According to Dalton's atomic theory, the smallest indivisible particle of matter that takes part in a chemical reaction is called what?

Introduction / Context:
Dalton proposed one of the earliest modern atomic theories to explain the laws of chemical combination, such as the law of conservation of mass and the law of constant proportions. His theory introduced the idea that matter is made up of very small, indivisible particles. This question asks what name Dalton gave to those smallest particles that participate in chemical reactions. Understanding this term is foundational for further study of atomic structure and chemistry.

Given Data / Assumptions:
- The theory under consideration is Dalton atomic theory, developed in the early nineteenth century.
- Dalton described matter as composed of tiny indivisible particles which cannot be created or destroyed in chemical reactions.
- The question seeks the term used for these smallest particles in his model.
- Options provided are atom, molecule, compound, and element.

Concept / Approach:
Dalton proposed that all matter consists of indivisible particles called atoms. According to his theory, atoms of a given element are identical in mass and properties, and chemical reactions involve rearrangement of these atoms. Molecules, compounds, and elements are related concepts, but the smallest indivisible particle in Dalton view is specifically the atom. Molecules are combinations of atoms, compounds are substances containing atoms of different elements, and an element is a pure substance made of only one type of atom. Therefore, the correct answer is atom.

Step-by-Step Solution:
Step 1: Recall the key postulates of Dalton atomic theory, especially that matter is made of very small particles that cannot be subdivided further in chemical processes. Step 2: Remember that Dalton named these small particles atoms, borrowing from earlier philosophical ideas but giving them a scientific basis. Step 3: Distinguish the term atom from molecule, which refers to two or more atoms bonded together, such as O2 or H2O. Step 4: Recognise that compound refers to a substance composed of atoms of different elements chemically combined in fixed proportions. Step 5: Recognise that element is a pure substance consisting of only one kind of atom, but the smallest particle itself is still called the atom. Step 6: Conclude that atom is the term used by Dalton for the smallest indivisible particle of matter.

Verification / Alternative check:
Textbooks in chemistry summarise Dalton theory by listing atoms as the fundamental particles that cannot be created, divided, or destroyed in chemical reactions. They also state that atoms combine in simple whole number ratios to form compounds. Historical discussions emphasise that Dalton revived and refined the ancient idea of indivisible atoms and provided quantitative evidence. Molecules and compounds are discussed later as structures formed from atoms. This hierarchy, consistently presented in educational resources, confirms that atom is the correct answer to the question.

Why Other Options Are Wrong:
Molecule: This is made up of one or more atoms, and is therefore not the smallest indivisible particle in Dalton sense.
Compound: A compound consists of molecules or formula units containing atoms of different elements, so it is not the smallest particle either.
Element: An element is a substance made of only one kind of atom, but the smallest unit that retains the chemical identity of the element is the atom itself, not the abstract concept of the element.

Common Pitfalls:
Students sometimes confuse atom and molecule, especially when learning about diatomic gases such as hydrogen and oxygen. They may think that the molecule is the smallest particle because it is the smallest unit of a substance that can exist independently. However, Dalton specifically focused on indivisible units in chemical reactions, which he called atoms. Modern science has discovered subatomic particles, but Dalton model remains a useful starting point. Keeping the definitions of atom, molecule, compound, and element clear helps avoid this confusion.

Final Answer:
According to Dalton atomic theory, the smallest indivisible particle of matter that takes part in a chemical reaction is called an Atom.