The existence of isotopes violates one of the original ideas of Dalton's atomic theory. Which one?

Dalton's Atomic Theory, proposed by the chemist John Dalton in the early 1800s, revolutionized the way we think about matter and its composition. The theory consists of several key postulates that describe the nature of atoms. Among these postulates are the ideas that elements are composed of tiny particles called atoms, and that the atoms of a particular element are identical in mass and properties. Additionally, Dalton posited that atoms of different elements can combine in fixed ratios to form compounds.

However, with the discovery of isotopes, it was found that not all atoms of the same element have identical mass, which contradicted one of Dalton's original assumptions. Despite this, Dalton's theory was crucial in developing the periodic table and understanding chemical reactions.

Isotopes are variants of a particular chemical element that have the same number of protons, or the same atomic number, but different numbers of neutrons. Consequently, while they share chemical properties, isotopes differ in atomic mass. For example, carbon has two stable isotopes: carbon-12 (with 6 protons and 6 neutrons) and carbon-13 (with 6 protons and 7 neutrons).

The existence of isotopes is evident in nature and can have significant implications, such as the use of carbon-14 in radiocarbon dating to determine the age of archaeological artifacts. Understanding isotopes is essential not only in chemistry but also in fields like geology and medicine, where isotopes are used for diagnostic imaging and cancer treatment.

The atomic mass of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. It reflects the relative abundance of each isotope of that element in nature. Since isotopes of the same element have different numbers of neutrons, they also vary in mass. The atomic mass unit (amu) is based on the mass of the carbon-12 isotope, which is defined as exactly 12 amu.

For instance, the atomic mass of chlorine is approximately 35.45 amu, which takes into account the relative abundances of its two main isotopes, chlorine-35 and chlorine-37. Accurately determining atomic mass is fundamental in various scientific calculations, such as stoichiometry in chemistry.

The atomic number is a fundamental property of an element and denotes the number of protons in the nucleus of an atom. It determines the identity of an element: for example, every atom with 6 protons is a carbon atom. The atomic number is unique to each element and is used to organize the periodic table.

Despite variations in neutrons and atomic mass among isotopes, the atomic number remains constant within a given element. This consistency allows chemists to predict the types of chemical reactions that an element can undergo, based on its position in the periodic table.