Periodic Classification Class 10 Notes, Formulas, Questions | SSC

As more elements were discovered, scientists looked for patterns to organise them. This chapter traces that search through Dobereiner's triads, Newlands' law of octaves and Mendeleev's periodic table, with the merits and demerits of each. It then explains Moseley's work, the modern periodic law and the structure of the long form of the periodic table. Finally it links the table to electronic configuration and studies trends in valency, atomic size and metallic and nonmetallic character.

Summary

Around 1800 only about 30 elements were known; today 118 are known. Elements were first classified simply as metals and nonmetals, and later metalloids were recognised. As information grew, scientists tried other ways of classification. In 1817 the German scientist Dobereiner made groups of three chemically similar elements called triads. When arranged in increasing atomic mass, the atomic mass of the middle element was roughly the mean of the other two, for example Li (6.9), Na (23.0), K (39.1). However, not all known elements could be fitted into triads.

In 1866 the English scientist John Newlands arranged the known elements, from hydrogen to thorium, in increasing order of atomic mass and found that every eighth element resembled the first, like the eighth note of a musical octave. This is the law of octaves (for example Na resembles Li, Mg resembles Be, Cl resembles F). Its limitations: it worked only up to calcium, two elements were placed in some boxes (Co and Ni, Ce and La), elements with different properties were put under the same note (Co and Ni with the halogens, while Fe was placed with O and S), and there was no room for newly discovered elements.

The Russian scientist Dmitri Mendeleev developed his periodic table between 1869 and 1872. He arranged the 63 then known elements in increasing order of atomic mass and grouped them using physical and chemical properties such as the formulae of their hydrides and oxides, melting and boiling points and densities. His periodic law states that the properties of elements are a periodic function of their atomic masses. Vertical columns are groups and horizontal rows are periods.

Merits of Mendeleev's table: atomic masses of some elements were corrected (beryllium from 14.09 to 9.4, placing it before boron); vacant places were left for undiscovered elements eka-boron, eka-aluminium and eka-silicon with predicted atomic masses 44, 68 and 72, later discovered as scandium, gallium and germanium with properties matching his predictions; and when the noble gases were discovered, a separate zero group was added without disturbing the table. Demerits: cobalt and nickel have the same whole number atomic mass, so their order was ambiguous; isotopes, with the same chemical properties but different atomic masses, could not be placed properly; the rise in atomic mass is not uniform, so the number of elements between two heavy elements could not be predicted; and hydrogen resembles both alkali metals and halogens, so its position was uncertain.

In 1913 Henry Moseley showed with X-ray tube experiments that the atomic number of an element equals the positive charge on its nucleus, that is the number of protons. Atomic number is therefore more fundamental than atomic mass. The modern periodic law states that the properties of elements are a periodic function of their atomic numbers. Arranging elements by atomic number removes most of Mendeleev's drawbacks, though the position of hydrogen is still uncertain.

The modern (long form) periodic table has 7 periods and 18 groups, plus the lanthanide and actinide series shown separately at the bottom, giving 118 places, all now filled. It is divided into four blocks: s-block (groups 1 and 2), p-block (groups 13 to 18), d-block (groups 3 to 12, the transition elements) and f-block (lanthanides and actinides). A zig-zag line in the p-block separates metals on the left from nonmetals on the right, with metalloids along the line. Elements beyond uranium (atomic number 92) are man-made, radioactive and short lived.

The position of an element depends on its electronic configuration. Elements of a group have the same number of valence electrons, so they show similar properties, and one shell is added on going down. Elements of a period have the same number of shells; the atomic number and valence electrons increase by one from left to right, and a new shell starts with each new period. The maximum capacity of a shell is 2n squared (K 2, L 8, M 18, N 32), so period 1 has 2 elements and period 2 has 8; period 3 also has only 8 because of the octet rule.

Periodic trends: valency equals the number of valence electrons or the number needed to complete the octet; it is the same within a group and changes across a period. Atomic radius, the distance between the nucleus and the outermost shell (measured in picometre), decreases from left to right because the nuclear charge increases while electrons enter the same shell, and increases down a group because a new shell is added. Metallic character (electropositivity, the tendency to lose electrons) decreases across a period and increases down a group. Nonmetallic character (electronegativity, the tendency to gain electrons) increases across a period and decreases down a group. Greater electropositivity or electronegativity means higher reactivity. In group 17, F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid. In group 2 the reactivity with water increases from Be to Ba.

