Periodic Table and Periodic Classification Notes

The periodic table is a chart that arranges all known chemical elements in order of increasing atomic number, so that elements with similar properties fall in the same vertical column. It is one of the most powerful tools in science, because the position of an element tells us a great deal about its behaviour: whether it is a metal or non-metal, how reactive it is, what kind of compounds it forms and even how large its atoms are. Questions on the periodic table appear frequently in SSC CGL, CHSL, MTS, railway, defence (NDA, CDS), state PCS and occasionally in UPSC Prelims, often on its history, groups, periodic trends and special elements. This note explains the subject step by step in simple language.

Quick Facts at a Glance

• Modern periodic law: Properties of elements are a periodic function of their atomic numbers (Henry Moseley, 1913).
• Mendeleev's periodic law: Properties are a periodic function of atomic masses (1869).
• Modern periodic table: 18 groups (vertical columns) and 7 periods (horizontal rows).
• Known elements: 118, from hydrogen (1) to oganesson (118).
• 2019 was celebrated as the International Year of the Periodic Table, marking 150 years of Mendeleev's table.

1. History of Periodic Classification

Dobereiner's Triads (1817–1829)

German chemist Johann Wolfgang Döbereiner noticed groups of three elements with similar properties, called triads. In each triad, the atomic mass of the middle element was roughly the average of the other two. Example: lithium (7), sodium (23) and potassium (39); (7 + 39) ÷ 2 = 23. Limitation: only a few triads could be found, so it did not work for all elements.

Newlands' Law of Octaves (1865)

English chemist John Newlands arranged elements in order of increasing atomic mass and found that every eighth element had properties similar to the first, like the eighth note in a musical scale. Limitation: it worked only up to calcium, and later elements did not fit the pattern.

Mendeleev's Periodic Table (1869)

Russian chemist Dmitri Mendeleev arranged the 63 elements known at the time in order of increasing atomic mass and grouped them by similar properties, especially the formulas of their oxides and hydrides. His great insight was to leave gaps for elements not yet discovered and to predict their properties. He named them eka-aluminium, eka-boron and eka-silicon; they were later discovered as gallium, scandium and germanium, with properties very close to his predictions. This proved the power of his table. He is called the Father of the Periodic Table.

Limitations of Mendeleev's table: the position of hydrogen was uncertain; isotopes (same element, different masses) had no proper place; and some elements had to be placed out of order of atomic mass, such as cobalt before nickel.

Modern Periodic Table

In 1913, Henry Moseley showed through X-ray experiments that the atomic number (number of protons) is the more fundamental property. Arranging elements by atomic number removed most of Mendeleev's anomalies. The modern long form of the periodic table is based on atomic number and electronic configuration.

2. Structure of the Modern Periodic Table

• Groups (1 to 18): Vertical columns. Elements in a group have the same number of valence electrons and similar chemical properties.
• Periods (1 to 7): Horizontal rows. The period number equals the number of electron shells. Period 1 has only 2 elements (hydrogen and helium); periods 6 and 7 are the longest.
• Lanthanides and actinides are placed in two separate rows at the bottom to keep the table compact.

Blocks of the Periodic Table

3. Important Groups

• Group 1 – Alkali metals: Lithium, sodium, potassium, rubidium, caesium, francium. Very reactive, soft metals that react vigorously with water; stored under kerosene. Hydrogen is placed in group 1 but is a non-metal.
• Group 2 – Alkaline earth metals: Beryllium, magnesium, calcium, strontium, barium, radium.
• Group 17 – Halogens: Fluorine, chlorine, bromine, iodine, astatine. "Halogen" means salt-former. Fluorine is the most reactive non-metal; bromine is the only non-metal that is liquid at room temperature.
• Group 18 – Noble gases: Helium, neon, argon, krypton, xenon, radon. They have complete outer shells and are very unreactive. Used in balloons (helium), advertising signs (neon) and bulbs (argon).

4. Periodic Trends

Why does atomic size decrease across a period? As we move right, protons are added to the nucleus while electrons are added to the same shell. The stronger nuclear pull draws the electrons closer, so the atom shrinks. Down a group, new shells are added, so atoms become larger.

