Periodic Classification of Elements

At present, 118 elements are known to us. Out of these 118, only 94 occur naturally. Each element has its own set of properties, and when you have that many of them, you need a sensible way to arrange them. This chapter traces how chemists slowly built the periodic table you use today, and why the idea still matters far beyond the exam hall. For more free study notes, see our Class 10 Science notes in Resources.

Why we classify elements

Elements are classified on the basis of similarities in their properties. Instead of memorising 118 separate elements, grouping the ones that behave alike lets us predict how an element will react just from its position. Several scientists attempted this classification over about a hundred years, and each attempt improved on the last.

Early attempts to classify elements

Dobereiner's Triads (1817)

In 1817, Johann Wolfgang Dobereiner tried to arrange elements with similar properties into groups of three. He called each group a triad. When the three elements of a triad were written in order of increasing atomic mass, the atomic mass of the middle element was roughly the average of the other two.

• Lithium, Sodium, Potassium: atomic masses 6.9, 23.0 and 39.0. Average of Li and K is (6.9 + 39.0) / 2 = 22.95, which is close to 23.0, the mass of sodium.
• Calcium, Strontium, Barium: atomic masses 40.1, 87.6 and 137.3. Average of Ca and Ba is (40.1 + 137.3) / 2 = 88.7, close to 87.6, the mass of strontium.
• Chlorine, Bromine, Iodine: atomic masses 35.5, 79.9 and 126.9. Average of Cl and I is (35.5 + 126.9) / 2 = 81.2, close to 79.9, the mass of bromine.

Limitation: Dobereiner could identify only three triads from the elements known at that time, so this system was not found to be very useful.

Newlands' Law of Octaves (1866)

In 1866, John Newlands arranged the then known elements in order of increasing atomic mass. He started with hydrogen, the lightest, and ended at thorium, the 56th element. He noticed that every eighth element had properties similar to the first, just like the eighth note in a musical scale. He called this the Law of Octaves. For example, sodium is the eighth element after lithium and behaves like it, and magnesium resembles beryllium in the same way.

Limitations:

• The Law of Octaves worked only up to calcium (atomic mass about 40). Beyond that, the pattern broke down.
• Newlands assumed only 56 elements existed in nature, so newly discovered elements did not fit the pattern.
• To force elements into the table, he sometimes placed two elements in the same slot, and put unlike elements together. Cobalt and nickel, for instance, ended up in the same column as fluorine, chlorine and bromine, which have very different properties.
• With the later discovery of the noble gases, the Law of Octaves stopped making sense. It worked well only for the lighter elements.

Mendeleev's Periodic Table

Dmitri Mendeleev arranged the elements using their atomic mass together with the similarity of their physical and chemical properties. He paid special attention to the compounds elements formed with oxygen and hydrogen, treating the formulae of their oxides and hydrides as a basic property for classification. From this he stated the Periodic Law:

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

His table had vertical columns called groups and horizontal rows called periods.

Achievements of Mendeleev's periodic table:

• A systematic study of elements: elements with similar properties were grouped together.
• Room for noble gases: when they were discovered, they fitted neatly into a new zero group without disturbing the rest of the table.
• Prediction of undiscovered elements: Mendeleev left gaps and predicted the properties of elements not yet found, naming them with the prefix Eka. His Eka-aluminium was later discovered as gallium, and his predictions were remarkably close.

For example, compare Mendeleev's predicted Eka-aluminium with the gallium that was actually discovered:

• Atomic mass: predicted about 68, actual 69.7.
• Formula of oxide: predicted E2O3, actual Ga2O3.
• Formula of chloride: predicted ECl3, actual GaCl3.

Limitations:

• Position of hydrogen: hydrogen resembles both the alkali metals and the halogens, so it had no fixed place.
• Position of isotopes: isotopes of an element have different atomic masses, yet they were not given different positions.
• Anomalous pairs: atomic mass did not always increase smoothly. Cobalt has a higher atomic mass than nickel, but was placed before it.
• Like elements in different groups: platinum and gold have similar properties but were placed in different groups.

The Modern Periodic Table

In 1913, Henry Moseley showed that the atomic number (symbol Z), the number of protons in an atom, is a more fundamental property than atomic mass. He modified Mendeleev's table, and the law was restated as the Modern Periodic Law:

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

This small change fixed most of Mendeleev's problems at once. Arranging by atomic number automatically sorted out the anomalous pairs and gave isotopes a single position, because isotopes share the same atomic number.

