BharatNotes Why this chapter matters for UPSC: This was Chapter 5 of NCERT's Class X Science in the 2020-21 edition (pp. 79-92). It is not in the current book: Reprint 2026-27 has 13 chapters and no periodic table chapter. The ideas still carry a lot of weight. The periodic table is the map behind every question on metals, non-metals, metalloids, rare earths and critical minerals, and its history is a short lesson in how science works: four attempts at classification, each fixing the faults of the last, and one bold prediction that came true.
Contemporary hook: Gallium and germanium, two of the elements Mendeléev predicted before anyone had found them, are on India's list of 30 critical minerals, which the Ministry of Mines placed before the Rajya Sabha on 24 July 2023. The Beyond-the-textbook box below traces the line from his gaps to that list.
🧠 First Principles — Read This First
Classification means finding the property that repeats. By the 1860s chemists knew dozens of elements and wanted a pattern that would let them study all of them at once. Each attempt in this chapter picked a basis (atomic mass first, atomic number later) and looked for properties that recur at regular intervals. When properties recur at regular intervals, they are periodic, and the table that shows the recurrence is a periodic table.
The modern answer is the electron arrangement. Elements in the same group have the same number of electrons in the outermost shell, so they react alike. Elements in the same period have the same number of occupied shells. Every trend in the chapter (valency, atomic size, the pull to lose or gain electrons, metallic character, the nature of oxides) follows from two things: how many shells an atom has, and how strongly its nucleus pulls on the outermost electrons.
PART 1 — Quick Reference
Table 1: Four attempts at classification
| Attempt | Basis | The idea | Why it fell short |
|---|---|---|---|
| Döbereiner's triads (NCERT: 1817) | Atomic mass within a group of three similar elements | In a triad written in order of increasing atomic mass, the middle element's atomic mass is roughly the average of the other two (Li 6.9, Na 23.0, K 39.0) | He "could identify only three triads from the elements known at that time", so the system "was not found to be useful" |
| Newlands' Law of Octaves (NCERT: 1866) | Increasing atomic mass, from hydrogen to thorium, "the 56th element" | "Every eighth element had properties similar to that of the first", like the eighth note in a musical scale | Worked only up to calcium; assumed only 56 elements existed; put two elements in one slot and unlike elements under the same note; made irrelevant by the noble gases |
| Mendeléev's Periodic Table (NCERT prints the 1872 table) | Increasing atomic mass plus similarity of chemical properties (formulae of hydrides and oxides) | "The properties of elements are the periodic function of their atomic masses" | No fixed place for hydrogen; isotopes did not fit; atomic masses do not rise in a regular way |
| Modern Periodic Table (Moseley, 1913) | Atomic number (Z) | "Properties of elements are a periodic function of their atomic number" | Hydrogen's position is still an anomaly (group 1 or group 17) |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, sections 5.1-5.3 (pp. 79-87).
Table 2: Mendeléev's table, what it achieved and where it failed
| Achievements (5.2.1) | Limitations (5.2.2 and 5.3) |
|---|---|
| Kept similar elements together even when that meant inverting the mass order: cobalt (58.9) placed before nickel (58.7) | Hydrogen resembles both the alkali metals and the halogens, so "no fixed position can be given to hydrogen". NCERT calls this "the first limitation" |
| Left gaps for undiscovered elements and predicted their properties, naming them with the Sanskrit numeral eka (one): eka-boron, eka-aluminium, eka-silicon | Isotopes, found later, have similar chemical properties but different atomic masses, so a mass-based table cannot place them |
| Scandium, gallium and germanium, discovered later, matched those predictions | Atomic masses do not increase regularly from one element to the next, so the table could not say how many elements lay between two known ones |
| The noble gases, found late, fitted into "a new group without disturbing the existing order" |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, sections 5.2.1-5.2.2 and 5.3 (pp. 83-85).
Table 3: Prediction and discovery (NCERT Table 5.5)
| Property | Eka-aluminium (predicted) | Gallium (discovered) |
|---|---|---|
| Atomic mass | 68 | 69.7 |
| Formula of oxide | E₂O₃ | Ga₂O₃ |
| Formula of chloride | ECl₃ | GaCl₃ |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, Table 5.5 (p. 84).
