Chemistry

Atoms and Molecules

Mole Concept

Banks have an extremely large number of coins of various denominations. Counting of such a large number of coins is a difficult job. So, in banks, they weigh coins, rather than count them. The bank people know that a fixed number of a particular coin will always have the same mass. This means that from the mass of a particular type of coins, they can find the number of coins. Thus bank people link the mass of coins to the number of coins present. In chemistry also, scientists link the mass of an element or compound to the number of atoms or molecules present in them. This is done through the 'mole'.
Scientists count atoms and molecules by weighing them. It has been found by performing experiments that if we weigh an element equal to its atomic mass in grams, then it contains

6.022X10

²³ atoms of the element. Now, the gram atomic mass of the element as well as

6.022X10

²³ atoms of the element, both represent 1 mole of element.Similarly, if we weigh a compound equal to its molecular mass in grams, then it contains

6.022X10

²³ molecules of the compound. In this case also, the gram molecular mass as well as

6.022X10

²³ molecules represent 1 mole of the compound.Thus, mole is a link between the mass of atoms (or molecules) and the number of atoms (or molecules).
The gram atomic mass of carbon element is 12 grams. Now, when a scientist is performing an experiment by weighing 12 grams of carbon, he knows that he is dealing with

6.022X10

²³ atoms of carbon. In other words, he is working with 1 mole of carbon. The gram molecular mass of water is 18 grams. So, when a scientist is using 18 grams of water in his experiment, he knows that he is carrying out the reaction by using

6.022X10

²³ molecules of water. In other words, the scientist is working with 1 mole of water. In the following discussion, we will start by considering mole as a .collection of

6.022X10

²³ particles (atoms,molecules or ions) of a substance. We will give the proper definition of mole after a while.
A group of $6.022X10$ ²³ particles (atoms, molecules or ions) of a substance is called a mole of that substance.
Thus, 1 mole of atoms =

6.022X10

²³ atoms
And, 1 mole of molecules =

6.022X10

²³ molecules
It is obvious that we can have a mole of atoms or a mole of molecules of a substance. For example,oxygen atom is O and oxygen molecule is O2. So, we can have a mole of oxygen atoms or a mole of oxygen molecules.
1 mole of oxygen atoms (O) =

6.022X10

²³ oxygen atoms
And, 1 mole of oxygen molecules (

O

²) =

6.022X10

²³ oxygen molecules
Thus, mole represents a definite number of particles, atoms or molecules, of a substance. This number of

6.022X10

²³, which represents a mole, is known as Avogadro number (L). We can now say that a mole represents an Avogadro number of particles of a substance. In addition to the definite number of particles of a substance, the mole also represents a definite amount (or mass) of a substance. This point will become more clear from the following discussion.

Notes

Atoms and Molecules - Notes

1. Drawbacks of Dalton’s Atomic Theory