Question

How was the problem of placement of isotopes in Mendeleeff's periodic table overcome in the modern periodic table?

Short answer

Answer: The problem of isotopes in Mendeleev's periodic table was overcome in the modern periodic table by arranging elements according to their atomic numbers instead of their atomic masses. This arrangement allowed all isotopes of a given element, which have the same atomic number but different atomic masses, to be placed under the same element in the periodic table, thus avoiding confusion and inconsistencies.

Step-by-step solution

The problem of isotopes in Mendeleev's periodic table

In Mendeleev's periodic table, elements were arranged in order of increasing atomic mass. This created a problem for the placement of isotopes, which are variants of a particular chemical element that have the same number of protons (atomic number) but different numbers of neutrons (different atomic mass). In Mendeleev's classification, isotopes of the same element would have been placed in different positions, leading to inconsistencies and confusion.

Introduction of the modern periodic table

The modern periodic table is based on Moseley's work on atomic numbers. In 1913, Henry Moseley discovered the concept of atomic numbers, which are the number of protons in the nucleus of an atom. This crucial discovery paved the way for the modern periodic table, in which elements are arranged according to their atomic numbers.

Overcoming the problem of isotopes in the modern periodic table

In the modern periodic table, elements are no longer arranged by increasing atomic mass, but by increasing atomic number. This arrangement solves the problem of isotopes since all isotopes of a given element have the same atomic number (number of protons) but different atomic masses (due to varying numbers of neutrons). As a result, isotopes of the same element can be placed under the same element in the periodic table, avoiding the confusion and inconsistencies caused by the arrangement in Mendeleev's table.

Key concepts

Isotopes

Isotopes are variants of a particular chemical element that have identical numbers of protons but differ in the number of neutrons within their atomic nuclei. This means that while they share chemical properties, their atomic masses vary. For example, carbon-12 and carbon-14 are both isotopes of carbon; they have the same atomic number (6) but different atomic masses (12 and 14, respectively).

The existence of isotopes posed a challenge for the earlier periodic table arrangements, which were based primarily on atomic mass. But with the advent of the modern periodic table, which uses atomic number as the organizing principle, isotopes of the same element are rightfully grouped together, acknowledging their shared properties while also recognizing their mass differences.

Mendeleev's Periodic Table

Mendeleev's periodic table, developed in 1869, was an early attempt to organize chemical elements based on their atomic masses. Dimitri Mendeleev, a Russian chemist, left gaps in his table for what he predicted would be undiscovered elements, and many of his predictions were eventually confirmed. However, his system struggled with certain irregularities, especially with isotopes, leading to the eventual refinement of the periodic table.

One of Mendeleev's key contributions was the recognition of the periodicity in the properties of elements, noticing that elements with similar chemical properties occurred at regular intervals when ordered by their atomic masses. His table set the foundation for the development of the modern periodic table.

Atomic Number

The atomic number is a fundamental property of an element and is denoted by the symbol 'Z'. It represents the number of protons found in the nucleus of an atom, and it defines the chemical identity of the element. All atoms of a particular element have the same atomic number. For instance, hydrogen, with an atomic number of 1, always has one proton in its nucleus.

This contrasts with the concept of atomic mass, which considers both protons and neutrons. The realization that atomic number, rather than atomic mass, is the key to the chemical properties of elements was revolutionary. It provided a more consistent and accurate way to organize the periodic table, leading to the clear placement of isotopes.

Henry Moseley

Henry Moseley, an English physicist, made a significant contribution to the field of chemistry with his research on atomic numbers. In 1913, through his experiments using X-ray spectroscopy, Moseley determined that each element has a unique charge in its nucleus, which corresponds to the number of protons—an element's atomic number. This was a crucial discovery that brought clarity to the periodic law and resolved many of the problems inherent in Mendeleev's table, specifically those involving isotopes.

Moseley's work led to the redefinition of the periodic table with atomic number as the organizing criterion. Sadly, Moseley's promising career was cut short when he died serving in World War I, but his legacy persists in the clarity and utility of the modern periodic table that students and scientists use today.