Molecular Formula of Inorganic Non-Metallic Binary Covalent Compounds Chemistry Tutorial

Key Concepts

The molecular formula of an inorganic non-metallic binary covalent compound⁽¹⁾, X_aY_b tells us the:
⚛ chemical symbols of the two non-metallic elements present in a molecule of the compound

X and Y

⚛ number of atoms of each element present in a molecule of the compound

a atoms of X

b atoms of Y

In inorganic binary covalent compounds the chemical symbols used in the molecular formula are placed in the following order, from the chemical symbol written first to the chemical symbol written last:⁽²⁾
C, P, N, H, S, O, I, Br, Cl, F

Apart from hydrogen, this order is based on the non-metallic element's location in the periodic table as shown below: ⁽³⁾

(Start with carbon, C, then follow the arrows around to fluorine, F )

Note that if the compound is named correctly,⁽⁴⁾ the chemical symbols of the elements will be in the same order as they appear in the name.

The number of atoms an element present in a molecule of the binary inorganic covalent compound is written as a subscript to the right of the chemical symbol for the element.
X_aY_b is made up of a atoms of element X, and, b atoms of element Y.

If only 1 atom of an element is present in the molecule, then the subscript 1 is omitted from the molecular formula.
XY is made up of 1 atom of element X and 1 atom of element Y.
X_aY is made up of a atoms of element X and 1 atom of element Y.
XY_b is made up of 1 atom of element X and b atoms of element Y.

Note that some binary inorganic non-metallic compounds have names that do not provide information about the number of atoms of each element present in the molecule.⁽⁵⁾
You will need to learn the names and molecular formulae of these compounds:

water: H₂O

ammonia: NH₃

hydrogen peroxide: H₂O₂

hydrogen sulfide : H₂S (may also be referred to systematically as dihydrogen sulfide)⁽⁶⁾

phosphane: PH₃

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Steps for Writing the Molecular Formula of Inorganic Non-metallic Binary Covalent Compounds Given the Name of the Compound

Follow the steps below to write the molecular formula of an inorganic non-metallic binary covalent compound given the name of the compound:

Check that the name is composed of two parts separated by a space:

Compound's name

name's first part name's last part

If it isn't, you have a "special case" so check one of the following:

water: H₂O

ammonia: NH₃

phosphane: PH₃

Identify the two prefixes in the compound's name and convert them to numbers using the table below⁽⁷⁾:

No. of atoms	1	2	3	4	5	6	7	8	9	10
Multiplicative prefix	mono	di	tri	tetra	penta	hexa	hepta	octa	nona	deca

If the compound is an oxide, it could be named as either "monooxide" or as "monoxide".

If a prefix is missing in the name it is presumed to have been "mono" UNLESS the compound is hydrogen sulfide which is the name commonly used for the the compound with the molecular formula H₂S

If the prefix is "per" this will be treated as a "special case": for example, hydrogen peroxide has the molecular formula H₂O₂

name's first part	name's last part
prefixxname	prefixynamide
a atoms of xname	b atoms of ynamide

Identify each of the two elements in the compound's name.

(i) The first part of the name will be the name of an element.

(ii) The second part of the name will be the name of an element with the end of the name replaced with "ide".
Reconstruct the name of this second element using the table below:

modified name of element	element's name
fluoride	flourine
chloride	chlorine
bromide	bromine
iodide	iodine
oxide	oxygen
sulfide	sulfur
hydride	hydrogen
nitride	nitrogen
phosphide	phosphorus
carbide	carbon

name's first part	name's last part
prefixxname	prefixynam~~ide~~
a atoms of xname	b atoms of yname

Use the Periodic Table of the Elements to find the chemical symbol of each element:

name's first part	name's last part
a atoms of xname	b atoms of yname
a atoms of X	b atoms of Y

Position the number of atoms of each element as a subscript number to the right of the symbol for the element:

name's first part name's last part

a atoms of X b atoms of Y

X_a Y_b

Check the molecular formula:
If only one atom of an element is present, remove the subscript 1

Assemble the final molecular formula by joining each part of the formula together in the correct order with no spaces between the two parts:
X_aY_b

name's first part	name's last part
a atoms of X	b atoms of Y
X_a	Y_b

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Worked Examples of Writing the Molecular Formula of Binary Inorganic Non-metallic Covalent Compounds

Question 1: Write the molecular formula for carbon monoxide.

Solution:

Check that the name is composed of two parts separated by a space:

carbon monoxide

name's first part name's last part

carbon monoxide

Identify the two prefixes in the compound's name and convert them to numbers using the table below:

No. of atoms	1	2	3	4	5	6	7	8	9	10
Multiplicative prefix	mono	di	tri	tetra	penta	hexa	hepta	octa	nona	deca

If the compound is an oxide, it could be named as either "monooxide" or as "monoxide".

