Question

(a) During the period from about 1800 to about 1865 , the atomic weights of many elements were accurately measured. Why was this important to Mendeleev's formulation of the periodic table? (b) What property of the atom did Moseley associate with the wavelength of X-rays emitted from an element in his experiments? (c) Why are chemical and physical properties of the elements more closely related to atomic number than they are to atomic weight?

Short answer

(a) Accurate atomic weight measurements during the 1800-1865 period were crucial for Mendeleev's formulation of the periodic table, as they allowed him to identify patterns and trends in element properties, organizing them according to increasing atomic weights. (b) Moseley associated the wavelength of X-rays emitted from an element with its atomic number, discovering that the square root of the frequency of X-rays emitted is proportional to the element's atomic number. (c) Chemical and physical properties of elements are more closely related to atomic number than atomic weight because atomic number directly impacts electron configuration, determining reactivity, bonding patterns, and other properties. While atomic weight can reveal trends, atomic number offers a more consistent basis for organizing elements in the periodic table.

Step-by-step solution

Question a: Importance of accurate atomic weight measurements

Measuring the atomic weights of elements with accuracy during the early 19th century was crucial for Mendeleev's formulation of the periodic table. Accurate atomic weights allowed Mendeleev to identify patterns and trends among the elements, which he organized according to their atomic weights in increasing order. The periodic table is based on the observation that the properties of elements are periodic functions of their atomic weights, and this observation would not have been possible without precise measurements of atomic weights.

Question b: Moseley's connection of atomic property to X-ray wavelength

In Moseley's experiments, he connected the wavelength of X-rays emitted from an element to a specific atomic property, which is the atomic number of the element. Moseley discovered that the square root of the frequency of X-rays emitted by an element is proportional to the element's atomic number. This relationship, known as Moseley's law, helped to solidify the concept of atomic number and its importance in ordering elements in the periodic table.

Question c: Relation of chemical and physical properties to atomic number and atomic weight

The chemical and physical properties of elements are more closely related to atomic number than atomic weight because atomic number represents the number of protons in an atom, which directly impacts its electron configuration. Electron configuration determines an element's reactivity, bonding patterns, and other properties. Although atomic weight is related to atomic number and can still reveal trends in elemental properties, it is less directly tied to the electron configuration – especially in the case of isotopes, which have the same atomic number but different atomic weights. Atomic number, therefore, offers a more consistent basis for organizing elements and understanding their properties within the periodic table.

Key concepts

Atomic Weights

In the early 19th century, the determination of atomic weights became an essential breakthrough for chemists. Accurate measurement of these weights allowed scientists like Dmitri Mendeleev to propose an organized system to classify elements. Mendeleev noticed a periodicity in elemental properties when they were arranged by atomic weights. He found that elements with similar properties appeared at regular intervals. This periodicity was crucial for the development of the periodic table, a tool that helps us predict and understand the behavior of elements.

It’s important to understand that atomic weight is the average mass of an element's atoms, considering the relative abundance of its isotopes. During Mendeleev's time, understanding isotopes was not fully developed, but the atomic weight concept was key in identifying patterns. This informed the placement of elements in a way that highlighted properties that repeated periodically. Such insights were only possible through accurate and meticulous measurement of atomic weights.

Mendeleev

Dmitri Mendeleev is celebrated for his role in developing the periodic table, which set a foundation for modern chemistry. In 1869, Mendeleev arranged elements in a table by increasing atomic weight. Importantly, he also left spaces open for elements that were yet to be discovered, predicting their existence and properties. This kind of foresight confirmed the usefulness of his periodic table.

Mendeleev's approach was revolutionary because it wasn't just based on atomic weight; it was rooted in the properties of elements. He understood the concept of periodicity, where certain characteristics recur at regular intervals. This concept allowed Mendeleev to predict the properties of undiscovered elements, and many of his predictions turned out to be remarkably accurate. Thus, Mendeleev’s periodic table was more than a list; it was a predictive tool that reshaped chemical research and education.

Moseley's Law

Henry Moseley, a physicist working in the early 20th century, played a pivotal role in refining the periodic table. Moseley's experiments involved measuring the X-ray spectra of elements, leading to the formulation of Moseley's Law. He discovered that the frequency of X-rays emitted by an element is related to its atomic number, not its atomic weight. His work demonstrated that elements should be arranged in the periodic table according to their atomic numbers, not weights.

Moseley’s Law, expressed as \(\sqrt{u}=a(Z-b)\), where \(u\) is the frequency of emitted X-rays, \(Z\) the atomic number, and \(a\) and \(b\) are constants, highlighted the importance of the atomic number. This discovery helped clarify the placement of elements, such as the correct ordering of the elements like iodine and tellurium, correcting the misplacements that occurred with a weight-based arrangement. Moseley’s insights emphasized that chemical properties are more closely tied to the atomic number, rather than to weight.

Atomic Number

The atomic number represents the number of protons within the nucleus of an atom. It is a fundamental characteristic that defines an element in the periodic table. Unlike atomic weight, the atomic number strictly defines an element, as it directly influences the electron configuration, thereby determining chemical behavior.

Chemical and physical properties of elements are highly dependent on electron configurations. Elements with the same atomic number share similar properties because they have the same number of electrons, which dictates how they interact chemically. For instance, elements are grouped into columns in the periodic table because they have the same valency, a trait determined by their electrons and thus, their atomic number.
Although isotopes of an element have different atomic weights, they share the same atomic number, illustrating why atomic number, rather than atomic weight, is more crucial for arranging elements in a meaningful way. The recognition and use of atomic number ultimately led to the modern layout of the periodic table, capturing the periodic nature of elemental properties accurately.