CHAPTER2 Flashcards
What are the differences in the meaning of the following terms: ‘peri- odic law’, ‘periodic system’ and ‘periodic table’?
The terms “periodic law”, “periodic system”, and “periodic table” are related but have different meanings. The periodic law refers to the approximate repetition of properties of chemical elements after certain intervals. The periodic system is a more general term that refers to the fundamental relationships that exist among elements and the ordering of elements by atomic number into natural groups. The periodic table is a 2D representation of the periodic system that attempts to graphically represent the periodic law. While the terms “periodic system” and “periodic table” are often used interchangeably, they are not synonyms 3.
Elaborate the following statement: “The periodic law has been de- scribed as the most characteristic and fundamental law of chemistry, but remains nevertheless approximate and vague.”
The periodic law is considered the most characteristic and fundamental law of chemistry because it describes the approximate repetition of properties of chemical elements after certain intervals. This law is fundamental because it underlies the organization of the periodic system and the periodic table, which are essential tools for understanding the properties of elements and their behavior in chemical reactions. However, the periodic law is also approximate and vague because the repetition of properties is not exact and the periodicity is not constant. The length of the periods varies, and the repetition is only approximate because there is a gradual change in the properties of elements in a group. Therefore, while the periodic law is a fundamental law of chemistry, it is not as precise or exact as some other laws in physics, for example. Nonetheless, the periodic law is still considered a law-like behavior because it displays a fundamental pattern that is essential to understanding the properties of chemical elements 2.
What are the main conclusions of Mendeleev’s first article on the pe- riodic law?
The main conclusions of Mendeleev’s first article on the periodic law are:
- The elements, when arranged according to their atomic weights, exhibit an evident periodicity of properties.
- The periodicity of properties is not affected by the chemical nature of the elements.
- The periodicity of properties is not affected by the physical state of the elements.
- The periodicity of properties is not affected by the number of atoms in the molecule.
- The periodicity of properties is not affected by the valency of the elements.
- The periodicity of properties is not affected by the position of the elements in the periodic system.
- The periodicity of properties is not affected by the position of the elements in the periodic table.
These conclusions were based on Mendeleev’s observation of the properties of the known elements and his attempt to organize them into a coherent system based on their atomic weights. His work led to the development of the periodic table and the periodic system, which are still used today to organize and understand the properties of chemical elements 15.
On which 2 properties did Mendeleev based his ordering of the chem- ical elements? Give a definition of these properties.
Mendeleev based his ordering of the chemical elements on two properties: atomic weight (atomic mass) and valency.
Atomic weight is the mass of an atom of a chemical element, expressed in atomic mass units (amu). It is determined by the number of protons and neutrons in the nucleus of the atom. Atomic weight is a fundamental property of an element that remains constant for a given element, and it is used to order the elements in the periodic table.
Valency is a measure of the combining power of an element, expressed as the number of hydrogen atoms with which it can combine, or the number of electrons it can donate or accept when forming a chemical bond. Valency is a property of an element that depends on its electronic configuration and determines its chemical behavior. Mendeleev used valency to predict the properties of undiscovered elements and to place them in the periodic table based on their expected chemical behavior.
Together, atomic weight and valency were used by Mendeleev to organize the chemical elements into groups with similar properties, leading to the development of the periodic table and the periodic system.15- 16.
Why must the periodic tables that decorate the walls in science muse- ums or university buildings, and typically display a variety of samples of the elements be considered as incorrect representations of the peri- odic law?
The periodic tables that decorate the walls in science museums or university buildings, and typically display a variety of samples of the elements, must be considered as incorrect representations of the periodic law because they often do not accurately reflect the underlying principles of the periodic system.
According to 3, the periodic system is a more general term that refers to the fundamental relationships that exist among elements, and the ordering of the elements by atomic number in such a way that they fall into natural groups. The periodic table, on the other hand, is a more restricted term that refers to a 2D representation of the periodic system, and an attempt to represent the periodic law graphically.
While there are more than 1000 versions of the periodic table that have been published, there is no ultimate or best periodic table, and the choice of which one to use depends on what one wants to highlight, such as chemical properties, electronic structure, or spectroscopic properties. Therefore, the periodic tables that decorate the walls in science museums or university buildings may not accurately reflect the underlying principles of the periodic system, and may be considered as incorrect representations of the periodic law 3.
Which problems led to a questioning of the validity of the periodic law and have threatened the periodic table? How have these problems been solved?
Several problems have led to a questioning of the validity of the periodic law and have threatened the periodic table. These problems include:
- Reversal of the pair tellurium-iodine and cobalt-nickel: The reversal of the pair tellurium-iodine and cobalt-nickel was originally contributed to errors in atomic weight. However, this problem was solved by using atomic number to order elements 18.
- Questionable placement of transition metals in 8th group: The questionable placement of transition metals in the 8th group violated the concept of single occupancy. However, this problem was solved by integrating transition metals in the long form of the periodic table 18.
- Position of hydrogen: The position of hydrogen is still a matter of discussion and has not been fully resolved 18.
- Position and number of rare-earth elements: The position and number of rare-earth elements were solved by the work of Moseley 18.
- Discovery of the noble gases: The discovery of the noble gases led to the addition of Group 0 to the periodic table 18.
- Existence of isotopes: The existence of isotopes was a problem that was solved by using atomic number to order elements 18.
- Radioactivity: Radioactive decay products are isotopes of known elements 18.
Overall, the problems that have threatened the periodic table have been solved by using atomic number to order elements, integrating transition metals in the long form of the periodic table, adding Group 0 to the periodic table, and recognizing the existence of isotopes and radioactive decay products 18. However, the position of hydrogen is still a matter of discussion.
Why is it not fair to give all credit for the discovery of the periodic law to Mendeleev? Why became Mendeleev’s name linked to the periodic table in the course of history?
It is not fair to give all credit for the discovery of the periodic law to Mendeleev because other scientists, such as Julius Lothar Meyer, also independently discovered the periodic system around the same time. However, Mendeleev’s version(s) of the periodic table created the biggest impact on the scientific community and became the most widely accepted version. Therefore, Mendeleev’s name became linked to the periodic table in the course of history.
According to 9, Julius Lothar Meyer, a German chemist, independently discovered the periodic system from Mendeleev. Meyer’s first table showing a regular variation of valency was published in 1864, which was the same year that Mendeleev published his first periodic table. However, Mendeleev’s version(s) of the periodic table created the biggest impact on the scientific community and became the most widely accepted version. Therefore, Mendeleev’s name became linked to the periodic table in the course of history, and he is considered as the leading discoverer of the periodic system 9.
