Periodic Table: History, Classification of Elements and the Modern Periodic Table
By Home Academy
The periodic table is one of the most important and fundamental tools in chemistry. It systematically arranges all known chemical elements in a way that reveals recurring trends in their properties. From early attempts to classify elements based on atomic mass to the modern arrangement based on atomic number, the periodic table has evolved through the contributions of many great scientists. Understanding its history, development, and scientific basis is crucial from an examination point of view.
Introduction to the Periodic Table
The periodic table is a tabular arrangement of chemical elements organized according to their atomic numbers, electronic configurations, and recurring chemical properties. Elements with similar properties are placed in the same vertical columns called groups, while horizontal rows are known as periods. The modern periodic table not only helps in identifying elements but also predicts their chemical behavior.
Early Attempts to Classify Elements
Before the discovery of the modern periodic table, scientists struggled to bring order to the growing number of known elements.
Antoine Lavoisier (1789)
Lavoisier classified elements into metals and non-metals. Although simple, this classification was the first scientific attempt to organize elements.
Johann Döbereiner’s Triads (1829)
Döbereiner grouped elements into sets of three called triads, where the atomic mass of the middle element was approximately the average of the other two. For example, lithium, sodium, and potassium formed a triad. This showed a relationship between atomic mass and properties but worked for only a few elements.
Newlands’ Law of Octaves (1864)
John Newlands arranged elements in increasing order of atomic mass and observed that every eighth element had properties similar to the first, similar to musical octaves. This law was applicable only up to calcium and failed for heavier elements but was important historically.
Mendeleev’s Periodic Table (1869)
Dmitri Mendeleev is regarded as the father of the periodic table. He arranged elements in order of increasing atomic mass and stated the Periodic Law, which said that the properties of elements are periodic functions of their atomic masses.
The most remarkable achievement of Mendeleev was that he left gaps for undiscovered elements and accurately predicted their properties. Elements like gallium, scandium, and germanium were discovered later and matched his predictions.
However, Mendeleev’s table had limitations. It could not explain the position of hydrogen, isotopes, and anomalies like argon and potassium.
Modern Periodic Law and Modern Periodic Table
Henry Moseley (1913)
Moseley resolved the problems of Mendeleev’s table by introducing atomic number as the basis of classification. He stated the Modern Periodic Law, which says that the properties of elements are periodic functions of their atomic numbers.
The modern periodic table is arranged in increasing order of atomic number, which explains electronic structure and chemical behavior accurately.
Structure of the Modern Periodic Table
The modern periodic table consists of 18 groups and 7 periods.
Elements are classified into metals, non-metals, and metalloids. Based on electronic configuration, elements are further divided into s-block, p-block, d-block, and f-block elements.
Noble gases are placed in Group 18 and are chemically inert due to their stable electronic configuration. Lanthanides and actinides are placed separately at the bottom to maintain the table’s structure.
Structure of the Modern Periodic Table
The modern periodic table is arranged according to increasing atomic number. It consists of 18 vertical columns called groups and 7 horizontal rows called periods. Elements in the same group show similar chemical properties due to the same number of valence electrons.
Classification of Elements
1. Metals
Metals are generally found on the left and center of the periodic table. They are good conductors of heat and electricity, malleable, ductile, and usually form positive ions (cations).
Examples include sodium, iron, copper, and aluminium.
2. Non-Metals
Non-metals are located on the right side of the periodic table. They are poor conductors of heat and electricity and usually form negative ions (anions) or share electrons.
Examples include oxygen, nitrogen, sulfur, and chlorine.
3. Metalloids
Metalloids are found along the zig-zag line separating metals and non-metals. They show properties intermediate between metals and non-metals.
Examples include boron, silicon, and germanium.
Block-wise Classification (Based on Electronic Configuration)
s-Block Elements
These elements have their valence electrons in the s-orbital and include Group 1 and Group 2 elements.
p-Block Elements
These include Groups 13 to 18. Their valence electrons enter the p-orbital. This block contains metals, non-metals, metalloids, and noble gases.
d-Block Elements (Transition Metals)
These include Groups 3 to 12. They have partially filled d-orbitals and show variable oxidation states, colored compounds, and catalytic properties.
f-Block Elements
These include lanthanides and actinides, placed separately at the bottom of the table to maintain its shape. Their valence electrons enter the f-orbital.
