Question

Which of the following of Dalton's proposals proved to be invalid? (a) Atoms of different elements combine to form compounds. (b) Atoms can combine in small whole number ratios. (c) Atoms are indivisible.

Short answer

Dalton's proposal that atoms are indivisible proved to be invalid.

Step-by-step solution

Understand Dalton's Proposals

Dalton's atomic theory proposed several ideas about the nature of matter. The relevant proposals here include: atoms are indivisible, atoms of different elements can combine in fixed ratios to form compounds, and atoms of an element are identical.

Analyze Option (a)

Option (a) states that atoms of different elements combine to form compounds. This aligns with Dalton's idea that atoms can come together as compounds and is considered valid in modern chemistry.

Analyze Option (b)

Option (b) states that atoms combine in small whole number ratios. This is consistent with Dalton's law of multiple proportions and is still a valid concept in chemistry, describing how elements form compounds.

Analyze Option (c)

Option (c) suggests that atoms are indivisible. This idea was later disproved with the discovery of subatomic particles such as electrons, protons, and neutrons, proving that atoms are indeed divisible into smaller particles.

Conclusion

After analyzing each option, it is clear that option (c) is invalid according to modern scientific understanding. Atoms are not indivisible as initially suggested by Dalton but contain even smaller particles.

Key concepts

Indivisibility of Atoms

The concept of atoms being indivisible was a cornerstone of Dalton's atomic theory. He proposed that atoms were the smallest unit of matter and could not be broken down into smaller parts. This idea was foundational in his time because it helped to explain the conservation of mass in chemical reactions.
However, scientific advancements uncovered that atoms are not truly indivisible. Around the late 19th and early 20th centuries, discoveries of subatomic particles like electrons, protons, and neutrons revealed that atoms are composed of even smaller components.
Scientists such as J.J. Thomson, who discovered the electron, showed that atoms could indeed be split into smaller parts, revolutionizing our understanding of atomic structure. Thus, while the indivisibility concept was pivotal at one point, it was eventually proven incorrect as our comprehension of atomic composition advanced.

Law of Multiple Proportions

Dalton's law of multiple proportions states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first are simple whole numbers. This principle supports the idea that compounds are formed by the combination of atoms in specific, consistent ratios, which is fundamental in chemical reactions.

• For instance, carbon and oxygen can combine to form both CO (carbon monoxide) and CO_2 (carbon dioxide).
• In CO, the ratio of the mass of oxygen to carbon is 1:1, while in CO_2, it is 2:1.

These simple ratios illustrate how distinct compounds result from different combinations of the same elements, underpinning Dalton's theory. This law remains a critical component of modern chemistry, emphasizing predictable patterns in element combination and compound formation.

Subatomic Particles

Subatomic particles are the building blocks of atoms, making up the fundamental structure of all matter. The three primary types of subatomic particles—electrons, protons, and neutrons—each play a specific role within an atom.

• Electrons are negatively charged particles that orbit the atom's nucleus.
• Protons are positively charged and reside in the nucleus, contributing to the atom's overall charge and identity.
• Neutrons are neutral particles, also located in the nucleus, and their presence affects the atom's mass and stability.

The balance and interaction between these particles determine the chemical behavior of atoms, such as bonding and reactions. Discoveries of subatomic particles have been crucial in expanding our understanding beyond Dalton's original atomic theory, shaping the complex models used in modern atomic chemistry, such as the quantum mechanical model.