Question

Which of the following of Dalton's proposals proved to be invalid? (a) Atoms are indestructible. (b) Atoms of the same element are identical. (c) Atoms can combine in more than one whole number ratio.

Short answer

Dalton's proposals (a) and (b) were proven invalid; atoms are destructible, and not all atoms of an element are identical due to isotopes.

Step-by-step solution

Identifying Dalton's Original Proposal

John Dalton, in his atomic theory, proposed that atoms are indestructible and indivisible particles. This means he believed that atoms could not be broken down into smaller parts during chemical reactions.

Testing the Proposal: Atoms are Indestructible

Modern science has shown that atoms are not indestructible. They can be split into smaller subatomic particles, such as protons, neutrons, and electrons through nuclear reactions. Thus, this proposal is not valid according to current scientific understanding.

Examining Atoms of the Same Element

Dalton also proposed that all atoms of a given element are identical in mass and properties. However, the discovery of isotopes, which are atoms of the same element with different masses due to varying numbers of neutrons, invalidates this part of the proposal.

Considering Atoms Combining in Ratios

Lastly, Dalton's idea that atoms can combine in more than one whole number ratio aligns with the principles of chemical combination, such as the law of multiple proportions. Therefore, this part of his proposal remains valid.

Key concepts

Indestructibility of Atoms

One of the core ideas proposed by John Dalton was that atoms are indestructible. Dalton believed that, during chemical reactions, atoms retain their identity and simply rearrange themselves to form new substances. He thought they could not be created, destroyed, or transformed into different types of atoms.
However, advancements in science have shown this is not entirely true. While chemical reactions do not destroy atoms, nuclear reactions do. In a nuclear reaction, an atom can be split into smaller particles like protons, neutrons, and electrons. This discovery was groundbreaking because it revealed that atoms have an internal structure, which can be altered. Despite this revelation, the principle of conservation of mass still holds in chemical reactions. Only nuclear reactions break this rule. Consequently, Dalton’s proposal that atoms are indestructible was one of the few elements of his theory proven to be incorrect.

Identical Atoms

Dalton also contended that all atoms of a given element are identical in mass and properties. According to his theory, this meant that every atom of carbon, for instance, would have the same weight and behave in the exact same manner.
However, the discovery of isotopes challenged this notion. Isotopes are atoms of the same element that have different numbers of neutrons, which gives them different masses. For example, carbon-12 and carbon-14 are isotopes of carbon with 12 and 14 neutrons, respectively.
Isotopes remain chemically similar, as their electron configurations stay the same, but the differing neutron content gives rise to variations in atomic mass. Therefore, Dalton’s idea was too simplistic and didn't account for these variations, proving once again how scientific theories are often revised as new discoveries are made.

Law of Multiple Proportions

Despite the inaccuracies in his theory, Dalton did accurately describe how atoms can combine in whole-number ratios—an idea that led to the law of multiple proportions. This law states that if two elements form more than one compound, the different masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
A practical example of this concept can be seen with carbon and oxygen. They can combine to form both carbon monoxide (CO) and carbon dioxide (CO₂). In carbon monoxide, the elements combine in a 1:1 ratio, while in carbon dioxide, they combine in a 1:2 ratio. This illustrates how the ratios of the elements' masses are simple and whole.

• Carbon and oxygen in CO: 1 to 1 ratio
• Carbon and oxygen in CO₂: 1 to 2 ratio

Understanding this principle helps elucidate the predictable and systematic ways in which elements react with one another, reinforcing the validity and utility of Dalton’s insights into combination ratios.