Question Video: Converting a Complex Chemical Name with Prefixes into a Chemical Formula Chemistry

What is the chemical formula of dichlorine heptoxide?


Video Transcript

What is the chemical formula of dichlorine heptoxide?

Recall the definition of a chemical formula is an expression of chemical symbols and numerical subscripts that represents the composition of one unit of a compound. An example of a chemical formula is CH4, which is a compound with the chemical name carbon tetrahydride, or more commonly known as methane. The chemical symbols C and H in the chemical formula represent the element carbon and hydrogen, which can be found on the periodic table in group 14, period two and group one, period one, respectively. The numerical subscript four indicates there are four hydrogen atoms in one unit, or one molecule, of the compound.

Notice there isn’t a subscript next to carbon’s chemical symbol. This is because a rule when writing chemical formulas is that subscript values equal to one are implied but not written in the chemical formula. So in the case of the carbon atom in this chemical formula, there’s actually an implied number one, which means there is one carbon atom in one unit of this compound.

To identify the chemical symbols and numerical subscripts for the chemical formula of dichlorine heptoxide, we will analyze the different parts of this chemical name. Firstly, the presence of two Greek numerical prefixes di- and hept- indicates we are dealing with a binary molecular compound, or a compound that is composed of atoms of two elements covalently bonded together. With nonmetals and metalloids consisting of the types of elements that covalently bond, for dichlorine heptoxide, we will be looking for the elements and their chemical symbols on the right-hand side of the periodic table, where all nonmetals and metalloids reside, with the exception of the element hydrogen, which is the one nonmetal on the left side of the periodic table.

Secondly, let’s break down the chemical name into the following parts to determine the chemical formula: prefix, root, and suffix. To determine the two elements present, we look to the root words. As a rule for determining formulas from names for binary molecular compounds, the first root word listed is the element’s full name, as can be found on the periodic table. Therefore, the root word in dichlorine is chlorine, which is an element on the periodic table with the chemical symbol Cl found in group 17, period three.

The second root word is truncated to only include the first portion of the element’s name and ends in the suffix -ide. Therefore, in heptoxide, the root word -ox- stands for the element oxygen, which has the chemical symbol O and is found in groups 16, period two on the periodic table. The order elements are written in their chemical name is the same order they’re written in their chemical formula. So we can start by writing the chemical symbols Cl and O next to one another.

Now that we know the chemical formula contains the elements chlorine and oxygen, we can use the Greek numerical prefixes in front of each root word to determine the number of atoms of each element in one unit of the compound. Referencing the Greek numerical prefix list used in naming binary molecular compounds, the di- in front of dichlorine indicates there are two chlorine atoms, with the number two written as a subscript to the right of the chlorine atom in the chemical formula.

Notice the prefix in heptoxide is written as hept- and not hepta- like it is shown in the table. This is because one of the naming rule states that if the prefix in front of oxygen or oxide ends in the letter A or O, the last letter of the prefix maybe dropped. So instead of writing the second portion of the name as heptaoxide, since the prefix hepta- ends with an A and oxide starts with an O, according to the rule, the last letter of the prefix, which is the letter A, is removed and it is written and read as heptoxide.

In any case, the presence of the prefix hept- indicates there are seven atoms of oxygen, which will also be written as a subscript to the right of the oxygen chemical symbol in the chemical formula. As a result, the chemical formula for dichlorine heptoxide is Cl2O7.

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