Modern periodic law groups periods and periodic trends explained
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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 and Modern Periodic Law
1. What is the periodic classification of elements?
The periodic classification of elements is the systematic arrangement of elements in the periodic table based on increasing atomic number and recurring chemical properties.
- Elements are arranged in order of increasing atomic number (Z).
- Elements with similar valence electron configurations are placed in the same group.
- This arrangement shows periodic trends such as atomic size, valency, and reactivity.
- It helps predict the properties of unknown or newly discovered elements.
2. What is the modern periodic law?
The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.
- Proposed by Henry Moseley in 1913.
- It corrected Mendeleev’s periodic law, which was based on atomic mass.
- When elements are arranged by increasing atomic number, similar properties recur at regular intervals.
- This law forms the basis of the modern periodic table.
3. What are groups and periods in the periodic table?
In the periodic table, groups are vertical columns and periods are horizontal rows.
- There are 18 groups and 7 periods in the modern periodic table.
- Elements in the same group have the same number of valence electrons and similar chemical properties.
- Elements in the same period have the same number of electron shells.
- Example: Group 1 elements (Li, Na, K) are alkali metals with valency +1.
4. Why are elements in the same group similar in properties?
Elements in the same group have similar properties because they have the same number of valence electrons in their outermost shell.
- Chemical properties depend mainly on valence electrons.
- For example, Group 17 elements have 7 valence electrons.
- They tend to gain one electron to form ions like Cl- and F-.
- This leads to similar reactivity and compound formation patterns.
5. What are the main periodic trends in the periodic table?
The main periodic trends are atomic radius, ionization energy, electronegativity, and metallic character.
- Atomic radius: Decreases across a period, increases down a group.
- Ionization energy: Increases across a period, decreases down a group.
- Electronegativity: Increases across a period, decreases down a group.
- Metallic character: Decreases across a period, increases down a group.
6. How does atomic size change across a period and down a group?
Atomic size decreases from left to right across a period and increases from top to bottom down a group.
- Across a period: Nuclear charge increases, pulling electrons closer to the nucleus.
- Down a group: New electron shells are added, increasing the distance from the nucleus.
- Example: Atomic size order in Group 1 is Li < Na < K.
7. What is the difference between Mendeleev’s periodic law and the modern periodic law?
The key difference is that Mendeleev’s law is based on atomic mass, while the modern periodic law is based on atomic number.
- Mendeleev’s periodic law: Properties are periodic functions of atomic masses.
- Modern periodic law: Properties are periodic functions of atomic numbers.
- The modern law explains isotopes and corrects position anomalies like cobalt and nickel.
8. What are s, p, d, and f blocks in the periodic table?
The s, p, d, and f blocks are classifications of elements based on the subshell in which the last electron enters.
- s-block: Groups 1 and 2 (valence electrons in s-orbital).
- p-block: Groups 13–18 (valence electrons in p-orbital).
- d-block: Transition elements (Groups 3–12).
- f-block: Lanthanides and actinides.
9. Why are noble gases placed in Group 18?
Noble gases are placed in Group 18 because they have completely filled valence shells, making them chemically stable.
- They have a stable electronic configuration (e.g., Ne: 2,8).
- They have very high ionization energy and almost zero electron affinity.
- They exist as monoatomic gases like He, Ne, and Ar.
- This stability explains their low reactivity.
10. How does the periodic table help predict the valency of an element?
The periodic table helps predict valency based on the number of valence electrons in a group.
- For Groups 1–4, valency equals the group number.
- For Groups 15–17, valency = 8 − group number.
- Example: Oxygen (Group 16) has 6 valence electrons, so valency = 8 − 6 = 2.
- This explains compound formation like H2O, where oxygen forms two bonds.