Mind map

History of the classification of elements

Important definitions

Formulas

Key points to remember

• 118 elements are known; around 1800 only about 30 were known.
• Dobereiner (German, 1817), Newlands (English, 1866), Mendeleev (Russian, 1869 to 1872), Moseley (English, 1913).
• Mendeleev arranged 63 elements by atomic mass and used formulae of hydrides and oxides.
• Eka-boron, eka-aluminium and eka-silicon became scandium, gallium and germanium.
• Mendeleev predicted eka-aluminium's atomic mass as 68; gallium's actual atomic mass is 69.7.
• The position of hydrogen is not settled even in the modern periodic table.
• s-block: groups 1, 2; p-block: groups 13 to 18; d-block: groups 3 to 12; f-block: lanthanides and actinides.
• Group 1: alkali metals; group 2: alkaline earth metals; group 17: halogens; group 18: noble gases (valency zero).
• Atomic radius decreases across a period and increases down a group.
• Metallic character increases down a group and decreases across a period; nonmetallic character does the opposite.
• Elements after uranium (Z = 92), that is 93 to 118, are man-made and radioactive.
• Be does not react with water, Mg reacts with steam, and Ca, Sr, Ba react with water at room temperature with increasing rates.

Solved examples

Example 1. Show that Li (6.9), Na (23.0) and K (39.1) form a Dobereiner's triad.

Mean of first and third \( = \dfrac{6.9 + 39.1}{2} = \dfrac{46.0}{2} = 23.0 \), which equals the atomic mass of Na. Since the three are chemically similar, they form a triad.

Example 2. Which of these are Dobereiner's triads? (1) Mg 24.3, Ca 40.1, Sr 87.6 (2) S 32.1, Se 79.0, Te 127.6 (3) Be 9.0, Mg 24.3, Ca 40.1

(1) \( \frac{24.3 + 87.6}{2} = 55.95 \ne 40.1 \): not a triad. (2) \( \frac{32.1 + 127.6}{2} = 79.85 \approx 79.0 \): triad. (3) \( \frac{9.0 + 40.1}{2} = 24.55 \approx 24.3 \): triad.

Example 3. Find the maximum electron capacity of the K, L and M shells.

Using \( 2n^2 \): K (n = 1) \( 2\times1^2 = 2 \); L (n = 2) \( 2\times2^2 = 8 \); M (n = 3) \( 2\times3^2 = 18 \).

Example 4. An element has electronic configuration 2,8,2. Find its atomic number, group, period and the element it resembles among N (7), Be (4), Ar (18), Cl (17).

Atomic number = 2 + 8 + 2 = 12 (magnesium). It has 2 valence electrons, so it is in group 2. It has 3 shells, so it is in period 3. Be (2,2) also has 2 valence electrons, so the element resembles beryllium.

Example 5. Deduce the valency of beryllium (Z = 4) and oxygen (Z = 8) from their electronic configurations.

Be: 2,2. It has 2 valence electrons which it can lose, so valency is 2. O: 2,6. It needs 2 more electrons to complete its octet, so valency is 2.

Example 6. Arrange O (66 pm), B (88), C (77), N (74), Be (111), Li (152) in decreasing order of atomic radius and state the trend.

Li (152) > Be (111) > B (88) > C (77) > N (74) > O (66). This is the order of period 2 from left to right, so atomic radius decreases from left to right in a period. Li is the biggest and O the smallest atom here.

Example 7. Arrange K (231 pm), Na (186), Rb (244), Cs (262), Li (152) in increasing order of atomic radius.

Li (152) < Na (186) < K (231) < Rb (244) < Cs (262). This matches group 1 from top to bottom, showing that atomic radius increases down a group. Cs is the biggest and Li the smallest.

Example 8. Among 7N, 6C, 8O, 5B, 13Al, which is the most electronegative? Explain.

B, C, N, O are in period 2 and Al is in period 3. Electronegativity increases from left to right in a period and decreases down a group. O is farthest right in period 2, so oxygen (2,6) is the most electronegative.

Important questions and answers

Q1. State the limitations of Newlands' law of octaves.

It applied only up to calcium. Two elements were placed in one box in some places (Co and Ni, Ce and La). Some dissimilar elements were placed under the same note, such as Co and Ni with the halogens, while Fe, similar to Co and Ni, was placed with O and S. There was no place for newly discovered elements, whose properties did not fit the law.

Q2. Write the merits of Mendeleev's periodic table.

