Naming Binary Covalent Compounds

Key Concepts

Binary means 2.
A binary covalent compound is a covalent compound made up of 2 different elements.

The first element in the formula is named first using the name of the element.

Symbol in Formula C N O F P S Cl Br I H

Name carbon nitrogen oxygen fluorine phosphorus sulfur chlorine bromine iodine hydrogen
The second element in the formula is named last and using the suffix 'ide'.

Symbol in Formula C N O F P S Cl Br I

Name carbide nitride oxide fluoride phosphide sulfide chloride bromide iodide

If the compound contains oxygen it will be named as an oxide UNLESS the compound contains oxygen and fluorine in which case it will named as a fluoride.

If there is more than one atom of an element in the compound, prefixes are used:

Number of atoms of the element	1	2	3	4	5	6
Prefix	mono	di	tri	tetra	penta	hexa
(the final vowel of the prefix is usually omitted if it sounds the same as the first letter in the name of the element, eg, monoxide not monooxide, but nitrogen triiodide not nitrogen trioided)

Exceptions:

The mono prefix is not usually used for the first named element and sometimes other prefixes are also omitted from the name of the first element.
The mono prefix may be used if there is more than compound possible, eg, CO and CO₂ are both possible oxides of carbon, CO is named carbon monoxide and CO₂ is named carbon dioxide.
Some compounds containing hydrogen have special names:

Formula H₂O NH₃ N₂H₄ PH₃ SiH₄

Name water ammonia hydrazine phosphine silane
Organic (carbon) compounds have their own naming system.

The first part of the formula will be the symbol for the less electronegative element.
The less electronegative element is usually named first, so, the symbol for the first named element is written first.
In general, an element to the left in the Periodic Table is less electronegative than one to the right in the Periodic Table (oxygen is an exception, it is more electronegative than all the other elements except fluorine).
In general, elements at bottom of a Group in the Periodic Table are less electronegative than the elements at the top of the Group.
nitrogen becomes N, sulfur beomes S, chlorine becomes Cl etc
The second part of the name will include the name of the element but ending end in 'ide', use the symbol for the element indicated:
oxide indicates oxygen, O, sulfide indicates sulfur, S, chloride indicates chlorine, Cl, etc
The prefixes for each element in the name are converted to subscript numbers in the formula:
eg, dinitrogen = N₂, trinitrogen = N₃, etc
eg, dioxide = O₂, trioxide = O₃, tetroxide = O₄ etc
Since the mono prefix is rarely used, assume if no prefix is present there is only one atom of that element present.

Name the compound with the formula NO₂

Identify the elements making up the formula:
N is the symbol for nitrogen
O is the symbol for oxygen
Determine which of the elements will be named first:
N lies to the left of O in the Periodic Table so it is less electronegative than O so nitrogen is named first.
O is more electronegative so it is named last and given the 'ide' suffix, ie, oxygen becomes oxide.
The name (without prefixes) looks like nitrogen oxide.
Use the number of atoms of each element present in the formula to find the prefixes:
N has no subscript so the prefix would be mono (but mono is not usually used), the first part of the name will be nitrogen.
O₂ tells us there are 2 atoms of oxygen present, the prefix will be di and the last part of the name will be dioxide.
Combine the first part of the name with the second part of the name:
nitrogen dioxide is the name given to the formula NO₂

Write the formula for sulfur dioxide

Find the symbol of each element present in the formula:
sulfur has the symbol S
oxide means oxygen is present and oxygen has the symbol O
Use the prefixes in the name to determine the number of atoms of each element present:
sulfur has no prefix, probably only 1 atom of sulfur is present, S
oxide has the prefix di so 2 atoms of oxygen are present, O₂
Determine the order of the elements in the formula:
S is less electronegative than O since it lies below O in the same Group of the Periodic Table, so S comes first in the formula
O₂ must come second in the formula
Combine the first part of the formula with the second part:
SO₂ is the formula for sulfur dioxide

Practice Questions

For AUS-e-TUTE members:

Click on the Name and Formula of Covalent Compounds drill link:
Name and Formula of Covalent Compounds drill
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Worked solutions are provided if you need some help!

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