How to write the formula and name of a molecule?

Name has a great significance in the world of elements. Before we proceed, I would like to tell you a few basic things. When atoms of two different elements bind together, they form one molecule and a number these molecules are collectively called as compound.

Just like some of us have two different types of names, a proper name and a nick name, the molecules have a scientific name and sometimes a common name as well. But unlike our names, a molecule’s name tells us a number of informations about that molecule. It tells us how many elements are present in that molecule and also gives information about the ratio of these elements. To write the name of a molecule in scientific notation is known as the molecular formula.

You have noticed that in a molecule atoms are present in a definite ratio. This ratio is similar to the ratio of their valencies. We will learn first to name simple ionic molecules which are made up of two elements.

If you bond Na with Cl. You know Na belongs to 1^st group and Cl belongs to the 17^th group. They have large difference in EN values that’s why they will form ionic bond. Valency of Na is 1 and valency of Cl is also 1, so they will combine in ratio of 1:1 and form a molecule of NaCl.

NaCl is the formula of the molecule which is formed on bonding of Na and Cl. It also tells us that 1 atom of Na and 1 atom of Cl were combined to form this molecule.

Did you notice that I wrote Na first and then Cl? Whenever you write a formula or name of a compound you will write electro positive (or less electronegative) element first and then the electronegative element.

Now let’s give a name to this molecule of NaCl.

1. Write the of name electro positive element- Sodium.
2. Then write the name of electronegative element- Chlorine,
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Chlorine, it becomes Chloride.
4. Write both of them together- name will be Sodium chloride.

Take another example of Mg and I. They have large EN difference so they will form ionic bond. Valency of Mg is 2 and valency of I is 1. ‘Mg’ needs to donate its 2 electrons and ‘I’ needs to get 1 electron. That means 2 atoms of ‘I’ will be required to fulfil the need of 1 ‘Mg’ atom.

Now write the formula of the molecule which will be formed by bonding of Mg and I.

1. First write the symbol of electropositive element- Mg
2. Write the symbol of electronegative element- I
3. Write the number of atoms in subscript to the symbol of elements (ignore if number of atom is 1) - MgI₂

Now name this molecule of MgI₂.

1. Electro positive element- Magnesium.
2. Electronegative element- Iodine,
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Iodine, it becomes Iodide.
4. Write both of them together- name will be Magnesium Iodide.

Now try one more combination: Ca and Cl.

1. Valency of Ca is 2 and valency of Cl is 1.
2. Formula will be- CaCl₂
3. Name will be Calcium chloride.

Now we will learn to name simple covalent molecules which are made up of 2 elements. Don’t worry, you don’t have to learn different rules of naming. Only a bit more information is to be added when you write the name of a covalent molecule.

In the name of a covalent molecule we have to add the information about number of atoms involved in the formation of molecule. Let’s take an example of B and Cl.

B and Cl both belong to p block. That’s why they will form covalent bond. Valency of B is 3 and valency of Cl is 1. I hope you will be able to understand now that, to fulfil the need of 1 atom of B, 3 atoms of Cl will be required.

In case of covalent molecules we have to add prefix to the name of elements corresponding to the number of their atoms like di, tri, tetra, penta, hexa, hepta, octa, nona, deca respectively for 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10.

Let’s try to name the BCl3.

1. Electro positive element- Boron.
2. Electronegative element- Chlorine.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Chlorine, it becomes chloride.
4. Add prefix ‘tri’ to Chloride because its 3 atoms are involved in the formation of BCl₃ molecule: trichloride.
5. Name of the BCl₃ molecule will be: Boron trichloride.

Let’s practice it with one more example: H and O.

You can work out the formula: H₂O. Now try to name it.

1. Electro positive element- Hydrogen.
2. Electronegative element- Oxygen.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Oxygen, it becomes oxide.
4. Add prefix ‘di’ to the Hydrogen because its 2 atoms are involved in the formation of H₂O molecule: Dihydrogen.
5. Name of the H₂O molecule will be: Dihydrogen oxide.

Let’s practice it with one more example: C and O.

You can work out the formula: CO₂.

Now try to name it.

1. Electro positive element- carbon.
2. Electronegative element- Oxygen.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Oxygen, it becomes oxide.
4. Add prefix ‘di’ to the oxide because its 2 atoms are involved in the formation of CO₂ molecule: dioxide.
5. Name of the CO₂ molecule will be: carbon dioxide.

The aim of bonding is to achieve octet, that means when you count electrons around any bonded atom in  Lewis dot structure it should be 8. In above examples I encircled the electrons around each atom. If you notice Boron in BCl₃ molecule,  it has only 6 electrons . This deficiency gives a special quality to this molecule. I will explain it in the coming post.

The atoms of two different elements combine to form a molecule.  This newly formed molecule is entirely different from parent elements. You can witness it in case of water molecule; hydrogen and oxygen are gases but water is liquid. What makes a molecule so different? Why parent elements lose their identity? In the next post we will try to solve the mystery of a molecule.   

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