Table 4: The Modern Periodic Table at a glance
| Feature | What NCERT says |
|---|---|
| Layout | 18 vertical columns (groups) and 7 horizontal rows (periods) |
| A group | Same number of valence electrons: groups "signify an identical outer-shell electronic configuration". The number of shells increases down a group |
| A period | Same number of occupied shells; valence electrons increase by one from left to right. "Each period marks a new electronic shell getting filled" |
| Shell capacity | 2n²: K shell 2 × 1² = 2, so the first period has 2 elements; L shell 2 × 2² = 8, so the second period has 8 |
| Elements per period | 2, 8, 8, 18, 18, 32, 32 (first to seventh) |
| Hydrogen | Can be placed "either in group 1 or group 17 in the first period" |
| Elements known | 118, of which "only 94 are naturally occurring" |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, pp. 79 and 87-88.
The period sizes add up to 2 + 8 + 8 + 18 + 18 + 32 + 32 = 118, the number of elements NCERT gives as known.
Table 5: Trends in the Modern Periodic Table
| Property | Left to right across a period | Down a group |
|---|---|---|
| Valence electrons | Increase by one at each step | Stay the same |
| Atomic size (radius) | Decreases: a larger nuclear charge pulls the electrons closer | Increases: new shells are added |
| Tendency to lose electrons (metallic character) | Decreases | Increases |
| Tendency to gain electrons (non-metallic character) | Increases | Decreases |
| Where the elements sit | Metals on the left, metalloids in the middle, non-metals on the right | Non-metals lie "on the right-hand side of the Periodic Table towards the top" |
| Oxides | Metal oxides basic, non-metal oxides acidic "in general" |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, section 5.3.2 (pp. 88-90). NCERT states the size and metallic-character trends and leaves the electron-gaining trend as Activity 5.11; the row above follows from its own reasoning.
Table 6: NCERT's atomic radii (picometres)
| Period II (Activity 5.7) | Li 152 | Be 111 | B 88 | C 77 | N 74 | O 66 |
|---|---|---|---|---|---|---|
| Group 1 (Activity 5.8) | Li 152 | Na 186 | K 231 | Rb 244 | Cs 262 |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, Activities 5.7 and 5.8 (pp. 88-89). NCERT prints the elements out of order and asks the reader to arrange them; the table shows the answer. Hydrogen's atomic radius is 37 pm (1 pm = 10⁻¹² m).
Table 7: NCERT lines to read with care
| NCERT says | The record |
|---|---|
| "In 1866, John Newlands ... arranged the then known elements" (p. 80) | Newlands' own letter to Chemical News, dated 8 August 1865, already proposes "to provisionally term the Law of Octaves" (quoted in the open textbook cited below), and Purdue University's chemistry history page dates his step to 1865. Know both years. |
| "In the year 1817, Johann Wolfgang Döbereiner" grouped elements into triads, and "could identify only three triads" (p. 79-80) | Purdue's history page gives a different account: "In 1829 Johann Wolfgang Dobereiner discovered the existence of families of elements", and its table shows five triads (adding S, Se, Te and Mn, Cr, Fe). That is one source against NCERT; it is shown here so that a different year in another book does not surprise you. |
| "Mendeléev's Periodic Table was published in a German journal in 1872" (p. 83) | 1872 is the date of the version NCERT prints. Mendeléev first published his periodic table in 1869, the year the United Nations used for the International Year of the Periodic Table (2019, the 150th anniversary). Asked when Mendeléev gave his table, answer 1869. |
| Mendeléev arranged elements by "their fundamental property, the atomic mass" (p. 82); then "the atomic number ... is a more fundamental property than its atomic mass" (p. 85) | Both lines are NCERT's. The second supersedes the first: atomic mass was the basis in Mendeléev's time, atomic number after Moseley (1913). |
| The cobalt-before-nickel inversion is listed among Mendeléev's achievements (5.2.1) | It was a choice that kept similar elements together, which is why NCERT counts it in his favour. Under his own law (properties follow atomic mass) it is still an anomaly, and Activity 5.3 asks how the Modern Periodic Table resolved it: by atomic number, cobalt (27) comes before nickel (28). |
| 2n² gives 8 for the L shell and 18 for the M shell, yet the third period has only 8 elements: "The reason for this you will study in higher classes" (p. 88) | Class IX gives part of the answer: "the outermost shell of an atom can accommodate a maximum of 8 electrons" (Bohr-Bury scheme). The full reason is left to higher classes. |
Source: NCERT, Science Class X, 2020-21 edition, ch. 5, pp. 79-88; NCERT, Science Class IX, ch. 4 "Structure of the Atom" (Reprint 2025-26); General Chemistry: Principles, Patterns, and Applications (v1.0), section 7.1 "The History of the Periodic Table" (Saylor Academy open textbook); Purdue University, Chemical Education, "John Newlands" and "Johann Wolfgang Dobereiner"; IUPAC, 28 December 2017. Atomic numbers of cobalt (27) and nickel (28): PubChem periodic table.
