The **key difference** between classical theory and quantum theory is that **classical theory describes the nature of macroscopic level, whereas quantum theory describes the nature of microscopic level**.

The classical theory and quantum theory are very important branches in physical chemistry since we can use them to describe the behaviour of objects. There is a significant difference between classical theory and quantum theory, including the predictability of events as well as the object that is considered, i.e. classical theory is applied for macroscopic objects where the quantum theory describes the behaviour if microscopic particles.

### CONTENTS

1. Overview and Key Difference

2. What is Classical Theory

3. What is Quantum Theory

4. Side by Side Comparison – Classical Theory vs Quantum Theory in Tabular Form

5. Summary

## What is Classical Theory?

The classical theory of mechanics is a theory that describes the motion of a macroscopic object. The type of macroscopic object may vary from projectiles to astronomical objects such as space crafts. The events of the movement are predictable according to classical theory. That means, if we know the initial state of the object, we can predict its state in the future and determine what was its state in the past; in other words, we can predict how an object will move and how it moved in the past.

Generally, classical theory gives extremely accurate results for large objects. However, this theory does not work for extremely massive objects and objects moving near the speed of light.

## What is Quantum Theory?

Quantum theory is a theory that describes the nature of objects at the atomic level. According to quantum theory, energy, momentum, and angular momentum are discrete values; we call it “quantized”. Here, objects have both wave and particle nature. Sometimes, this is the only theory that can describe the behaviours of subatomic particles – electrons, protons, etc.

Furthermore, this theory is important in determining how atoms bind to each other via covalent bonding. In addition, the fields where quantum theory is applied include electronics, cryptography, quantum computing, etc.

## What is the Difference Between Classical Theory and Quantum Theory?

Classical theory of mechanics is a theory that describes the motion of a macroscopic object while quantum theory is a theory that describes the nature of objects at atomic level. Therefore, the key difference between classical theory and quantum theory is that classical theory describes the nature of macroscopic level whereas quantum theory describes the nature of microscopic level. Further, classical theory does not describe wave-particle duality while quantum theory does not describe wave-particle duality.

Moreover, a further difference between classical and quantum theory is that future events are predictable if we use classical theory, but according to quantum theory, the events are unpredictable.

## Summary – Classical Theory vs Quantum Theory

Classical theory and quantum theory are important theories in physical chemistry. The key difference between classical theory and quantum theory is that classical theory describes the nature of macroscopic level, whereas quantum theory describes the nature of microscopic level.

##### Reference:

1. “Quantum Mechanics.” Wikipedia, Wikimedia Foundation, 3 June 2019, Available here.

##### Image Courtesy:

1. “Energy levels of sodium atom” By I, Orci (CC BY 2.5) via Commons Wikimedia

2. “Projectile-Motion—Video” By Zappys Technology Solutions (CC BY 2.0) via Flickr

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