Question

The first periodic law stated by Mendeleev was (a) there is no correlation in the properties and atomic weights of the elements (b) the properties of the elements are a periodic function of their atomic numbers (c) the properties of the elements are a periodic function of their atomic weights (d) the properties of the elements are a periodic function of their empirical formula.

Short answer

The first periodic law stated by Mendeleev is that the properties of the elements are a periodic function of their atomic weights.

Step-by-step solution

Understanding the First Periodic Law

First, it's essential to recognize what the First Periodic Law, as stated by Dimitri Mendeleev, actually claims. This law introduced the concept that the properties of the elements have a specific pattern or periodicity when they are arranged by increasing atomic weight.

Evaluate the Given Options

Analyze each of the provided options in the context of historical scientific knowledge at the time of Mendeleev’s discovery. Mendeleev’s original periodic table was based on atomic weights, not atomic numbers, because the concept of atomic number was not yet known.

Select the Correct Answer

Match the historically accurate definition of the First Periodic Law with the options provided. Considering Mendeleev’s emphasis on atomic weights, the correct statement regarding his law must be identified.

Key concepts

Mendeleev's Periodic Table

Discovered by Dimitri Mendeleev in the 1860s, Mendeleev's periodic table is a milestone in the history of chemistry. Mendeleev arranged the known chemical elements in a tabular format, organizing them in order of increasing atomic weight, which refers to the mass of an atom's nucleus expressed in atomic mass units. While modern periodic tables are now organized by atomic number, Mendeleev's table was a groundbreaking step towards the periodic law we understand today.

Through his observations, Mendeleev discerned that the elements' properties repeated periodically, leading to a predictive tool allowing him to even forecast the existence and properties of then-undiscovered elements. His initial table faced skepticism but was later validated as a fundamental scientific development as it accounted for the recurrent chemical and physical properties that could be observed and measured.

The Significance of Mendeleev's Work

Mendeleev's periodic table was significant for several reasons. It did not just classify the elements based on their atomic weight but also provided a way to predict the discovery of new elements. Furthermore, Mendeleev's table highlighted the importance of periodicity in understanding chemical behavior, which laid the groundwork for chemical periodicity and reactivity observed in modern chemistry.

Atomic Weight

The concept of atomic weight, or atomic mass, is central to understanding the structure of Mendeleev's periodic table. Atomic weight represents the average mass of atoms of an element, measured in atomic mass units (amu), basing on a relative scale with carbon-12 isotope assigned a mass of 12 amu. Mendeleev's pioneering investigation utilized these weights to order the elements and even adjust misinterpretations in the measured weights known at his time.

Keep in mind that in modern times, we usually refer to 'relative atomic mass' which is slightly more accurate, considering isotopic distribution in a natural sample of the element. Mendeleev's table banked on the relative atomic weights to draw periodic trends before the discovery of protons or the realization that atomic number accurately defines an element's identity.

How Atomic Weight Informed Element Properties

The ordering of elements by atomic weight revealed a sequential 'periodicity' to their properties. Elements with similar chemical behavior appeared at regular intervals, groupings today known as chemical families, like the noble gases or the halogens. This periodicity was the essence of Mendeleev's table and showed that atomic weight was a key to unlocking the systematization of elements.

Periodicity of Elements

Periodicity in chemistry refers to the recurring trends that are apparent in the properties of the elements. It explains why certain characteristics of elements appear at regular intervals when they are arranged in order of their atomic weights (or atomic numbers in the modern context). Mendeleev's recognition of this concept was revolutionary; he saw that elements with similar chemical and physical properties occurred periodically in the list of elements when they were ordered by increasing atomic weight.

For students, understanding periodicity helps in predicting the behavior of an element based on its position in the periodic table. Each period, or row, in the table represents one complete sequence from the smallest to the largest principal quantum number, which signifies electron shells filling from inner to outer layers in atoms.

Examples of Periodic Trends

Such trends include atomic and ionic radius, ionization energy, affinity, and electronegativity. For instance, electronegativity tends to increase across a period and decrease down a group. Recognizing these trends allows chemists to anticipate how an element might react or what properties it may have, reinforcing periodicity as a fundamental concept in chemistry.