If a prefix is missing in the name it is presumed to have been "mono".

name's first part	name's last part
(mono)carbon	monoxide
1 atom of carbon	1 atom of oxide

Identify each of the two elements in the compound's name.

(i) The first part of the name will be the name of an element.

(ii) The second part of the name will be the name of an element with the end of the name replaced with "ide".
Reconstruct the name of this second element using the table below:

modified name of element	element's name
fluoride	flourine
chloride	chlorine
bromide	bromine
iodide	iodine
oxide	oxygen
sulfide	sulfur
hydride	hydrogen
nitride	nitrogen
phosphide	phosphorus
carbide	carbon

name's first part	name's last part
1 atom of carbon	1 atom of ox~~ide~~
1 atom of carbon	1 atom of oxygen

Use the Periodic Table of the Elements to find the chemical symbol of each element:

name's first part	name's last part
1 atom of carbon	1 atom of oxygen
1 atom of C	1 atom of O

Position the number of atoms of each element as a subscript number to the right of the symbol for the element:

name's first part name's last part

1 atom of C 1 atom of O

C₁ O₁

Check the molecular formula: If only one atom of an element is present, remove the subscript 1

name's first part name's last part

C₁ O₁

C O

Assemble the final molecular formula by joining each part of the formula together in the correct order with no spaces between the two parts:
CO is the molecular formula for carbon monoxide

name's first part	name's last part
1 atom of C	1 atom of O
C₁	O₁

name's first part	name's last part
C₁	O₁
C	O

Question 2: Write the molecular formula for carbon dioxide.

Solution:

Check that the name is composed of two parts separated by a space:

carbon dioxide

name's first part name's last part

carbon dioxide

Identify the two prefixes in the compound's name and convert them to numbers using the table below:

No. of atoms	1	2	3	4	5	6	7	8	9	10
Multiplicative prefix	mono	di	tri	tetra	penta	hexa	hepta	octa	nona	deca

If the compound is an oxide, it could be named as either "monooxide" or as "monoxide".

If a prefix is missing in the name it is presumed to have been "mono".

name's first part	name's last part
(mono)carbon	dioxide
1 atom of carbon	2 atoms of oxide

Identify each of the two elements in the compound's name.

(i) The first part of the name will be the name of an element.

(ii) The second part of the name will be the name of an element with the end of the name replaced with "ide".
Reconstruct the name of this second element using the table below:

modified name of element	element's name
fluoride	flourine
chloride	chlorine
bromide	bromine
iodide	iodine
oxide	oxygen
sulfide	sulfur
hydride	hydrogen
nitride	nitrogen
phosphide	phosphorus
carbide	carbon

name's first part	name's last part
1 atom of carbon	2 atoms of ox~~ide~~
1 atom of carbon	2 atoms of oxygen

Use the Periodic Table of the Elements to find the chemical symbol of each element:

name's first part	name's last part
1 atom of carbon	2 atoms of oxygen
1 atom of C	2 atoms of O

Position the number of atoms of each element as a subscript number to the right of the symbol for the element:

name's first part name's last part

1 atom of C 2 atoms of O

C₁ O₂

Check the molecular formula: If only one atom of an element is present, remove the subscript 1

name's first part name's last part

C₁ O₂

C O₂

Assemble the final molecular formula by joining each part of the formula together in the correct order with no spaces between the two parts:
CO₂ is the molecular formula for carbon dioxide

name's first part	name's last part
1 atom of C	2 atoms of O
C₁	O₂

name's first part	name's last part
C₁	O₂
C	O₂

Question 3: Write the molecular formula for dioxygen difluoride.

Solution:

Check that the name is composed of two parts separated by a space:

dioxygen difluoride
name's first part		name's last part
dixoygen		difluoride

Identify the two prefixes in the compound's name and convert them to numbers using the table below:

No. of atoms	1	2	3	4	5	6	7	8	9	10
Multiplicative prefix	mono	di	tri	tetra	penta	hexa	hepta	octa	nona	deca

If the compound is an oxide, it could be named as either "monooxide" or as "monoxide".

If a prefix is missing in the name it is presumed to have been "mono".

name's first part	name's last part
dioxygen	difluoride
2 atoms of oxygen	2 atoms of fluoride

Identify each of the two elements in the compound's name.

(i) The first part of the name will be the name of an element.