Group-wise Explanation and Names (Very Important for Exams)
Group 1 – Alkali Metals
These elements include lithium, sodium, potassium, rubidium, cesium, and francium.
They are called alkali metals because they form alkaline hydroxides when reacting with water. They are soft, highly reactive metals and are never found free in nature.
Group 2 – Alkaline Earth Metals
This group includes beryllium, magnesium, calcium, strontium, barium, and radium.
They form alkaline oxides and hydroxides and are harder and less reactive than alkali metals.
Group 13 – Boron Group
This group includes boron, aluminium, gallium, indium, and thallium.
Boron is a metalloid, while the rest are metals. These elements generally show +3 oxidation state.
Group 14 – Carbon Group
This group includes carbon, silicon, germanium, tin, and lead.
Carbon is a non-metal essential for life. Silicon and germanium are metalloids, while tin and lead are metals.
Group 15 – Nitrogen Group (Pnictogens)
This group includes nitrogen, phosphorus, arsenic, antimony, and bismuth.
Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is a metal.
Group 16 – Oxygen Group (Chalcogens)
This group includes oxygen, sulfur, selenium, tellurium, and polonium.
Oxygen and sulfur are non-metals and are essential for life. These elements usually show −2 oxidation state.
Group 17 – Halogens
This group includes fluorine, chlorine, bromine, iodine, and astatine.
They are called halogens meaning salt-producers. They are highly reactive non-metals and usually show −1 oxidation state.
Group 18 – Noble Gases
This group includes helium, neon, argon, krypton, xenon, and radon.
They are called noble gases because they are chemically inert due to completely filled valence shells (duplet in helium and octet in others). They exist as monoatomic gases.
Lanthanides and Actinides (f-Block Elements)
Lanthanides
These are rare-earth elements from atomic number 57 to 71. They are used in magnets, lasers, and electronic devices.
Actinides
These elements range from atomic number 89 to 103. Most are radioactive and are used in nuclear reactors and research.
Why Lanthanides and Actinides Are Placed Separately
They are placed separately at the bottom of the periodic table to avoid increasing the width of the table and to maintain a clear and systematic structure.
Exam Key Points to Remember
18 groups and 7 periods
Group 1 → Alkali metalsGroup 17 → Halogens
Group 18 → Noble gases
Modern periodic table is based on atomic number
f-block elements placed separately
Clssification of Elements by Scientists (Chronological & Exam-Focused Table)
| Scientist | Year | Major Contribution / Work | Basis of Arrangement of Elements | Total Number of Elements Considered |
|---|---|---|---|---|
| Antoine Lavoisier | 1789 | First scientific classification of elements; published list in Traité Élémentaire de Chimie | Classified elements into metals and non-metals based on physical and chemical properties | 33 elements |
| Johann Döbereiner | 1829 | Proposed Döbereiner’s Triads, showing relationship between atomic mass and properties | Groups of three elements with similar properties; atomic mass of middle element ≈ average of other two | Few elements (about 20–30) forming 5 known triads |
| John Newlands | 1864 | Gave the Law of Octaves | Arranged elements in increasing order of atomic mass; every 8th element showed similar properties | 56 elements |
| Dmitri Mendeleev | 1869 | Created first widely accepted periodic table; predicted undiscovered elements | Arranged elements in increasing order of atomic mass; similar properties placed in same group | 63 elements |
| Henry Moseley | 1913 | Established the Modern Periodic Law using X-ray spectra | Arranged elements in increasing order of atomic number | All known elements of the time (about 75) |
Exam Tips (Very Important)
33 elements → Lavoisier
Law of Octaves → Newlands (every 8th element)Predicted elements → Mendeleev
Atomic number basis → Moseley
Modern Periodic Table is based on Moseley’s law, not Mendeleev’s
Important Scientists and Their Contributions (Exam Focus)
Antoine Lavoisier introduced basic classification of elements.
Johann Döbereiner proposed triads.
John Newlands gave the Law of Octaves.
Dmitri Mendeleev created the first widely accepted periodic table and predicted new elements.
Henry Moseley introduced atomic number and established the modern periodic law.
Importance of the Periodic Table
The periodic table helps predict the properties of elements, understand trends like atomic size and electronegativity, and study chemical reactions. It is essential in chemistry, physics, material science, medicine, and environmental studies.
Important Facts About the Periodic Table
The modern periodic table contains 118 confirmed elements, arranged in order of increasing atomic number.
