Question

Dalton assumed that all atoms of the same element were identical in all their properties. Explain why this assumption is not valid.

Short answer

The existence of isotopes invalidates Dalton's assumption that all atoms of the same element are identical in all their properties. Isotopes have the same chemical properties but different atomic masses which result in different physical properties, such as density and boiling points, and in some cases, radioactivity. This discrepancy is evident in hydrogen's three isotopes - protium, deuterium, and tritium - which have varying numbers of neutrons and display different properties.

Step-by-step solution

Understanding Dalton's Atomic Theory

Dalton's Atomic Theory states that all matter is made up of small, indivisible particles called atoms, and that atoms of the same element are identical in all their properties. This theory was widely accepted because it provided a simple and elegant explanation for the observed behavior of elements and their compounds.

Introduction to Isotopes

Isotopes are variations of the same element that contain the same number of protons but different numbers of neutrons in their atomic nuclei. This results in the isotopes having different atomic masses while still possessing the same chemical properties. The existence of isotopes contradicts Dalton's assumption that all atoms of the same element are identical in all their properties, as isotopes have different atomic masses.

Discovery of Isotopes

In the early 20th century, scientists discovered that the naturally occurring elements often consist of mixtures of isotopes. This was first observed using techniques such as mass spectrometry, which measures the mass-to-charge ratio of ions. The measurements showed that some elements, like chlorine, had more than one atomic mass present in the samples, indicating the presence of isotopes.

Examples of Isotopes and their Properties

There are numerous examples of isotopes in nature, such as hydrogen. Hydrogen has three isotopes: protium (\(^1\text{H}\)), deuterium (\(^2\text{H}\)), and tritium (\(^3\text{H}\)). Protium has no neutrons, deuterium has one neutron, and tritium has two neutrons in their nuclei. While these isotopes of hydrogen have the same chemical properties, their differing atomic masses result in different physical properties, such as density and boiling points. Additionally, tritium is radioactive, which is another property that sets it apart from the other two isotopes.

Conclusion

The discovery of isotopes and the differences in their properties invalidates Dalton's assumption that all atoms of the same element are identical in all their properties. Although the chemical properties of isotopes are similar, their differing atomic masses result in varied physical properties, and in some cases, radioactivity. Therefore, we can conclude that Dalton's assumption is not completely valid.