Atoms and Molecules: Historical journey through Ancient insights to Atomic Theories

In ancient times Indian and Greek philosophers have always wondered about the unknown and unseen form of matter. Around 500 BC, the idea of divisibility of matter was considered in India. Atoms and molecules are the fundamental building blocks of matter.

What Philosophical insights led to the emergence of Atoms and Molecules as Fundamental elements in the Study of Matter?

• Maharishi Kanad: An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (Padarth), a stage will come when further division will not be possible. 
  • He named these smallest particles Parmanu. 
• Pakudha Katyayama: Another Indian philosopher, Pakudha Katyayama, elaborated this doctrine and said that these particles normally exist in a combined form which gives us various forms of matter. 
• Democritus and Leucippus: Greek philosophers, Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further and Democritus called these indivisible particles Atoms (meaning indivisible). 
  • All this was just based on philosophical considerations.
• Understanding Differences: By the end of the 18th century, scientists recognised the difference between elements and compounds and started finding out how and why elements combine and what happens when they combine. 
• Antoine L. Lavoisier: He laid the foundation of chemical sciences by establishing two important laws of chemical combination. 

Can Atoms and Molecules explain the Laws of Conservation of Mass and Definite Proportions in Chemical Combinations?

• The following two laws were established after much experimentation by Lavoisier and Joseph L. Proust.
• Law of Conservation of Mass in Chemical Transformations:

• It states that mass can neither be created nor destroyed in a chemical reaction.

• Law of Constant Proportions or the law of definite proportions  in Chemical Compounds:

• • Developing Understanding: Lavoisier, along with other scientists, noted that many compounds were composed of two or more elements and each such compound had the same elements in the same proportions, irrespective of where the compound came from or who prepared it. 
  • Statement: This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”. 
    • Example: In a compound such as water, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained i.e the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. 
    • Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass. 
• To explain this, British chemist John Dalton provided the basic theory about the nature of matter. 

Dalton’s Atomic Theory and the Intricate Patterns of Atoms and Molecules:

• Laws of Mass Conservation and Definite Proportions: Dalton’s atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions. 
• Essence of Dalton’s Atomic Theory: According to the theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. 

• Postulates of Dalton’s Atomic Theory and the Consistency of Matter’s Building Blocks:  

• • Atom: All matter is made of very tiny particles called atoms, which participate in chemical reactions. 
  • Indivisible: Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. 
  • Properties: Atoms of a given element are identical in mass and chemical properties. 
    • Atoms of different elements have different masses and chemical properties. 
  • Compound:  Atoms combine in the ratio of small whole numbers to form compounds. 
  • The relative number and kinds of atoms are constant in a given compound.

How did the discovery of charged particles reshape our understanding of atoms and molecules in the 19th century?

• Challenging the Structure and Properties of Atoms: 19th Century Scientific Journey: At the end of the 19th century the major challenge was to reveal the structure and the properties of the atom. 
• Indivisibility of Atoms through the Conduction of Electricity: One of the first indications that atoms are not indivisible, comes from studying static electricity and the condition under which electricity is conducted by different substances. 
  • Many scientists contributed in revealing the presence of charged particles in an atom. 
• J.J. Thomson: Electron and Challenging the Notion of Indivisible Atoms: In 1900, J.J. Thomson identified that the atom was an indivisible particle but contained at least one subatomic particle – the electron (represented as ‘e– ’).
• E. Goldstein’s 19th Century Revelation in Subatomic Exploration: Even before the electron was identified, E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. 
• Discovery of Proton: Canal Rays Connection: Canal rays were positively charged radiations which ultimately led to the discovery of another subatomic particle named proton (represented as p+) which had a charge, equal in magnitude but opposite in sign to that of the electron. 
  • Its mass was approximately 2000 times that of the electron. 
• Particle Charges and Mass Disparities: The mass of a proton is taken as one unit and its charge as plus one whereas the mass of an electron is considered to be negligible and its charge is minus one.
• Neutrality in Atoms: It seemed that an atom was composed of protons and electrons, mutually balancing their charges.

DOWNLOAD PDF