Key extremes: Fluorine is the most electronegative element; caesium (among stable elements) is among the most electropositive; helium has the highest ionisation energy.

5. Metals, Non-Metals and Metalloids

Metals lie on the left and centre of the table, non-metals on the right, and metalloids along a zig-zag line between them. Metalloids, such as boron, silicon, germanium, arsenic, antimony and tellurium, show properties of both metals and non-metals. Silicon and germanium are semiconductors and form the basis of electronic chips.

6. Interesting Facts About Elements

• Hydrogen is the lightest and most abundant element in the universe; oxygen is the most abundant element in the Earth's crust, followed by silicon; nitrogen makes up about 78% of the atmosphere.
• Aluminium is the most abundant metal in the Earth's crust; iron is the most widely used metal.
• Mercury is the only metal that is liquid at room temperature; gallium and caesium melt just above room temperature.
• Silver is the best conductor of electricity, followed by copper.
• Tungsten has the highest melting point among metals and is used in bulb filaments.
• Diamond and graphite are two forms (allotropes) of carbon; diamond is the hardest natural substance, while graphite conducts electricity.
• Helium was first detected in the Sun's spectrum during a solar eclipse observed from India in 1868, before it was found on Earth.

7. New Elements and Naming

New elements, mostly very heavy and short-lived, are created in particle accelerators. The International Union of Pure and Applied Chemistry (IUPAC) approves their names and symbols. In 2016, the seventh period was completed with the naming of four elements: nihonium (113, after Japan), moscovium (115, after Moscow), tennessine (117, after Tennessee) and oganesson (118, after the physicist Yuri Oganessian). Elements are usually named after places, scientists, mythological concepts or properties, such as einsteinium (Einstein), curium (the Curies) and mendelevium (Mendeleev).

8. Why the Periodic Table Matters

The periodic table is not just a list to memorise; it is a map of chemical behaviour. Chemists use it to predict how elements react, design new materials and search for substitutes for scarce elements. For example, rare earth elements (mostly lanthanides) are vital for magnets in wind turbines, electric vehicle motors and electronics, and their supply has become a strategic issue globally. India's National Critical Mineral Mission (2025) aims to secure such minerals, which links basic chemistry to UPSC topics on economic security and green energy.

9. Practice Questions

1. Which element is the most reactive non-metal? Answer: Fluorine.
2. In which group are the alkaline earth metals placed? Answer: Group 2.
3. What is the number of elements in the first period? Answer: Two.
4. Which scientist arranged elements by atomic number? Answer: Henry Moseley.
5. Which block contains the transition elements? Answer: d-block.

Memory Tricks

To remember the first twenty elements in order (H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca), many students use the line: "Hi Hello Little Bear, Brown Cat Never Offers Food; Nice Naughty Mice Always Sing Pretty Songs; Clever Artists Know Cats". For periodic trends, remember that atomic size goes down and left to grow larger, while electronegativity and ionisation energy go up and right to grow stronger. Fluorine sits near the top-right corner, which is why it is the most electronegative element.

Related Notes

• Atomic Structure, Carbon Compounds and Fuels
• Chemistry Basics: Acids, Bases, Salts and Metals
• Physics Basics Notes
• All General Science Notes

Frequently Asked Questions

Who is known as the Father of the Periodic Table?

Dmitri Mendeleev.

What is the modern periodic law?

The properties of elements are a periodic function of their atomic numbers.

Who proposed the modern periodic law?

Henry Moseley, in 1913.

How many groups and periods are in the modern periodic table?

18 groups and 7 periods.

What were Dobereiner's triads?

Groups of three similar elements where the middle element's atomic mass was about the average of the other two.

Which elements did Mendeleev predict?

Eka-aluminium, eka-boron and eka-silicon, later discovered as gallium, scandium and germanium.

Which is the most electronegative element?

Fluorine.

Which non-metal is liquid at room temperature?

Bromine.

Why are noble gases unreactive?

Because their outermost electron shells are completely filled.

How does atomic size change across a period?

It decreases from left to right because of increasing nuclear charge.

What are metalloids?

Elements with properties of both metals and non-metals, such as silicon, germanium and arsenic.

Which elements are called transition metals?

The d-block elements in groups 3 to 12, such as iron, copper and zinc.