3.6: Molecular Compounds - Formulas and Names (2023)

learning goals

• Derive names for common types of inorganic compounds using a systematic approach

Compounds of two elements

When two non-metallic elements form a molecular compound, different combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO₂. Because they are different substances with different properties, they cannot have the same name (they cannot be called carbon oxide). To deal with this situation, we use a naming method somewhat similar to that for ionic compounds, but with additional prefixes to indicate the number of atoms of each element. The name of the most metallic element (the leftmost and/or bottom of the periodic table) comes first, followed by the name of the most nonmetallic element (the rightmost and/or top) with its ending changed to the suffix -Idea. The ordinal numbers of each element are denoted by the Greek prefixes shown in table \(\PageIndex{3}\).

If there is only one atom of the first element, the prefixMono- is usually excluded from this part. Therefore, CO is called carbon monoxide and CO₂It's called carbon dioxide. When two vowels are next to each other, theAin the Greek prefix is ​​usually omitted. Some other examples are shown in table \(\PageIndex{4}\).

There are some common names that you will come across as you continue your study of chemistry. For example, although NO is often referred to as nitric oxide, its real name is nitric oxide. In the same way N₂O is known as laughing gas, although our rules call for the name nitrous oxide. (And h₂O is usually referred to as water, not dihydrogen monoxide.) You should memorize the common names of compounds as you encounter them.

binary acids

Some hydrogen-containing compounds belong to an important class of substances known as acids. The chemistry of these compounds will be explored in more detail in later chapters of this text, but for now suffice it to say that many acids liberate hydrogen ions, H⁺, when dissolved in water. To denote this distinctive chemical property, a mixture of water and an acid is given a name derived from the name of the compound. If the connection abinary acid(composed of hydrogen and another non-metallic element):

1. The word "hydrogen" is changed to the prefixWater-
2. The name of the other non-metallic element is modified by adding the suffix -ic
3. The word “acid” is added as the second word

For example, when HCl (hydrogen chloride) gas is dissolved in water, the solution is calledhydrochloric acid. Several other examples of this nomenclature are shown in table \(\PageIndex{5}\).

Oxyazidos

Many compounds that contain three or more elements (e.g. organic compounds or coordination compounds) are subject to special naming rules that you will learn about later. However, let's briefly touch on the important connections known asoxyacids, compounds containing hydrogen, oxygen, and at least one other element, bonded together to give the compound acidic properties (you'll learn the details of this in a later chapter). Typical oxyacids consist of hydrogen combined with a polyatomic ion containing oxygen. To name oxyacids:

1. Omit "hydrogen".
2. Start with the root name of the anion
3. Substitute -he atecom -ic, or -I youcom -us
4. Add "acid".

For example, think of H₂CO₃(which you might be tempted to call "bicarbonate"). To call it right, "hydrogen" is omitted; O -he ateof carbonate is replaced by -ic; and acid is added - hence it is called carbonic acid. You can find more examples in table \(\PageIndex{6}\). There are some exceptions to the general naming method (e.g. H₂THEN₄is called sulfuric acid, not sulfuric acid, and H₂THEN₃it is sulphurous, non-sulphurous, acidic).