Difference Between Dalton’s Atomic Theory and Modern Atomic Theory

Key Difference – Dalton’s Atomic Theory vs Modern Atomic Theory
 

Dalton’s atomic theory is the oldest theory about the atom. In 1808, John Dalton published his theory, which was composed of several postulates that were built based on his experiments and laws of chemical combination.  A number of scientists later contributed to the development of modern atomic theory, which is different from Dalton’s atomic theory and has more advanced facts about the atom and its behavior. The key difference between Dalton’s Atomic Theory and Modern Atomic Theory is that the structure and properties of atom according to Dalton’s theory are different from the structure and properties proposed by the modern atomic theory.

CONTENTS

1. Overview and Key Difference
2. What is Dalton’s Atomic Theory
3. What is Modern Atomic Theory
4. Side by Side Comparison – Dalton’s Atomic Theory vs Modern Atomic Theory in Tabular Form
5. Summary

What is Dalton’s Atomic Theory?

Dalton’s atomic theory is a set of postulates proposed to describe the structure and properties of an atom. The development of this first atomic theory was influenced by facts like the dissolution of different gases in water in different proportions, the composition of tin oxide with  88% Tin while the rest being oxygen, etc. Then Dalton proposed the following postulates.

• All matter is made out of atoms which are indivisible.
• Atoms of one element are identical to each other in their mass, size, and shape.
• Atoms can combine with each other in small whole numbers.
• Atoms can neither be created nor destroyed.
• An atom is the smallest unit of matter that can take part in a chemical reaction.

These postulates above does not explain the structure or properties of an atom in detail.

Figure 01: Some atoms and molecules with their structures according to Dalton’s atomic theory.

What is Modern Atomic Theory?

Since there were so many defects in Dalton’s atomic theory, scientists started to carry out more experiments to explain the exact structure and the properties of an atom. This led to the development of the modern atomic theory. The modern atomic theory indicated the defects of Dalton’s atomic theory. These defects can be stated as below.

• Atoms are not indivisible; they are composed of subatomic particles.
• There can be atoms of the same element which are not identical. These are called isotopes.
• Atoms are not always combined in small numbers. In polymers, a large number of atoms are combined to create the molecule.
• Atoms can be destroyed by fission (ex: atom bomb).
• Sometimes, subatomic particles take place in certain reactions. (ex: radioactive decay)

Apart from these, the modern atomic theory explains more details about the atom and its behavior. Some of these details are listed below.

• Atoms are composed of subatomic particles such as electrons, protons and neutrons.
• Protons and neutrons together form the core of the atom where electrons are found in orbitals around the nucleus, which looks like a cloud.
• The orbitals occupied by electrons are energy levels which indicate the energy of a certain electron.
• These energy levels are composed of sub-energy levels.
• The fundamental characteristic that all the atoms of the same element share is the number of protons. Atoms of the same element can have different numbers of electrons which are called ions and different numbers of neutrons which are called isotopes.
• Compounds can be made out of the same element or different elements.
• When all the elements are considered together, their atoms have properties that vary periodically.

Figure 02: The structure of Helium atom according to the modern atomic theory.

What is the difference between Dalton’s Atomic Theory and Modern Atomic Theory?

Summary – Dalton’s Atomic Theory vs Modern Atomic Theory

Even though there were no well-equipped laboratories, Dalton was able to build up a theory on atoms, which are invisible to the eye. This led to the development of modern atomic theory, which can explain almost everything about the structure and properties of atoms. There is a large difference between Dalton’s atomic theory and modern atomic theory since the structure and properties of atom according to Dalton’s theory is different from the structure and properties proposed by the modern atomic theory.

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Reference:

1.Gillaspy, Rebecca. “Modern Atomic Theory: Electron Clouds, Schrodinger & Heisenberg.” Study.com. N.p., n.d. Web. Available here.07 June 2017.
2.Shrestha, Binod. “Postulates of Daltons Atomic Theory.” Chemistry LibreTexts. N.p., 21 Mar. 2017. Web. Available here. 07 June 2017.

Image Courtesy:

1.” Daltons particles” By John Dalton; Published in New System of Chemical Philosophy  (Public Domain) via Commons Wikimedia
2. “Helium atom QM” By User: Yzmo – Own work (CC BY-SA 3.0) via Commons Wikimedia

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