PART 2 — Concepts & Narrative
Making order out of chaos: early attempts (5.1)
NCERT opens with a shop. Soaps are kept in one place and biscuits in another, and bathing soaps are stacked apart from washing soaps. Chemists wanted the same kind of order for the elements, and the earliest attempt sorted them simply into metals and non-metals. As more elements and properties became known, finer schemes were tried.
Döbereiner's triads (5.1.1). In NCERT's account, in 1817 the German chemist Johann Wolfgang Döbereiner grouped elements with similar properties in threes. Written in order of increasing atomic mass, the middle element's mass came out close to the average of the other two. Lithium (6.9), sodium (23.0) and potassium (39.0) show it: the average of 6.9 and 39.0 is about 23. In NCERT's Table 5.1, calcium, strontium and barium (40.1, 87.6, 137.3) and chlorine, bromine and iodine (35.5, 79.9, 126.9) form triads; nitrogen, phosphorus and arsenic (14.0, 31.0, 74.9) do not. NCERT's Table 5.2 lists three triads: Li, Na, K; Ca, Sr, Ba; Cl, Br, I. NCERT's biographical box adds that Döbereiner studied pharmacy at Münchberg and chemistry at Strasbourg, became a professor at the University of Jena, and made "the first observations on platinum as a catalyst".
Triad: a group of three elements with similar properties in which, arranged by increasing atomic mass, the middle element's atomic mass is roughly the average of the other two. NCERT's verdict: only three triads could be found among the elements then known, so the scheme "was not found to be useful".
Newlands' Law of Octaves (5.1.2). Encouraged by Döbereiner, other chemists tried to link properties with atomic mass. In NCERT's account, in 1866 John Newlands, an English scientist, arranged the known elements from hydrogen, the lightest, to thorium, "the 56th element". Every eighth element resembled the first, as every eighth note in a musical scale repeats the first (sa, re, ga, ma, pa, da, ni, or do, re, mi, fa, so, la, ti). Lithium and sodium resemble each other, and sodium is the eighth element after lithium; beryllium and magnesium also resemble each other.
The scheme broke down quickly. It held only up to calcium. Newlands assumed only 56 elements existed and that no more would be found, but new elements kept appearing that did not fit. To make his table work he placed two elements in one slot: cobalt and nickel share a slot (as do cerium and lanthanum), and they sit in the same column as fluorine, chlorine and bromine, whose properties are very different. Iron, which resembles cobalt and nickel, lies far away from them. When the noble gases were discovered, the Law of Octaves "became irrelevant". In NCERT's summary, it "worked well with lighter elements only".
Law of Octaves: Newlands' observation that, with elements arranged by increasing atomic mass, every eighth element has properties similar to the first. It works for the lighter elements only.
Mendeléev's Periodic Table (5.2)
Dmitri Ivanovich Mendeléev (1834-1907), a Russian chemist born in Tobolsk in Western Siberia, gets the main credit for classifying the elements. When he started, 63 elements were known. He looked at how their atomic masses related to their physical and chemical properties, and among chemical properties he concentrated on the compounds each element forms with hydrogen and oxygen. He chose these two "as they are very reactive and formed compounds with most elements". He wrote the properties of each element on a card, sorted the 63 cards by similar properties and pinned them on a wall. Most elements fell into place in order of increasing atomic mass, and elements with similar properties recurred at intervals.
Mendeléev's Periodic Law: "the properties of elements are the periodic function of their atomic masses". His table has vertical columns (groups) and horizontal rows (periods). At the head of each column he wrote the formulae of the oxide and hydride, with R standing for any element of the group: the hydride of carbon, CH₄, appears as RH₄, and its oxide CO₂ as RO₂.
Why hydrides and oxides were a good basis. NCERT asks the reader to work this out after the Modern Periodic Table has been explained. The formula of an element's oxide or hydride depends on how many bonds it forms, and that depends on its valence electrons. So by grouping elements with the same formulae (RO₂, RH₄ and so on), Mendeléev was in effect grouping elements with the same outer-shell electrons, decades before anyone knew about electrons in shells. That is why his groups survive in the modern table. The same logic answers NCERT's in-text question on predicting oxide formulae: potassium K₂O, carbon CO₂, aluminium Al₂O₃, silicon SiO₂, barium BaO.
