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Chemical Formula

The combination of similar or different atoms form molecules. The number of atoms constituting a molecule is known as its atomicity. The only example of monatomic molecule is Noble gas because they do not have a tendency to form chemical bond with other atoms. Diatomic molecules consist of two similar or different atoms like the oxygen molecule ($O_2$) or carbon monoxide ($CO$).

Similarly, some molecules contain three atoms like the ozone molecule ($O_3$). Hence, the number of atoms present in a molecule determines the atomicity of the molecule. A compound is made up of more than one element which is chemically combined. In other words, a compound is a type of matter composed of atoms of two or more elements chemically combined in fixed ration. Now there must be a way to express a compound just like symbols of atoms.

You know a compound is made up of different atoms only and each atom has a certain symbol. So a compound can be represented by using them. The chemical representation of compound is known as a chemical formula. It identifies each constituent element by its chemical symbol and indicates the number of atoms of each element found in each molecule of that compound. Let’s discuss few more examples of chemical formulas of compounds.

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What is a Chemical Formula?

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A chemical formula is the representation of the elements present in a molecule or compound.

A chemical formula shows the ratio of elements present in a compound.
Example for chemical formula would be like nitric acid : HNO3 where we have one hydrogen atom, one nitrogen and three oxygen atoms combining together to form one mole or molecule of nitric acid.
The cationic part here in this chemical formula is H+ and the anionic part is NO3-

Writing a Chemical Formula

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Writing a chemical formula, one needs to understand the oxidation number of the elements involved in the chemical formula to be written. Two kinds of compounds are formed.
  1. Covalent bond – Bond formed by sharing electrons. It is necessary to know the valency of the elements.
  2. Ionic bond – Bond formed by one element donating an electron and the other element accepting it. It is necessary to know the oxidation number on the elements to write the chemical formula.

Chemical formula for covalently bonded compound


In a covalently bonded compound, the chemical formula simply signifies the ratio in which the elements are combined.

Use the below widget to find the chemical formula.

Solved Examples

Question 1: Write the formula for Phosphorus trichloride.
Solution:
Phosphorus is – P
Tri chloride signifies three moles of chlorine. So, - 3Cl, written as: Cl3
The formula would be : PCl3


Question 2: Write the formula for Di nitrogen Tetra oxide
Solution:
In this case, both Nitrogen and oxide (oxygen) has a prefix attached to it, to indicate the number of that particular element.
Di = 2 and tetra = 4
So, there are two nitrogen and 4 oxygen in it.
The formula would be: N2O4


Question 3: Write the formula for Sulfur dioxide
Solution:
Here, we know that sulfur does not have any prefix attached to it. That indicates that there’s only one mole of sulfur.But, for oxygen, there is a prefix attached to it- ‘di’ , indicating there are two moles of oxygen.So, the formula for this compound would be: SO2


Naming Chemical Formulas

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If instead of the name, the formula of a compound is given, then a prefix is to be attached to the element’s name to signify its numbers.

Example: How to write the name for CCl4
Solution
Looking at the formula, there is one mole of carbon and four moles of chlorine.
So, carbon need not have any prefix attached. But there are four chlorine atoms, so a prefix “Tetra” has to be attached before chloride. The name of this compound would be: Carbon tetrachloride.

Example: How do we name the compound: SiO2
Solution
We can see that there are two elements in it : silicon and oxygen. There are two atoms of oxygen and one atom of silicon. So, the name of this compound would be Silicon Dioxide. So, covalently bonded compounds are named by attaching an appropriate prefix to the element applicable.
Some covalent compounds, such as the hydrocarbons, carbohydrates, and all organic chemicals cannot be named using the same method. Organic chemistry employs a very different nomenclature for these compounds. Every group of organic compounds has their own nomenclature rules.

Some important chemical formula

List of covalent bonded chemical compounds

Name of the compound
Formula
Sucrose (common sugar) C12H22O11
Urea
(NH2)2 CO
Glucose
C6H12O6
Methanol (alcohol) CH3OH
Methane CH4

Chemical Formula for Sugar

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The chemical formula for sugar or sucrose is C12H22O11 where we have 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms forming one mole of sugar compound.
The common name for sucrose is sugar or saccharose. Sucrose is basically made out of two simple sugars, fructose and glucose, held together by a glycosidic bond.

