Why Periodic Classification Matters in Chemistry
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Need for Periodic Classification of Elements
Chemists discovered that all the substances are made up of atoms or elements. Slowly they discovered many elements. In 1789, Antoine Lavoisier published a list of 33 elements. With the discovery of many elements chemists felt the need of classification of elements for their easy understanding and comparison. So, Antoine Lavoisier attempted to group all 33 elements into gases, metals, nonmetals and earth metals. Slowly by 1865, 63 elements were discovered. By now, chemists felt the need of periodic classification of elements as now it was very difficult to study the properties of these chemical elements individually. Scientists were trying to classify elements in a periodic manner of the basis of their various properties.
In the year 1869, Dmitri Mendeleev arranged all 63 elements in rows or columns in order of their atomic weight. He left the space for corresponding elements in his periodic table which were not even discovered then. Although he was able to predict the properties of those elements through his periodic classification of elements.
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Mendeleev’s Periodic Table from his book-‘An attempt towards a chemical conception of the Ether’
By 1911, scientists discovered atomic structure and atomic nucleus. Now they know the terms like isotope, proton etc. Mendeleev’s periodic table was not able to describe all this and was not able to assign position to isotopes and a perfect position to hydrogen according to its properties. So, in 1913 Mendeleev’s periodic table was perfected by Henry Moseley. He arranged all the elements in order of their increasing atomic numbers. This improved all the flaws of the Mendeleev Periodic Table. Such a position of hydrogen was fixed as its atomic number 1, there was no need for a separate position for isotopes as the arrangement was according to the atomic number of elements.
Presently, 118 chemical elements are known and are arranged in the Modern Periodic table. Out of 118 elements 98 elements occur naturally while 20 elements are man made in laboratories.
Significance of the Periodic Classification of Elements
Periodic table is very important as they classify elements according to their properties and provide us a great deal of information about elements and how they relate to one another. The organized classification of chemical elements in the periodic table has following advantages:
It enables chemists to easily understand properties of elements.
It comes very handy while performing experiments.
It enables chemists to compare the properties of elements.
It gives systematic and orderly information about elements and their compounds as well.
It enables chemists to predict the properties of even those elements which have not yet been discovered.
It provides information which can be used to easily balance the chemical equations.
It gives a proper explanation of the difference and similarity between properties of elements in groups and rows.
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FAQs on Periodic Classification of Elements Explained
1. What is the periodic classification of elements?
The periodic classification of elements is a systematic method of arranging all known chemical elements into a table. This arrangement is based on their increasing atomic number, electron configuration, and recurring chemical properties. Elements with similar properties are placed in the same vertical column, called a group, making it easier to study and predict their behaviour.
2. What is the Modern Periodic Law, and how does it differ from Mendeleev's Periodic Law?
The Modern Periodic Law states that the physical and chemical properties of elements are a periodic function of their atomic number. This is the primary difference from Mendeleev's Periodic Law, which proposed that properties were a function of atomic mass. By using atomic number, the modern law successfully resolved inconsistencies in Mendeleev's table, such as the position of isotopes and the incorrect order of certain element pairs like Argon and Potassium.
3. How are elements organised into periods and groups in the modern periodic table?
Elements in the modern periodic table are organised into horizontal rows and vertical columns:
- Periods: These are the 7 horizontal rows. The period number corresponds to the highest principal energy level (n) or shell that contains electrons for an element in that row.
- Groups: These are the 18 vertical columns. Elements within the same group share similar chemical properties because they have the same number of valence electrons (electrons in the outermost shell).
4. Why do elements belonging to the same group show similar chemical properties?
Elements in the same group exhibit similar chemical properties because they have an identical valence shell electronic configuration. Chemical reactions primarily involve the loss, gain, or sharing of these outermost valence electrons. Since elements in a group have the same number of valence electrons, they tend to react in similar ways to achieve a stable electron configuration, resulting in comparable chemical behaviour and valency.
5. What are s, p, d, and f-block elements?
Elements are classified into four blocks based on the subshell in which the last electron, or differentiating electron, enters:
- s-block: The last electron enters the 's' subshell. These are elements of Group 1 (alkali metals) and Group 2 (alkaline earth metals).
- p-block: The last electron enters the 'p' subshell. These include elements from Groups 13 to 18.
- d-block: The last electron enters the 'd' subshell. These are the transition metals, found in Groups 3 to 12.
- f-block: The last electron enters the 'f' subshell. These are the inner-transition elements (Lanthanoids and Actinoids) placed separately at the bottom of the table.
6. How does atomic radius vary across a period and down a group? Explain the reason.
The trend in atomic radius is as follows:
- Across a Period (left to right): The atomic radius generally decreases. This is because, while electrons are added to the same shell, the nuclear charge (number of protons) increases. The stronger attraction from the nucleus, known as increased effective nuclear charge, pulls the electron shells closer.
- Down a Group: The atomic radius increases. This is due to the addition of a new principal electron shell for each successive element. The effect of adding a new, more distant shell outweighs the increase in nuclear charge, leading to a larger atomic size.
7. What is electronegativity, and why are noble gases not typically assigned electronegativity values?
Electronegativity is a measure of the ability of an atom in a chemical compound to attract a shared pair of electrons towards itself. Noble gases (Group 18) are generally not assigned electronegativity values because they have a stable, complete octet (or duet for Helium) in their valence shell. Due to this stability, they have almost no tendency to attract electrons and rarely form chemical bonds.
8. What is the 'diagonal relationship' in the periodic table? Provide an example.
The diagonal relationship refers to the similarity in properties between certain diagonally adjacent elements in the second and third periods of the periodic table. This similarity arises because these elements have a comparable ionic size and charge-to-radius ratio (ionic potential). A classic example is the relationship between Lithium (Li) from Period 2, Group 1, and Magnesium (Mg) from Period 3, Group 2, who share many similar chemical characteristics.
9. What are isoelectronic species? Give an example and explain why their sizes differ.
Isoelectronic species are atoms or ions that have the same number of electrons. For instance, N³⁻, O²⁻, F⁻, Ne, and Na⁺ are all isoelectronic as they each have 10 electrons. However, their sizes differ significantly because they have different numbers of protons in their nucleus. The species with more protons (like Na⁺ with 11 protons) will exert a stronger pull on the 10 electrons, resulting in a smaller ionic radius compared to a species with fewer protons (like N³⁻ with 7 protons).