Achievements (5.2.1). Mendeléev let chemistry overrule arithmetic. In a few places he put an element of slightly greater atomic mass before one of slightly lower mass so that similar elements stayed together: cobalt (58.9) before nickel (58.7). He also left gaps. Instead of treating them as defects, he predicted elements not yet discovered and named each by adding the Sanskrit numeral eka (one) to the element above the gap: eka-boron, eka-aluminium and eka-silicon. Scandium, gallium and germanium, discovered later, had the predicted properties (Table 3 compares eka-aluminium with gallium). NCERT calls this "convincing evidence for both the correctness and usefulness" of the table; the success of the predictions led chemists to accept the table and to recognise Mendeléev as the originator of the concept. Finally, the noble gases (helium, neon, argon), discovered late because they are very inert and present in extremely low concentrations in the atmosphere, could be placed in a new group without disturbing the existing order.
Eka: the Sanskrit numeral for "one", which Mendeléev added to the name of the element above a gap to name the missing element below it. Eka-aluminium is the element one place below aluminium.
Limitations (5.2.2). Hydrogen fits nowhere for sure. Its electronic configuration resembles the alkali metals', and like them it combines with halogens, oxygen and sulphur to give compounds with similar formulae (HCl and NaCl, H₂O and Na₂O, H₂S and Na₂S). But like the halogens it exists as diatomic molecules and forms covalent compounds with metals and non-metals. NCERT: "no fixed position can be given to hydrogen", and this was "the first limitation". Isotopes, discovered long after Mendeléev's table, were a second problem: isotopes of an element have similar chemical properties but different atomic masses. The third: atomic masses do not rise in a regular way from one element to the next, so a mass-based table cannot tell how many elements remain to be found between two known ones, especially among the heavier elements.
The Modern Periodic Table (5.3)
In 1913 Henry Moseley showed that the atomic number (Z) of an element, the number of protons in its nucleus, is a more fundamental property than its atomic mass. The Periodic Law was restated on that basis.
Modern Periodic Law: "Properties of elements are a periodic function of their atomic number." Atomic number rises by exactly one from each element to the next, so there are no hidden gaps of unknown size.
How atomic number cleared up Mendeléev's problems. NCERT says the Modern Periodic Table "takes care of three limitations" of Mendeléev's, and Activity 5.3 points to them. Cobalt and nickel: by atomic number cobalt (27) comes before nickel (28), so the order no longer needs inverting. Isotopes: chlorine-35 and chlorine-37 both have 17 protons, so they take the same place (Activity 5.2). Regular steps: no element can have atomic number 1.5, so nothing can sit between hydrogen and helium, and the number of elements between any two is fixed. Hydrogen's position remains the one anomaly: it can go in group 1 or group 17.
Position of elements (5.3.1). The modern table has 18 groups and 7 periods. The first three elements of group 1 (lithium, sodium, potassium) each have one valence electron, and fluorine and chlorine in group 17 each have seven. So the elements of a group share an outer-shell configuration, and the number of shells grows down the group. Across the second period (Li, Be, B, C, N, O, F, Ne) the number of shells stays at two while the valence electrons rise by one at each step. Na, Mg, Al, Si, P, S, Cl and Ar are in the third period because their electrons fill the K, L and M shells.
The number of elements in a period follows from how electrons fill the shells. The 2n² rule gives 2 for the K shell (first period, 2 elements) and 8 for the L shell (second period, 8 elements). The third to seventh periods hold 8, 18, 18, 32 and 32 elements. NCERT leaves the reason for these numbers to higher classes (Table 7).
Trends in the Modern Periodic Table (5.3.2)
Valency. Valency is decided by the number of valence electrons. Magnesium (12) is 2, 8, 2 and has valency 2. Sulphur (16) is 2, 8, 6; it needs two more electrons to complete its octet, so its valency is also 2. Across the third period, valency rises from 1 (sodium) to 4 (silicon) and then falls back through 3, 2 and 1 to 0 for argon, whose shell is full. Down a group the valency stays the same, because the valence electrons do.
Why atoms shrink across a period. Moving right, each element has one more proton and one more electron, but the new electron goes into the same shell. The larger nuclear charge pulls all the electrons of that shell closer, so the atom gets smaller: lithium 152 pm to oxygen 66 pm in the second period. Moving down a group, a new shell is added at each step. The outer electrons are farther from the nucleus, so the atom grows (lithium 152 pm to caesium 262 pm) "in spite of the increase in nuclear charge".