Wrong atomic masses of some elements were corrected, e.g. Be from 14.09 to 9.4. Gaps were left for undiscovered elements and their properties were predicted; eka-boron, eka-aluminium and eka-silicon were later found as Sc, Ga and Ge with matching properties. Noble gases, discovered later, were fitted into a new zero group without disturbing the table.

Q3. Write the demerits of Mendeleev's periodic table.

Co and Ni have the same whole number atomic mass, so their order was ambiguous. Isotopes have different atomic masses but the same chemical properties, so their placement was a problem. The non-uniform rise in atomic mass made it impossible to predict how many elements lie between two heavy elements. Hydrogen resembles both alkali metals and halogens, so its position was uncertain.

Q4. Write a short note on the structure of the modern periodic table.

Elements are arranged in increasing atomic number. There are 7 periods and 18 groups, with lanthanide and actinide series below, making 118 places. It has four blocks: s (groups 1, 2), p (13 to 18), d (3 to 12, transition elements) and f (lanthanides, actinides). A zig-zag line in the p-block separates metals (left) from nonmetals (right), with metalloids along it.

Q5. How are isotopes placed in Mendeleev's and the modern periodic table?

In Mendeleev's table, based on atomic mass, isotopes such as Cl-35 and Cl-37 would need different places although their chemical properties are the same, which was a problem. In the modern table, based on atomic number, both have Z = 17 and so occupy the same place.

Q6. Give reason: Atomic radius decreases from left to right in a period.

Across a period, the atomic number and so the nuclear charge increase by one unit at a time, but the added electron enters the same outermost shell. The greater nuclear charge pulls the electrons closer to the nucleus, so the atomic size decreases.

Q7. Give reason: Atomic radius increases down a group.

On going down a group a new shell is added at each step, so the distance between the outermost electrons and the nucleus increases. Hence the atomic size increases in spite of the increased nuclear charge.

Q8. Give reason: Metallic character decreases from left to right in a period.

Across a period the nuclear charge increases and the atomic radius decreases, so the effective nuclear charge on the valence electrons increases. The atom holds its valence electrons more strongly and its tendency to lose them, the metallic character, decreases.

Q9. Give reason: Elements in the same group have the same valency.

Valency depends on the number of valence electrons. All elements of a group have the same number of electrons in their outermost shell, so they have the same valency.

Q10. Give reason: The third period contains only eight elements although the M shell can hold 18 electrons.

The number of elements in the first three periods is decided by the shell capacity and the octet rule. Because of the law of electron octet, only 8 electrons are filled in the outermost M shell in the third period, so it has only eight elements.

Q11. Give reason: Metallic character increases down a group.

Down a group a new shell is added, so the valence electrons are farther from the nucleus and the effective attraction on them decreases, making them easier to lose. Also, after losing them the complete octet of the penultimate shell becomes the outermost shell, giving a stable cation. So the tendency to lose electrons increases.

Q12. Why was atomic number accepted as the basis of classification instead of atomic mass?

Moseley showed in 1913 that the atomic number equals the positive charge, or number of protons, on the nucleus. It is a more fundamental property than atomic mass and also decides the number of electrons and the electronic configuration, so properties are predicted more accurately on its basis.

Exam tips

• Learn names with nationality and year: Dobereiner 1817, Newlands 1866, Mendeleev 1869 to 1872, Moseley 1913.
• Remember the eka trio with masses: eka-boron 44 (Sc), eka-aluminium 68 (Ga), eka-silicon 72 (Ge).
• For trend questions, first write electronic configurations to find group and period, then apply the trend.
• Make a small table of the four trends (radius, valency, metallic, nonmetallic) across a period and down a group.
• In 'give reason' answers mention nuclear charge, number of shells and the outermost shell explicitly.

FAQs

Why is the position of hydrogen uncertain?

Hydrogen has formula H2 like F2 and Cl2 of the halogens, but it also forms compounds like HCl, H2O, H2S similar to NaCl, Na2O, Na2S of alkali metals. So it fits both group 1 and group 17.

What are the blocks of the modern periodic table?

s-block (groups 1, 2), p-block (groups 13 to 18), d-block (groups 3 to 12) and f-block (lanthanide and actinide series).

Which group has valency zero?

Group 18, the noble gases, because their outermost shell is complete.

Is the modern periodic table complete?

Yes. It has 118 places and the formation of all 118 elements has now been established experimentally.

These notes were prepared with AI help from the Balbharati Science and Technology Part 1 textbook (2026-27) and checked against it.