Which of the following is the major drawback of Dalton's atomic theory? 

Atoms are indivisible. 

The correct answer is option 2. Atoms are indivisible.

Dalton's atomic theory, proposed by the English chemist John Dalton in the early 19th century, outlined several fundamental principles about the nature of matter:

1. Atoms are indivisible:  Dalton proposed that atoms are the smallest, indivisible particles of matter and cannot be further divided into smaller parts. This concept implied that atoms of a given element have identical properties.

2. Atoms of an element have exactly the same mass:  Dalton suggested that all atoms of a particular element have the same mass, providing a basis for understanding chemical reactions and the formation of compounds.

3. Atoms combine in fixed ratios to form compounds: Dalton proposed that atoms combine with one another in simple whole-number ratios to form compounds. This principle is known as the law of multiple proportions.

While Dalton's atomic theory laid the groundwork for modern atomic theory and provided valuable insights into the nature of matter, it also had some limitations and drawbacks, one of which was the assumption that atoms are indivisible. This assumption was later disproven with the discovery of subatomic particles, such as electrons, protons, and neutrons, which are the constituents of atoms.

Subsequent experimental evidence, particularly from studies of radioactivity and subatomic particles, revealed that atoms are indeed divisible and composed of smaller particles. For example, the discovery of the electron by J.J. Thomson in the late 19th century demonstrated that atoms contain negatively charged particles orbiting around a positively charged nucleus.

Therefore, the major drawback of Dalton's atomic theory is the assertion that atoms are indivisible, which was contradicted by later experimental findings. This limitation led to the development of more refined models of atomic structure, such as the nuclear model proposed by Ernest Rutherford and the quantum mechanical model developed in the early 20th century. These models provided a more accurate description of the structure of atoms and their behavior, incorporating the existence of subatomic particles and their interactions.