The first definition of an element was made by

Analyze the question: It asks who first defined an element. This is a history-of-chemistry question.

Concept: The idea of an "element" developed over time:

• Ancient Greeks believed in four "elements": earth, air, fire and water.
• Robert Boyle (1661), in his book The Sceptical Chymist, rejected the Greek idea. He defined an element as a substance that cannot be broken down into simpler substances by chemical means. This is regarded as the first modern definition of an element.
• Antoine Lavoisier (1789) later compiled a list of elements, treating them as substances that could not be decomposed further. His work built on Boyle's idea.
• John Dalton (1803-1808) proposed the atomic theory, in which elements are made of identical atoms.
• J. J. Thomson (1897) discovered the electron.

Solution:

• Dalton: gave the atomic theory, not the first definition. ✗
• Lavoisier: gave the first list of elements and refined the definition, but came after Boyle. ✗
• Boyle: gave the first definition of an element. ✓
• Thomson: discovered the electron. ✗

The correct answer is **C. R.

Tip: Link each scientist to one key word:

• Boyle = first definition of an element (and gas pressure-volume law)
• Lavoisier = oxygen and conservation of mass
• Dalton = atomic theory
• Thomson = electron