How do you know the Order of Elements in a Chemical Formula

What is Chemical Formula?

A chemical formula is a representation of a chemical substance using letters for atoms and subscript numbers to show the numbers of each type of atoms that are present in the substance.
Based on the chemical formula of a substance, we know the composition of the substance.
(a) The elements that make up the substance.
(b) The ratio or number of atoms of each element in the substance.

Chemical formulae of elements

The chemical formula of an element is a statement of the composition of its molecule in which symbol tells us the element and the subscript tells us how many atoms are present in one molecule. One molecule of hydrogen element contains two atoms of hydrogen, therefore, the formula of hydrogen is H₂.

For elements that exist as atoms, their chemical formulae represent their atoms.
For elements that exist as molecules, their chemical formulae represent their molecules. Subscript number at the right bottom shows the number of atoms in each molecule.

Example: The formula H₂ indicates that one molecule of hydrogen element contains 2 atoms of hydrogen. 2 H represents 2 separate atoms of hydrogen; H₂ represents 1 molecule of hydrogen and 2H₂ represents 2 molecules of hydrogen.

Table: Chemical formulae of some elements

Element	Chemical Formula	Meaning of chemical formula
Carbon	C	Carbon is made up of carbon atoms.
Argon	Ar	Argon is made up of. argon atoms.
Copper	Cu	Copper is made up of copper atoms.
Oxygen gas	O₂	Each oxygen molecule consists of two oxygen atoms.
Chlorine gas	Cl₂	Each chlorine molecule consists of two chlorine atoms.

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Chemical formulae of compounds

Chemical formula of a compound tells us the composition of the compound.
However, the chemical formula does not show the type of particles in the compound.
Example: Water is a compound made up of 2 atoms of hydrogen element and 1 atom of oxygen element, so the formula of water is written as H₂O. In the formula H₂O, the subscript 2 indicates 2 atoms of hydrogen. In the formula of water, oxygen O is written without a subscript and it indicates 1 atoms of oxygen.

Table: Chemical formulae of some compounds

Compound	Chemical formula	Meaning of chemical formula
		Elements present		Ratio of the number of atoms of elements
Water	H₂O	Hydrogen, oxygen	H:O = 2:1
Ammonia	NH₃	Nitrogen, hydrogen	N:H = 1:3
Sulphuric acid	H₂SO₄	Hydrogen, sulphur, oxygen	H:S:O = 2:1:4
Zinc hydroxide	Zn(OH)₂	Zinc, oxygen, hydrogen	Zn:O:H = 1:2:2
Magnesium nitrate	Mg(NO₃)₂	Magnesium, nitrogen, oxygen	Mg;N;O = 1:2:6

Rules for writing a chemical formula

We first write the symbols of the elements which form the compound
Below the symbol of each element, we write down its valency.
Finally, we cross-over the valencies of the combining atoms. That is, with first atom we write the valency of second atom (as a subscript); and with the second atom we write the valency of first atom (as subscript).

Example 1:
How do you know the Order of Elements in a Chemical Formula 1
Example 2:

Example 3:

Example 4:

Example 5:

Writing the formula of a compound

Step-1 :
Write the symbols of formulae of the ions of the compound side by side with positive ion on the left hand side and negative ion on right hand side.
Step-2 :
Enclose the polyatomic ion in a bracket.
Step-3 :
Write the valency of each ion below its symbol
Step-4 :
Reduce the valency numerals to a simple ratio by dividing with a common factor, if any.
Step-5 :
Cross the valencies. Do not write the charges positive or negative of the ions.
Example: Formula of barium nitrate.
Step-1 : Writing the formula of the ions :
Ba²⁺                  NO₃^–
Step-2 : Ba²⁺                  (NO₃)^–
Step-3 : Ba²⁺                  (NO₃)^–
2 1
Step-4 : Not applicable, because ratio is already simple
Step-5 :
How do you know the Order of Elements in a Chemical Formula 6
Thus, the formula of barium nitrate is Ba(NO₃)₂

Significance of the formula of a substance

Formula represents the name of the substance.
Formula represents one molecule of the substance.
Formula also represents one mole of molecules of the substance. That is, formula also represents 6.022 × 10²³ molecules of the substance.
Formula gives the name of all the elements present in the molecule.
Formula gives the number of atoms of each element present in one molecule.
Formula represents a definite mass of the substance.