Sucrose Molecule

Chemical Formula for Salt

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The chemical formula for salt or common salt is NaCl where the sodium atom plays the role of cation and the chlorine atom is the anion. The chemical formula for salt would be any combination which takes place by the action of an acid on any metal.

This result is the formation of the respective salt of the metal that is acted upon by the acid.

Example : If metal magnesium (Mg) is acted upon by sulfuric acid, then we have a salt magnesium sulfate or MgSO4. This magnesium salt is also known as Epsom salt.

Mg + H2SO4 MgSO4 + H2

Common salt or NaCl is basically formed when sodium metal (Na) is acted upon by Hydrochloric acid resulting in the formation of NaCl which is also known as common salt.

2Na + 2HCl 2NaCl + H2

Chemical Formulas for Ionic Compounds

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Naming an ionic compound is very easy. We need not attach a prefix to it since that are named as such. But, if the name of the compound is given, writing the formula can be a little confusing, since we need the exact oxidation number on both elements present.

An ionic compound is made from an anion or the negative ion and the cation or the positive ion. It is understood that the total charge on an ionic compound should always be zero. So, it should be taken care of, while writing the chemical formula.
So, knowing the oxidation number (number of electrons an ion, a positive or a negative ion is capable of donating or accepting) is very important while writing the chemical formula.

Chemical formula of salt

Solved Examples

Question 1: Write the formula for common salt
Solution:
This compound, we know is the common salt. So, common salt has a formula of NaCl. And it is named as: Sodium – Na and Chloride- Cl.
Sodium Chloride – NaCl.


Question 2: Let us try to identify and write the formula for Magnesium Iodide.
Solution:
 Magnesium is in the second group of the periodic table. Elements, or the metals in the second group exhibits +2 as their oxidation state.
So, Mg+2
Now, chlorine is in the 17th group or in the VII A group.
The elements of this group, the halogens, always exhibit -1 as their oxidation state.
So, Cl-1
Now, bringing both together and remembering the fact that the compound will have a zero charge.
                                   

We need 2 chlorine (-1) x 2 = -2 to satisfy the charge on Mg - +2
Therefore, we can write the formula as

Mg1Cl2 or just: MgCl2 

Note that in an ionic compound, we do not use any prefix in naming the formula.
This compound is called as Magnesium chloride and not magnesium dichloride.


Question 3: Write the formula for Ferric chloride and ferrous chloride
Solution:
Sometimes, an element can have more than one oxidation state, like a transition metal: Iron. Iron has +2 and +3 oxidation state. In this case, we mention the oxidation state in roman letters in brackets, while writing the name of the compound.

Iron (III) chloride has a formula of
                                    
Formula: FeCl3
Iron (II) chloride has a formula of
                                 
Formula: FeCl2




Question 4: Write the formula for Chromium Oxide
Solution:
Write the formula for: Chromium (III) Oxide.
Oxygen mostly exhibits a charge or oxidation state of -2.
And oxidation state of chromium is given as +3. (in the brackets, in roman numerals).
So, we can write the formula as
                         
Formula: Cr2O3


Complex Chemical Formula

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There are certain anions, known as complex anions and cations, which have a nomenclature of its own too. To write the formula or name of such compounds, it is very important to know these complex anions and cations.

List of Complex Cations/Anions


Anion / Cation
Name
NH4+ Ammonium ion –Cation
CH3+ Methyl ion- cation
SO42- Sulphate ion – Anion
SO32- Sulphite ion – Anion
NO32- Nitrate ion – Anion
NO22- Nitrite ion – Anion
CrO42- Chromate ion – Anion
Cr2O72- Dichromate ion – anion

So, to name a compound with these complex ions, we take into account their charge, which is a constant and then name them.

Common Chemical Formulas

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The list of common chemical formulas are given below.

H2SO4 – Hydrogen Sulphate or most popularly called as Sulfuric acid.
HNO3 – Hydrogen Nitrate or nitric acid.
K2CrO3 – Potassium Chromate
NH4Cl – Ammonium chloride
MgCr2O7 - Magnesium dichromate

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