Nouns can be classified into two groups, countable and uncountable. Countable nouns typically occur as discrete units that can be counted, whereas uncountable nouns, also called mass nouns, are typically substances or concepts.
Countable: electron, flask, joule
Uncountable: oxygen, heat, patriotism
Countable nouns can be made plural. Uncountable nouns are not used in the plural.
Countable: polar regions, transistors, earthquakes
Uncountable: carbon dioxide, infrared radiation, equipment
You can refer to discrete units of uncountable nouns by adding a counting word, such as a unit of measurement, or the general word piece.
a carbon dioxide; an equipment
a molecule of carbon dioxide; a piece of equipment
Although it is often easy to distinguish between countable and uncountable nouns, the distinction is not always obvious for all nouns. In addition, many nouns can be used as either countable or uncountable elements, but with different meanings. For this reason, you may find it useful to consult a specialized dictionary when you are not sure whether a particular noun is countable or uncountable or whether it has special meanings in one category or the other.