In chemistry, the naming of molecular compounds helps in the determination of the number of atoms and the nature of a compound. A molecular compound is formed when two or more atoms are bonded together in a fixed proportion.
Since ancient times, elements and compounds have been represented by symbols. John Dalton (1766-1844) had a unique way of representing the elements and compounds of his time.
The representation symbols of some chemical elements and compounds by John Dalton are given below.
Later on, the way of naming elements and compounds got changed from symbols to writings. The works of 19th-century Swedish chemist Jason Jakob Berzelius led to this invention. He started a system of naming elements via letters. These letters would most probably be the first letters of the names of elements. According to Berzelius, hydrogen, oxygen, nitrogen, carbon, phosphorous, and sulfur were named H, O, N, C, P, S respectively.
Rules for naming molecular compounds
Molecular compounds are basically covalent compounds. They are mainly composed of nonmetals. For example, sulfur hexafluoride (SF6) and dinitrogen pentoxide (N2O5).
Following are the rules for naming the chemical molecular compounds:
1. For the first element, start with its original name
The first element of any molecular compound can be pronounced with its original name. For example, in the case of SF6, the first element is sulfur. The name of this compound starts with sulfur, i.e. sulfur hexafluoride.
Similarly, in the case of N2O5. The first element is nitrogen. There are two nitrogen atoms in the compound. So the name of this compound starts with dinitrogen. Di is a prefix here. It is used first as the number of nitrogen is more than one in this case. For example Dinitrogen pentoxide, etc.
2. For the second element, end with the -ide name
This step involves the addition of -ide at the end of elements, such as sulfide (sulfur), oxide (oxygen), etc.
The second element ends with the -ide name. For example, in the case of dinitrogen trioxide (N2O3). The second element is oxygen. At the start of oxygen, (tri) is used as a prefix, and at the end of oxygen, (ide) is used. For example, Dinitrogen pentox(ide).
3. Use of prefixes for identifying the number of atoms
Prefixes are used to determine the number of atoms involved in molecular compounds. Common prefixes include mono, di, tri, tetra, penta, etc.
For example, there are two (di) nitrogen and three (tri) oxygen involved in the case of N2O3. So the name of this molecular compound is dinitrogen trioxide.
4. Do not use “mono” as a prefix for the first element
There is no need to add mono with the first element. The prefix can be used when there is more than one atom of the first element present. For example, the molecular name of (CO) is carbon monoxide, and (CF4) is carbon tetrafluoride, etc.
5. Turn “oa” and “oo” into a single “o”
Sometimes, while naming the chemical compounds, a problem arises when compounds with “ao” and “oo” are written with -ide name. Conventionally, they are converted into a single “o”. For example, the “ao” in the dichlorine hept-ao-xide (Cl2O7). This “ao” in the Cl2O7 can be converted into “o”. As the result, the correct name of this compound would be dichlorine hept-o-xide.
Examples of molecular compounds with their names:
- Carbon monoxide CO
- Carbon dioxide CO2
- Dichlorine heptoxide Cl2O7
- Tetraphosphorus hexoxide P4O6
- Dinitrogen trioxide N2O3
- Carbon disulfide CS2
- Silicon tetraiodide SiI4, etc
How to name simple molecular compounds?
There are several basic and common molecular compounds, such as hydrogen chloride (HCl). Being simple, they just have one name. There is no need to attach any prefixes or suffixes in order to indicate the number of molecules.
The common chemical name for H2O is water instead of di-hydrogen oxide. Naming these kinds of molecules simply involves naming each part of it separately, its elemental name plus its chemical symbol.
- chlorine + oxygen = hypochlorous acid
- nitrogen + hydrogen = ammonia
If more than one atom exists in a molecule. Then there is a need to add a prefix to indicate the number of atoms.
Examples of simple molecular compounds are:
- Water, H2O
- Hydrochloric acid, HCl
- Ammonia, NH3
- Methane, CH4, etc
How to name complex molecular compounds?
Complex molecular compounds are composed of two or more different elements linked together. While naming them, it is a common practice in chemistry not to include symbols for each individual element. But rather use a single word (in italics) that reflects an overall category or class of elements. However, before naming any complex molecular compound, the number of atoms present must be known.
They can also be named according to their geometric structure using structural diagrams. These diagrams consist of capital letters or numbers which denote points on a line representing one or more chemical bonds between atoms within a molecule.
Other naming considerations for molecular compounds
Different naming systems are used in different regions and contexts. For example, IUPAC rules apply mostly in organic chemistry. Whereas, IUPAP rules apply mostly in physics and astronomy. So, sometimes two or more names will be equally valid for a single substance. One way of dealing with that is writing out all possible names for a substance.
CAS Number: A Unique Chemical Identifier
Chemical Abstract Service registry number of simply CAS number is a unique numerical identifier that can give particular information about a certain chemical compound. It serves as a unique code for a particular compound just like a person’s social security number or a car’s license plate number. In fact, it is more useful because of being recognized internationally.
It is a naming mechanism for chemical compounds, an alternative to the IUPAC system.
What are the rules for naming compounds?
- The first element starts with its name.
- The second element ends with its -ide name.
- Use prefixes to identify the number of atoms.
How do you write and name a molecular compound using prefixes?
According to the rules of naming molecular compounds, prefixes are used before naming the element. They show the number of atoms. For example, N2O3 (dinitrogen trioxide).
What is the importance of naming compounds?
By naming we can identify the compounds, their arrangement of atoms, the number of atoms, the shape of molecules, etc.
What is the name of the molecular compound PCl5?
The molecular name of PCl5 is phosphorus pentachloride.
What is the name of the molecule C6H6?
To determine a chemical name, you must know which element is bonded to each carbon atom. For example, C6H6 means there are six carbon atoms bonded together. Each of these carbons has one hydrogen atom bonded to it. Knowing these two things makes it easier for us to identify the molecule. The name of this molecule is benzene. It has great importance in organic and pharmaceutical chemistry.
What is the chemical name of Cl2O7?
The chemical name of this compound is ‘dichlorine heptoxide’.
What is the chemical name for P4O10?
The name of this molecule is tetra phosphorous deca oxide. It is also named phosphorous pentoxide because it is actually P2O5.
The 100 Most Important Chemical Compounds: A Reference Guide By Richard L. Myers (Boston University)
- Naming Molecular Compounds (courses.lumenlearning.com)