Naming Molecular Compounds: Videos & Practice Problems

Molecular compounds, also known as covalent compounds, consist of nonmetals bonded together and require numerical prefixes for naming due to their ability to combine in various proportions. The naming process involves three steps: the first nonmetal is named normally (excluding the prefix 'mono'), the second nonmetal retains its base name but ends with 'ide' and can use any numerical prefix. When the prefix ends with 'a' and is followed by an 'o', the 'a' is dropped to avoid awkward pronunciation. For example, 'tetroxide' becomes 'tetraoxide' when adhering to this rule. Understanding the correct application of these rules is crucial for accurately naming covalent compounds.

Naming Molecular Compounds

Molecular compounds, also known as covalent compounds, consist solely of nonmetals bonded together. Naming these compounds involves the use of numerical prefixes, which are essential because these compounds can form in various proportions. The prefixes range from mono (1) to deca (10). Some commonly recognized prefixes include penta for 5, hexa for 6, hepta for 7, and nona for 9.

To name a molecular compound, follow these steps: First, name the first nonmetal using its standard name and apply the appropriate numerical prefix, except for mono, which is omitted if it is the first element. For the second nonmetal, retain its base name, apply the relevant numerical prefix, and change the ending to ide. It is important to note that if the letter 'a' from the prefix is adjacent to an 'o' from the base name, the 'a' should be dropped. For instance, in the case of tetroxide, the prefix tetra (meaning 4) is combined with the base name for oxygen, which is ox. Since 'a' and 'o' are next to each other, the name simplifies to tetroxide.

Understanding these naming conventions allows for accurate communication of molecular compounds, facilitating further exploration and application of chemistry concepts.

Naming Molecular Compounds Example

When writing chemical formulas for compounds, it's essential to understand the prefixes that indicate the number of atoms present in each element. For example, in the compound disulfur monochloride, the prefix "di-" signifies two sulfur atoms, represented as S2, while "mono-" indicates one chlorine atom, leading to the formula S2Cl.

Next, consider tetraphosphorus pentaselenide. The prefix "tetra-" means four, so there are four phosphorus atoms, denoted as P4. The prefix "penta-" indicates five selenium atoms, represented as Se. Therefore, the complete formula is P4Se5.

Lastly, for dibromoheptoxide, "di-" indicates two bromine atoms, which is Br2. The prefix "hept-" means seven, and "oxide" refers to oxygen, resulting in seven oxygen atoms, or O7. Thus, the formula for dibromoheptoxide is Br2O7.

In summary, when constructing formulas for molecular compounds, the first nonmetal retains its elemental name and uses numerical prefixes (except for "mono-"), while the second nonmetal also uses numerical prefixes but changes its ending to "-ide."

Give the systematic name for the following compound:SeF₆

• A. selenium fluoride
• B. selenium hexafluoride
• C. monoselenium hexafluoride
• D. monoselenium fluoride
• E. selenium fluorite

Answer: (B)

Give the systematic name for the following compound:IO₅

• A. iodine oxide
• B. monoiodine oxide
• C. iodine pentoxide
• D. iodide pentoxide
• E. monoiodide pentoxide

Answer: (C)

Give the systematic name for the following compound:N₂S₄

• A. nitrogen tetrasulfide
• B. dinitrogen tetrasulfide
• C. dinitrogen sulfide
• D. nitrogen sulfide
• E. nitrogen sulfite

Answer: (B)

Here’s what students ask on this topic:

How do you name covalent compounds?

Naming covalent compounds involves a few simple rules. First, the name of the compound is constructed by using the full name of the first element (or a modified version if it's a polyatomic group), followed by the name of the second element with an "-ide" suffix. For example, CO is carbon monoxide.

If there are multiple atoms of an element, you use prefixes to indicate the number of atoms. These prefixes are: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), and deca- (10). For instance, $N_2{O}_5$ is dinitrogen pentoxide. Note that the prefix mono- is often omitted for the first element, so CO is carbon monoxide, not monocarbon monoxide.

Remember that these rules apply to binary covalent compounds, which are compounds composed of two different nonmetal elements. When naming compounds with more than two elements, such as those containing polyatomic ions, the naming can become more complex and often involves memorizing the names of common polyatomic ions.

How is naming ionic and covalent compounds different?

When naming ionic compounds, which consist of metals and nonmetals, you first name the cation (metal) and then the anion (nonmetal). For the anion, you use the root of the element's name and add the suffix '-ide.' For example, NaCl is named sodium chloride. If the ionic compound contains a polyatomic ion (a charged group of covalently bonded atoms), you use the name of that ion directly, such as in sodium sulfate (${\mathrm{Na}}_2{\mathrm{SO}}_4$).

In contrast, when naming covalent compounds, which are formed between nonmetals, you use prefixes to denote the number of atoms of each element present in the compound (mono-, di-, tri-, etc.). The first element retains its name, and the second element's name is modified to end with '-ide' just like in ionic compounds. However, the prefix 'mono-' is typically omitted for the first element. For example, ${\mathrm{CO}}_2$ is carbon dioxide, and $P_4{O}_{10}$ is tetraphosphorus decoxide. Remember that these naming conventions do not apply to compounds with transition metals, which often require Roman numerals to indicate the charge on the cation, such as in copper(II) oxide for ${\mathrm{Cu}}_{\mathrm{O}}$.

What are the rules for naming covalent compounds?

In covalent compounds, atoms share electrons to achieve stability. When naming these compounds, there are specific rules to follow:

1. The element with the lower group number in the periodic table (or the one that is furthest left if both are in the same group) is written first. If both elements are in the same group, the one with the higher period number is named first.
2. The second element is named by taking its root and adding the suffix "-ide."
3. Prefixes are used to denote the number of atoms of each element in the compound (mono- for one, di- for two, tri- for three, tetra- for four, penta- for five, etc.). The prefix "mono-" is often omitted for the first element.
4. If the prefix ends in a vowel and the element name starts with a vowel, the final vowel of the prefix is often dropped to make the name easier to pronounce.

For example, $\mathrm{CO}$ is carbon monoxide, not monocarbon monoxide, and ${\mathrm{CO}}_2$ is carbon dioxide.