[Chemistry Class Notes] on Dalton’s Atomic Theory Pdf for Exam

The introduction to the early theory of the atom was done by a scientist named John Dalton (1766-1844). He was a British physicist, chemist, and meteorologist who is well known for many of his contributions to the pioneering research of atoms, the law of partial pressures, Daltonism, etc. Dalton’s atomic model showed the way to many future works, researches regarding atomic theory, even though his conclusions were rather incorrect. He considered the atom as the smallest, indivisible unit of matter and wrote several postulates. Dalton’s model suggested the atom to be a ball-like structure that cannot be further divided. He also symbolized different atoms. Each atom was circular and bears different symbols.

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Dalton’s Atomic Model

The matter has been a subject of fascination since the beginning. Democritus was known to be the first to suggest that matter is made up of particles. Dalton’s model came almost two millennia later and brought further light to the topic. However, the ideas from researches of methane and ethylene might have helped define Dalton’s atomic theory at the time.  

 

The theory of Dalton was published in the paper “New Chemical Philosophy”. Dalton’s idea for the theory is believed to be inspired by the physical properties of gases. Although connections of his work have been made with several other chemists of the time. The definition of dalton’s atomic theory brought the novel concept of calculating relative atomic weights. 

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Postulates of Dalton’s Atomic Theory

The law of conservation of mass and constant proportions are the basis that helps explain Dalton’s atomic theory. Based on these laws Dalton’s atomic theory states the following postulates:

• Atoms are considered to be a matter which is made up of very small, indivisible particles.

• All the atoms in an element are identical. So, they have the same size, shape, mass, and chemical properties. Different elements have different properties i.e. masses, sizes, shapes, and other chemical properties.

• Atoms can neither be created nor be destroyed or subdivided into a chemical reaction.

• Atoms of different elements combine in whole numbers in a simple but fixed ratio to form compounds. Different types of atoms are joined together to form compounds.

• A chemical reaction is a rearrangement of atoms, where the formation of new products occurs due to the rearrangement of atoms in the reactant. The number of atoms before and after a chemical reaction remains the same. 

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Limitations of Dalton’s Atomic Theory

Although Dalton’s atomic theory marked a significant turning point in the research regarding the matter, the theory wasn’t entirely faultless. In further research, his theories were proven wrong.    

• As it was found, later on, atoms are not indivisible. Subatomic particles like protons, electrons, and neutrons have been discovered since then.

• Dalton’s atomic theory states atoms of an element are identical in mass. However, a single element having different atomic masses has been observed. These are called isotopes.

• According to the theory of Dalton, compounds are formed when atoms combine in whole numbers. This fails to explain the formation of complex organic compounds.

• Dalton’s model failed to explain the existence of isobars. Atoms of different elements when having the same mass, are called isobars.

• The theory fails to explain the difference in properties of allotropes. It can not prove the difference in properties of charcoal, diamond, and graphite; allotropes of carbon. 

 

Influences on Modern Atomic Theory

Dalton’s atomic theory contributed a lot to modern atomic theory. Dalton’s model was revolutionary for the period and gave much to the new chemists to research upon. The Atomic theory got modified with the contribution of many after Dalton, namely, Chadwick, JJ Thompson, Ernest Rutherford, Niels Bohr etc. Later, JJ Thompson discovered electrons and Rutherford worked on the model to discover the nucleus. 

 

Finally, Niels Bohr’s model and the Quantum mechanical model provided the modern atomic model as it is known today. Modern atomic theory, though evolved in two centuries, holds much of Dalton’s atomic theory.