Position of elements in the modern periodic table

• Elements are arranged in order of increasing atomic number.
• The table has 7 periods (horizontal rows) and 18 groups (vertical columns).
• An element is placed in a period according to the number of electron shells it has.
• The first period is the shortest, with only two elements: hydrogen and helium.
• The sixth period is one of the longest, and the lanthanides and actinides are shown separately at the bottom of the table to keep it a manageable width.
• Metals such as sodium and magnesium sit towards the left, while non-metals such as sulphur and chlorine sit towards the right.
• A zig-zag line separates metals from non-metals. The elements along it, such as boron, silicon, germanium, arsenic, antimony, tellurium and polonium, are semi-metals or metalloids.
• Elements in the same group have the same number of valence electrons. Fluorine and chlorine both belong to group 17 and each has 7 valence electrons in its outermost shell.
• So a group signals a common outer-shell electronic configuration, while the number of shells increases as you move down a group.

Periodic trends: how properties change

Because elements are arranged so neatly, their properties change in a regular, predictable way as you move across a period or down a group. These patterns are called periodic trends.

Valency (the combining capacity, set by the number of valence electrons):

• Down a group: stays the same, because the number of valence electrons stays the same.
• Across a period: first increases and then decreases.

Atomic size or atomic radius (the distance from the centre of the nucleus to the outermost shell):

• Down a group: increases, because new shells are added, pushing the outer electrons further from the nucleus. Example: Li < Na < K < Rb < Cs < Fr.
• Across a period: decreases, because the growing nuclear charge pulls the electrons closer in. Example: Li > Be > B > C > N > O > F.

Metallic character (the tendency to lose electrons):

• Down a group: increases, because it becomes easier to lose the outer electrons.
• Across a period: decreases, because it becomes harder to lose electrons.

Non-metallic character (the tendency to gain electrons):

• Down a group: decreases.
• Across a period: increases.

Electronegativity (the tendency to attract a shared pair of electrons):

• Down a group: decreases. Example: Li > Na > K > Rb > Cs.
• Across a period: increases. Example: Li < Be < B < C < N < O < F.

Ionisation energy (the energy needed to remove an electron) and electron affinity both increase across a period and decrease down a group.

Two summary points worth remembering: metals are electropositive while non-metals are electronegative, and the oxides of metals are basic in nature while the oxides of non-metals are acidic.

Why it still matters

It is easy to think of the periodic table as a finished, framed chart on the lab wall. It is not. Both ends of the story are alive right now.

The table is still growing. Scientists are trying to make brand new elements that no one has ever seen. Element 119 and element 120 would open a completely new eighth row of the table. In 2024, a team at the Lawrence Berkeley National Laboratory in the United States fired a beam of titanium-50 atoms at plutonium-244 for 22 days and produced element 116, livermorium, a result published in July 2024. Making element 120 is thought to be 10 to 20 times harder, so the very table you revise for your board exam is not actually complete.

The elements are now a matter of national policy. Where an element sits, its group and its properties, is exactly what makes it useful, and today some elements are fought over. In 2025 India launched its National Critical Mineral Mission with a government outlay of about 16,300 crore rupees up to 2030 to 2031, aimed at securing 30 minerals it calls critical, including lithium, cobalt, nickel and the rare earth elements. These are the metals inside electric vehicle batteries, solar panels and wind turbines. The clean energy shift, and India's push to make more of it at home, is in a real sense a race for particular squares of the periodic table. You can see more of these science-to-world connections on the Learnacy Hub.

Key takeaways

• Classification groups elements by shared properties so their behaviour can be predicted.
• Dobereiner's triads and Newlands' octaves were early, limited attempts that worked only for lighter elements.
• Mendeleev arranged elements by atomic mass and left gaps that correctly predicted undiscovered elements such as gallium.
• Moseley's atomic number gave the modern table: 7 periods, 18 groups, and clear periodic trends.
• The table is still an active field, from making new superheavy elements to securing the critical minerals that power clean energy.

Sources

1. US Department of Energy, New Progress Toward the Discovery of New Elements
2. Live Science, Scientists 1 step closer to a superheavy element that adds a new row to the periodic table
3. DD News, India's Critical Mineral Mission: charting a self-reliant green future
4. Press Information Bureau, Government of India, National Critical Mineral Mission