Classification of Elements by Nature
Out of the 118 elements:
Metals: about 88 elements
Non-metals: 17 elementsMetalloids: 7 elements
Metalloids include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.
Structure-Related Facts
The modern periodic table has 18 groups and 7 periods.
Groups are vertical columns; periods are horizontal rows.Elements in the same group have similar chemical properties.
Period number indicates the number of electron shells.
Group number (for s and p block) indicates the number of valence electrons.
Block-Wise Distribution of Elements
s-block: Groups 1 and 2 → mostly metals
p-block: Groups 13 to 18 → metals, non-metals, metalloids, noble gasesd-block: Groups 3 to 12 → transition metals
f-block: Lanthanides and actinides → inner transition metals
Metals – Key Facts
Metals occupy the left and center of the periodic table.
Most elements in the table are metals.They are generally solid at room temperature (except mercury).
Metals are good conductors of heat and electricity.
They usually form positive ions (cations).
Non-Metals – Key Facts
Non-metals are found on the right side of the table.
Many non-metals exist as gases (oxygen, nitrogen).They are poor conductors of heat and electricity.
They usually form negative ions (anions) or share electrons.
Hydrogen is a non-metal but placed in Group 1 due to electronic configuration.
Metalloids – Key Facts
Metalloids lie along the zig-zag (staircase) line.
They show both metallic and non-metallic properties.Most metalloids are semiconductors.
Silicon and germanium are widely used in electronics.
Group-Specific Facts
Group 1 (Alkali metals): 6 elements, highly reactive metals.
Group 2 (Alkaline earth metals): 6 elements, less reactive than Group 1.Group 17 (Halogens): 5 elements, highly reactive non-metals.
Group 18 (Noble gases): 6 elements, chemically inert.
Noble Gases Facts
Noble gases have completely filled valence shells.
They exist as monoatomic gases.Helium has a duplet configuration, others have an octet.
Xenon and krypton can form compounds under special conditions.
Lanthanides and Actinides Facts
Lanthanides: 14 elements (atomic number 57–71).
Actinides: 14 elements (atomic number 89–103).Most actinides are radioactive.
They are placed separately to maintain table shape.
Special Facts for Exams
The periodic table is based on Henry Moseley’s modern periodic law.
Mercury is the only metal liquid at room temperature.Bromine is the only liquid non-metal.
Francium is the most reactive metal.
Fluorine is the most reactive non-metal.
Rare Earth Minerals (REM) – Important Facts
What are Rare Earth Minerals?
Rare earth minerals are a group of 17 metallic elements that occur together in nature and are essential for modern technology. They are called “rare” not because they are scarce, but because they are difficult to extract and separate.
List of Rare Earth Elements (17 Elements)
Lanthanides (elements)
Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium
Plus 2 Elements
Scandium and Yttrium
Classification of Rare Earth Elements
Light Rare Earth Elements (LREE): Lanthanum to Samarium
Heavy Rare Earth Elements (HREE): Europium to Lutetium + YttriumImportant Rare Earth Minerals (Ores)
Monazite – most important source
BastnaesiteXenotime
Loparite
👉 Monazite sand is rich in rare earths and also contains thorium.
Uses of Rare Earth Minerals
Rare earth elements are critical for high-technology and strategic industries:
Mobile phones and computers
Permanent magnets (EVs, wind turbines)Solar panels and LED lights
Nuclear reactors
Missiles, radar, and defense systems
Medical imaging (MRI)
Catalysts in petroleum refining
Rare Earth Minerals in India (Exam Important)
India has one of the world’s largest reserves of monazite.
Major Locations
Kerala coast
Tamil Nadu coastOdisha coast
Andhra Pradesh coast
India has about 6–7% of global rare earth reserves.
Global Distribution – Key Fact
China dominates rare earth production (over 60%)
Other countries: Australia, USA, India, Brazil, Vietnam👉 This makes rare earths geopolitically important.
Why Rare Earth Minerals are Important
Essential for green energy transition
Critical for defense and national securityBackbone of modern electronics
Strategic minerals for future technologies
Why They Are Called “Rare”? (Trick Question)
They are not rare in quantity
They are rare because they are:
Found in low concentrationsDifficult and costly to process
Environmentally challenging to extract
Quick Exam Facts (One-Liners)
Total rare earth elements → 17
Most important ore → MonaziteIndia’s main source → Beach sands
Used in EVs & wind turbines → Neodymium magnets
Radioactive element associated → Thorium