(ii) The second part of the name will be the name of an element with the end of the name replaced with "ide".
Reconstruct the name of this second element using the table below:

modified name of element	element's name
fluoride	flourine
chloride	chlorine
bromide	bromine
iodide	iodine
oxide	oxygen
sulfide	sulfur
hydride	hydrogen
nitride	nitrogen
phosphide	phosphorus
carbide	carbon

name's first part	name's last part
2 atoms of oxygen	2 atoms of fluor~~ide~~
2 atoms of oxygen	2 atoms of fluorine

Use the Periodic Table of the Elements to find the chemical symbol of each element:

name's first part	name's last part
2 atoms of oxygen	2 atoms of fluorine
2 atoms of O	2 atoms of F

Position the number of atoms of each element as a subscript number to the right of the symbol for the element:

name's first part name's last part

2 atoms of O 2 atoms of F

O₂ F₂

Check the molecular formula: If only one atom of an element is present, remove the subscript 1
We do not need to change the subscripts.

Assemble the final molecular formula by joining each part of the formula together in the correct order with no spaces between the two parts:
O₂F₂ is the molecular formula for dioxygen difluoride

name's first part	name's last part
2 atoms of O	2 atoms of F
O₂	F₂

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Footnotes: reference "Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005" (Red Book)

(1) Compounds containing carbon are called organic compounds. You can find out more about the IUPAC conventions for naming organic compounds in the Introduction to Naming Organic Compounds Tutorial.
Inorganic compounds, in general, do not contain carbon but there are notable exceptions such as carbon dioxide (CO₂) and carbon monoxide (CO)
Inorganic chemistry and organic chemistry are two separate areas within chemistry because the chemistry of carbon is quite different to the chemistry of other elements.
A binary compound is made up of two (bi) parts, in this tutorial the two parts will be two different non-metals.

(2) In constructing a stoichiometric name for an inorganic binary non-metallic covalent compound, one element is designated as the electropositive constituent and the other element is designated as the electronegative constituent.
The electropositive constituent is, by convention, the element that occurs first in the sequence given in the diagram.
The order of the elements in the diagram has its basis in consideration of the electronegativity of each element, that is, electronegativity generally decreases going down each group from top to bottom, and increases across a period from left to right.
The same order is used to write molecular formula of binary inorganic molecules, and, to name these molecules.

(3) This eliminates the previous problems of using strict "electronegativity" of elements to order the names of the elements.
Note that compounds containing oxygen are longer named as oxides UNLESS oxygen appears last in the molecular formula based on the diagram. Previously, compounds containing chlorine and oxygen would have been named as oxides, now they are named as chlorides.

(4) We are using compositional nomenclature to name our binary inorganic compounds, but this is not the only IUPAC system available for nomenclature (and it is not even the most widely used system because it provides only minimal information about the molecule).
There are 3 primary IUPAC systems for naming an inorganic compound:
(a) Compositional nomenclature : requires no understanding of the structure of the connections within the compound.
(Strictly speaking we are using a specific type of compositional nomenclature known as stoichiometric nomenclature because we will be indicating the number of atoms of each element using multiplicative prefixes).
(b) Substitutive nomenclature : requires an understanding of connections within the structure of the compound
(c) Additive nomenclature : requires an understanding of connections within the structure of the compound and this is the most generally applicable system for inorganic compounds

(5) Some names that have been in common useage for a long time are no longer acceptable IUPAC names. Some examples are:
(a) nitric oxide for NO is no longer acceptable. Acceptable names include nitrogen monoxide and nitrogen monooxide.
(b) nitrous oxide for N₂O is no longer acceptable. An acceptable IUPAC name is dinitrogen oxide.
(c) phosphine for PH₃ is no longer acceptable. The acceptable IUPAC name for this compound as a parent hydride is phosphane.

(6) Herein lies the reason why the "hydrogen names" for some ions are named without a space between "hydrogen" and the name of the rest of the ion.
Consider the compositional names "hydrogen sulfide" and "hydrogen sulfide(2-)".
Hydrogen sulfide could have the formula HS^- as well as the formula H₂S.
Hydrogen sulfide(2-) could also refer to the formula HS^- as well as H₂S.
H₂S is therefore named dihydrogen sulfide or more commonly hydrogen sulfide, while HS^- is named hydrogensulfide, monohydrogensulfide, or hydrogensulfide(1-) (hydrogen names) to avoid confusion.
The "hydrogen name" hydrogensulfide (all one word with no spaces) will ONLY apply to a hydrogen sulfide molecule (H₂S) that has lost a proton to become the hydrogensulfide ion, HS^-.

(7) More multiplicative prefixes are given in the table below:

No. atoms	Prefix	No. atoms	Prefix	No. atoms	Prefix
11	undeca	21	henicosa	60	hexaconta
12	dodeca	22	docosa	70	heptaconta
13	trideca	23	tricosa	80	octaconta
14	tetradeca	30	triaconta	90	nonaconta
15	pentadeca	31	hentriaconta	100	hecta
16	hexadeca	35	pentatriaconta	200	dicta
17	heptadeac	40	tetraconta	500	pentacta
18	octadeca	48	octatetraconta	1000	kilia
19	nonadeca	50	pentaconta	2000	dilia
20	icosa	52	dopentaconta

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