Chemical formulae of ionic compounds

Ionic compounds consist of cations and anions.
(a) Cations are positively-charged ions.
(b) Anions are negatively-charged ions.
To construct chemical formulae of ionic compounds, we need to know the formulae of cations and anions.

Below tables show the formulae of some common cations and anions.
Table: Formulae of some common cations.

Charge of cation	Cation (positive ion)	Formula of cation
+ 1	Sodium ion	Na⁺
	Potassium ion	K⁺
	Silver ion	Ag⁺
	Hydrogen ion	H⁺
	Ammonium ion	NH₄⁺
	Copper(I) ion	Cu²⁺
+2	Calcium ion	Ca²⁺
	Magnesium ion	Mg²⁺
	Zinc ion	Zn²⁺
	Barium ion	Ba²⁺
	Iron(II) ion	Fe²⁺
	Copper(II) ion	Cu²⁺
	Tin(II) ion	Sn²⁺
	Lead(II) ion	Pb²⁺
+3	Iron(III) ion	Fe³⁺
	Aluminium ion	Al³⁺
	Chromium(III) ion	Cr³⁺
+4	Tin(IV) ion	Sn⁴⁺
+4	Lead(IV) ion	Pb⁴⁺

Charge of anion	Anion (negative ion)	Formula of anion
-1	Fluoride ion	F^–
	Chloride ion	cl^–
	Bromide ion	Br^–
	Iodide ion	I^–
	Hydroxide ion	OH^–
	Nitrate ion	NO₃^–
	Nitrite ion	NO₂^–
	Hydride ion	H^–
	Ethanoate ion	CH₃COO^–
	Manganate(VII) ion	MnO₄^–
	Chlorate(I) ion	ClO^–
	Bromate(I) ion	BrO^–
	Iodate(I) ion	IO^–
	Chlorate(V) ion	ClO₃^–
	Bromate(V) ion	BrO₃^–
	Iodate(V) ion	IO₃^–
-2	Oxide ion	O²^–
	Carbonate ion	CO₃^2-
	Sulphide ion	S^2-
	Sulphate ion	SO₄²^–
	Sulphite ion	S₂O₃^2-
	Thiosulphate ion	O₃²^–
	Chromate(VI) ion	CrO₄^2-
	Dichromate(VI) ion	Cr₂O₇^2-
-3	Phosphate ion	PO₄^3-

Even though ionic compounds contain charged particles, their chemical formulae are electrically neutral. This is because the total positive charges are equal to the total negative charges.
Steps in constructing the chemical formula of an ionic compound:
1. From its name, write the formulae of its cation and anion.
2. Determine the number of cations and anions by balancing the positive and negative charges.
3. Write the number of cations and anions as subscript numbers.
  Ex: Construct the chemical formula of iron(II) chloride.
  Solution:
There is an easier way to determine the number of cations and anions. You only need to interchange the value of the charges of the ions as shown in the following examples.
However, when the cation and anion have the same value of charges, you need to determine the simplest ratio of the ions.

Some ions such as OH^–, CO₃^2-, SO₄²^– and NO₃^– are made up of more than one element. Brackets are used to show the number of these ions in formulae.
Table: Usage of brackets in formulae

Formula of ionic compound	Usage of brackets in the formula
Mg(OH)₂	To show that there are 2 groups of OH^–
AI₂(CO₃)₃	To show that there are 3 groups of CO₃^2-
(NH₄)₂SO₄	To show that there are 2 groups of NH₄⁺
Cu(NO₃)₂	To show that there are 2 groups of NO₃^–