Question

Based on results of his study of atomic x-ray spectra, Moseley discovered a relationship that replaced atomic mass as the criterion for ordering the elements. By what criterion are the elements now ordered in the periodic table? Give an example of a sequence of element order that was confirmed by Moseley's findings.

Short answer

Elements are now ordered by atomic number. For example, despite having a lower atomic mass, iodine is placed after tellurium in the periodic table due to its atomic number.

Step-by-step solution

Understand Moseley's Discovery

Moseley discovered that the elements should be ordered by their atomic number, not their atomic mass. The atomic number corresponds to the number of protons in the nucleus of an atom.

Define the Atomic Number

The atomic number is the number of protons in the nucleus of an atom. It uniquely identifies an element and determines its position in the periodic table.

Significance of Atomic Number over Atomic Mass

Ordering elements by atomic number resolves anomalies where ordering by atomic mass led to incorrect positioning of elements. For example, iodine (I) has a lower atomic mass than tellurium (Te), but a higher atomic number. Moseley's findings confirmed that iodine should come after tellurium.

Example Sequence Confirmed by Moseley

Consider the elements: Tellurium (Te) with atomic number 52 and Iodine (I) with atomic number 53. Despite iodine having a lower atomic mass, it is placed after tellurium in the periodic table due to its higher atomic number.

Key concepts

atomic number

The atomic number is a fundamental concept in chemistry and physics. It represents the number of protons found in the nucleus of an atom. Each element on the periodic table has a unique atomic number. For example, Hydrogen has an atomic number of 1, meaning it has one proton in its nucleus.

The atomic number is crucial because it defines the identity of an element. If you change the number of protons, you change the element itself. For instance, adding a proton to a hydrogen atom (atomic number 1) would turn it into helium (atomic number 2).

It was Henry Moseley who highlighted the significance of the atomic number in arranging the periodic table. He showed that organizing elements by their atomic number, rather than atomic mass, provided a more accurate representation.

Using x-ray spectra, Moseley found that the atomic number correlates directly with certain properties of the elements, making it a better criterion for ordering them.

periodic table

The periodic table is an organized chart of all known elements. Elements are arranged in order based on their atomic number, thanks to Moseley’s work.

The periodic table has rows called periods and columns called groups or families. Elements in the same group share similar chemical properties. This structure helps scientists understand and predict the behavior of elements.

Before Moseley’s discovery, the periodic table was arranged by atomic mass. However, this method led to inconsistencies. Moseley corrected these issues by using atomic numbers as the organizing principle.

For example, tellurium (Te) and iodine (I) were originally misplaced when organized by atomic mass. Tellurium has a higher atomic mass than iodine, but when arranged by atomic number, iodine (atomic number 53) correctly follows tellurium (atomic number 52).

This reordering helped solidify the periodic table’s ability to accurately represent elemental properties and relationships.

atomic mass

Atomic mass refers to the total number of protons and neutrons in an atom's nucleus. It is sometimes called atomic weight, though this term can be a bit misleading. The atomic mass is typically measured in atomic mass units (amu).

While the atomic mass is important, it's not the best criterion for ordering elements in the periodic table. This realization came about because atomic mass doesn’t always increase uniformly across elements.

For example, iodine has a lower atomic mass (126.90 amu) compared to tellurium (127.60 amu), yet it should be placed after tellurium in the periodic table. Moseley's findings showed that atomic number more accurately reflects an element’s properties and position than atomic mass does.

Thus, while atomic mass gives us valuable information about the makeup of an atom, it is the atomic number that determines the order of elements in the periodic table.

x-ray spectra

X-ray spectra are patterns of X-rays emitted by atoms when electrons transition between different energy levels. These spectra can be used to determine various properties of elements, including atomic number.

Moseley studied the x-ray spectra of different elements and discovered a direct relationship between the frequency of the emitted x-rays and the atomic number of the element. This relationship provided strong evidence that the atomic number is a fundamental property of elements and should be used to organize the periodic table.

By examining the x-ray spectra, Moseley could accurately determine the atomic number of various elements. This breakthrough helped to correct the anomalies found when elements were arranged by atomic mass. For instance, the x-ray spectra showed that iodine should indeed follow tellurium, despite iodine’s lower atomic mass.

Moseley’s work with x-ray spectra thus played a pivotal role in refining the periodic table, ensuring that elements were placed in their correct positions based on their atomic number.