Chapter 3, Problem 25A

Interpretation Introduction

Interpretation: “Dalton’s atomic theory was modified after the discovery that several isotopes of an element may exist” should be discussed.

Concept introduction:  Theory is a group of connected ideas or concepts intended to describe something.  It involves question- “why” or “how anything/ somethings happens. It is based on inference.

Answer to Problem 25A

Scientist James Chadwick modified Dalton’s theory by discovering neutrons and protons.

Explanation of Solution

John Dalton, British teacher and scientist, explained concepts behind most natural materials are mixtures of pure substance, pure substances are either elements or combinations of elements called compound. Main ideas of Dalton’s atomic theory are mentioned below:

• This theory states that all matter is made of indestructible and indivisible atoms.
• All atoms of element have identified mass and properties.
• Each element has different atoms.
• Atoms can engage in a chemical reaction.
• Compound contain a definite and small number of atoms.

John Dalton presumed that all of the atoms of certain element were identical. This idea continued for over $100$ years, until the scientist James Chadwick proposed his own theory. According to the Chadwick’s theory, nuclei of the most atoms contain neutrons and protons.

So, the theory of Dalton’s atomic theory changed to "all atoms of same element contains equivalent number of protons as well as electrons. Also, atoms of a given element may have unlike numbers of neutrons."

Conclusion

Dalton’s atomic theory was modified after the discovery that several isotopes of an element may exist.