↪ The foundation of modern chemistry was laid by John Dalton (1776-1844), an Englishman, a Manchester School teacher.
↪ He put forward what is now known as Dalton's Atomic Theory.
⁕ Postulates of the Theory
↪ John Dalton has published a book in 1808 A.D., "The Theory of Chemical
Reactivity" in which he had explained the existence of atom in scientific way.
↪ This was one of the most popular book of nineteenth century.
At present, Dalton's atomic theory can be summarized as follows:
1. Matter is composed of extremely small, indivisible, invisible particles called atoms.
2. Atoms of an element are similar in all respect while atoms of different elements are different in terms of properties and weight.
3. Atoms cannot be destroyed and created i.e. atoms of one of the element cannot be changed into atoms of another element. This is also called law of indestructibility of matter.
4. Atoms can combine in simple whole number ratio to give compound atom. During combination, the atoms of one element are not changed into those of another element.
⁕ Limitations of Theory
Dalton's atomic theory has great importance in the development of modern science. However, at present this theory is modified in different aspects. According to Dalton's atomic theory, atom is the smallest particle of an element. But, nowadays we know that, atom is not the smallest particle of an element. It has been found that, an atom contains approximately forty subatomic particles. Similarly, atoms of an element may be different in terms of mass number as well as physical properties. We know an element may have different types of isotopes. However, atoms of an element have identical chemical properties. The atoms of different elements may have the same mass number i.e. isobars. Isobars have similar nuclear properties. Nowadays atoms of one of the element can be changed into the atoms of another element by nuclear reactions. For example, nitrogen atom can be changed into oxygen atom bombarding by α-particles i.e.
Atoms can combine between themselves to give molecule (compound atom). The combining ratio may be simple or complex however, it is integral ratio. Above explanation shows that, Dalton's atomic theory is no more applicable in its original form, it has several draw backs as
1. It could not explain the cause of chemical reaction.
2. It could not explain the existence of isotopes and isobars.
3. The idea of compound atoms has not applied properly.
4. It could not explain Gay Lussac's law of gaseous volume.
It has undergone modification at present. However, the key idea of tiny nature of atom and their combining capacity explained by Dalton remain unchanged. This theory can explain laws of chemical combination.
⁕ The main modification made in theory are:
1. The atoms are no longer supposed to be indivisible.
2. Atoms of same elements may not be similar in all respect. For example; isotopes of Cl-35 and Cl-37.
3. Atoms of different elements may be similar in one or more respect. For example; isobars, Ca-40 and Ar–40.
4. The ratio in which atoms combine may not be simple. For example; the ratio C, H and O atom in C12H22O11 is 12 : 22 : 11 which is not simple.
5. By nuclear reaction, atoms of one element can be changed into atoms of other.
6. Mass and energy are inter convertible.