents the covalent molecule that is formed when two chlorine atoms bond with one another.
As only one type of element is present in this Lewis structure, only one elemental symbol, " Cl ," is written in the corresponding chemical formula. Furthermore, because this Lewis structure contains two chlorine atoms, a subscript of " 2 " should be written on c h l o r i n e ' s elemental symbol.
The resultant chemical formula, Cl 2 , accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule. For a covalent molecule, the information represented in its chemical name must also be a direct reflection of its Lewis structure. Therefore, the chemical name of a covalent molecule must contain information that indicates the identities of its constituent elements and usually reflects how many of each of those elements are present within the molecule.
Note that, if written properly, the chemical formula for a covalent molecule also contains this information and, therefore, can be used as the basis for developing a chemical name. Elemental names are incorporated into a covalent molecule's chemical name in the order in which their corresponding elemental symbols appear in the chemical formula.
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