Science

Since ancient times, philosophers had thought of the idea that everything around us is made of tiny particles which cannot be subdivided. This was verified by scientists in the 18th century.

The idea that all matter around us is made up of atoms and molecules gives rise to two important conclusions. They are, law of conservation of mass, and law of constant proportions. The first states that "mass can neither be created nor destroyed in a chemical reaction".

We can think about this intuitively, for better understanding. A chemical reaction is simply a rearrangement of molecular structures. The molecules themselves do not change in any way - only the way they connect with each other does; and it is these molecules which give mass to any matter. So, it makes sense that if the molecules are not altered, the mass too should remain constant before, during and after a chemical reaction.

The second law which is central to chemistry is “In a chemical substance the elements are always present in definite proportions by mass”. This is the law of constant proportions. Again, in simple terms, we can say that any chemical substance is gonna have a fixed ratio of elements in it. For example, water will always have the formula H2O, so, for every atom of oxygen, a water molecule will have two Hydrogen atoms - otherwise, it simply won't be a water molecule! (eg. if a molecule had two atoms of oxygen for two atoms of hydrogen, the molecule would be called H2O2, which is another compound named hydrogen peroxide (and not water).

Atomic mass

In our daily lives, we use units like grams, kilograms and sometimes even tonnes to weigh objects. However, these units are far too large for measuring the weight (or, more accurately, mass) of molecules. For example, the weight of one C12 (an isotope of Carbon) atom is of the order of 10-26 kilograms. These units are simply too inconvenient to be used by chemists.

Therefore, we need a new set of units to weigh atoms and molecules. The unit of mass is amu, short for “atomic mass unit”, and is equal to 1/12th the weight of a C12 atom. In these units, the weight of a Hydrogen atom is nearly 1, that of Helium is 4, and so on. The important thing here is that these numbers are much easier to work with than if we had used kilograms or grams. Following are some elements and their atomic weights:

Note that the unit of atomic mass is u.

Molecules

It must be noted that atoms usually cannot exist on their own. Except for noble gases, which are very stable, the isolated atom of an element is too reactive and it will combine with other atoms to form a molecule. The textbook definition of a molecule is “the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance”.

We stress on the capability of a molecule to independently exist, because this stability is what atoms seek when they combine to form molecules. Also, a molecule is like the smallest unit of a substance – so it has to represent the nature of the substance it belongs to. Hence it should exhibit all the properties of the substance.

So, atoms of same or different elements come together, in a fixed ratio, to form compounds.

Some of these compounds are Ionic compounds, which means they are made up of ions. Ions are simply atoms carrying a net charge over them, positive or negative. For example, Na+ and Cl- react to form NaCl, which is sodium chloride, usually known as common salt.

But how do these elements react to form compounds? IN other words, how can we determine the ratio in which these elements (or ions) combine?

One way to answer this question is by using the concept of valency. With each atom or ion, we can associate a fixed number, known as its valency, and that helps us in determining how it reacts with other elements.

Before going ahead, it is useful to memorize the following valency values. The advantage of that is, we can predict the composition, or “formula” of a large number of compounds simply by using the valency of elements/ions shown below:

Note that some ions may show more than one valency (eg. Iron and Copper), but let’s ignore that for now to keep the discussion simple. 

Now suppose we wanted to derive the formula for water, but we only knew that water is made up of hydrogen and oxygen. That is, we do not know the ratio in which H and O combine. This problem can be solved by using valency.

Valency of Hydrogen is 1, while that of Oxygen is 2. This means one H atom needs only one other atom to satisfy its valency, but Oxygen needs two atoms. Note that the requirement of Oxygen can be satisfied by two Hydrogen atoms, each of whose valency will be satisfied by the single oxygen atom. Hence, the formula for water is H2O. 

This suggests a shortcut for deriving formulae, as shown below:

• Step 1: Write the symbols of individual atoms with their valencies under it.

• Step 2: Exchange the valencies to get the formula.

Here is a worked-out example for Hydrogen Sulphide:

Mole Concept

Earlier, we have discussed how atomic mass is measured in special units, amu. This is because atoms are far too light to be weighed in regular units. Similarly, atoms are so tiny that counting them in regular units can be tricky. For example, the number of Carbon atoms that collectively weigh 12g is 6.023 x 1023 atoms. So, we use a unit called “moles”. 1 mole of any atom or molecule simply means 6.023 x 1023 entities of that atom or molecule. Thus, 1 mole of H2O has 6.023 x 1023 molecules of H2O. Note that this number is referred to as the Avogadro Number.

Another significance of the mole concept is that it relates quantity to mass. For example, 1 mole of H2O weighs 2x1 + 1x16 = 18 grams. Thus, we can deal in moles while working with equations, and can convert that to mass easily. The following chart should explain it further:

We can use this to think about equations in terms of moles. For example, we know the equation for formation of water:

2H2 + O2 🡪 2H2O

We can interpret this reaction as “two moles of hydrogen molecules react with one mole of oxygen molecules to form two moles of water molecules”.

Or, in terms of mass, (since one mole of molecules is equivalent to the molecular mass of that molecule in grams), “four grams of hydrogen reacts with 32 grams of oxygen to give36 grams of water”. Thus, the advantage of mole concept is two-fold – it provides an easier way to express the quantity of atoms and molecules, and it also works as a conversion factor from atomic mass unit to grams.

Summary

• Law of conservation of mass states that "mass can neither be created nor destroyed in a chemical reaction".

• Law of constant proportions states that “In a chemical substance the elements are always present in definite proportions by mass”.

• A molecule is the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

• The unit for measuring atomic mass is amu (u), and number of atoms or molecules is moles (NA).

• Valency is the combining capacity of an element.