Atomic size. Atomic size means the radius of an atom: the distance from the centre of the nucleus to the outermost shell of an isolated atom. Hydrogen's radius is 37 pm (picometres). Size decreases across a period and increases down a group (Table 6).
Metallic and non-metallic properties. In the third period, sodium and magnesium on the left are metals, sulphur and chlorine on the right are non-metals, and silicon in the middle is a semi-metal or metalloid because it shows some properties of both. In the Modern Periodic Table "a zig-zag line separates metals from non-metals", and the borderline elements are the metalloids.
Metalloid (semi-metal): an element intermediate in properties between metals and non-metals. NCERT's list: "boron, silicon, germanium, arsenic, antimony, tellurium and polonium".
Metals tend to lose electrons when forming bonds; they are electropositive. Across a period the effective nuclear charge acting on the valence electrons rises, so "the tendency to lose electrons will decrease". Down a group the outer electrons are farther from the nucleus, the effective pull on them falls, and they are lost more easily. Hence "metallic character decreases across a period and increases down a group". Non-metals are electronegative: they tend to gain electrons. The same reasoning runs in reverse for them, which is why NCERT places non-metals on the right of the table, towards the top. The trends also predict the oxides: metal oxides are basic and non-metal oxides acidic, in general.
Effective nuclear charge: the net pull of the nucleus felt by the valence electrons. It rises across a period (more protons, same shell) and falls in its effect down a group (outer electrons farther away). It is the single idea behind the size, metallic-character and electron-gaining trends.
What happened to this chapter?
This chapter was printed as Chapter 5 (pp. 79-92) in the 2020-21 edition of NCERT's Class X Science. It is absent from the current book: Reprint 2026-27 lists 13 chapters, and its imprint records a "Revised Edition October 2022". It was dropped in NCERT's 2022 rationalisation of textbooks; the front matter explains the rationalisation in general terms and gives no reason chapter by chapter.
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021); NCERT, Science Class X, Reprint 2026-27, front matter (Contents and imprint).
Beyond the textbook: from Mendeléev's gaps to India's critical minerals
The gaps. Mendeléev's three famous gaps were filled by scandium, gallium and germanium. On the dates, a standard open chemistry textbook gives gallium "(Discovered in 1875)" and germanium "(Discovered in 1886)", both after his first table of 1869.
The list. On 24 July 2023 the Ministry of Mines told the Rajya Sabha that "Government has released a list of 30 critical minerals for India". Gallium and germanium are on it. So are four of NCERT's seven metalloids (silicon, germanium, antimony and tellurium); boron, arsenic and polonium are not. "REE", the rare-earth elements, is one entry on the list.
The rare earths. The third predicted element, scandium (eka-boron), counts as a rare-earth element. The Indian Bureau of Mines defines REEs as "a group of 17 elements, namely, yttrium (Y), scandium (Sc) and lanthanides (15 elements in the periodic table with atomic numbers 57 to 71".
The anniversary. On 20 December 2017 the UN General Assembly proclaimed 2019 the International Year of the Periodic Table of Chemical Elements, to "coincide with the 150th anniversary of the discovery of the Periodic System by Dmitry Mendeleev in 1869" (IUPAC).
Source: PIB (Ministry of Mines), "Thirty Critical Minerals List Released", 24 July 2023, Release ID 1942027; Indian Bureau of Mines, Indian Minerals Yearbook 2024 (63rd edition), Rare Earths; General Chemistry: Principles, Patterns, and Applications (v1.0), section 7.1 (Saylor Academy open textbook); IUPAC, "The United Nations Proclaims the International Year of the Periodic Table of Chemical Elements", 28 December 2017.
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — Döbereiner, Newlands and Mendeléev in sequence; eka-elements and gallium; Moseley and atomic number; groups and periods; trends in size and metallic character; NCERT's list of metalloids; the position of hydrogen.
- GS3 (Science and technology) — elements as materials: silicon and germanium as semiconductors' raw materials, gallium, rare earths.
- GS3 (Economy and resources) — critical minerals: India's list of 30 (stated in the Rajya Sabha, 24 July 2023) and the rare-earth elements.
- Essay — prediction as the test of a scientific idea; classification as a way of making order out of chaos.
Past questions on these themes: Prelims 2026 asked about the rare-earth elements and India's National Critical Mineral Mission.
Frames for Mains Answers
1. A theory proves itself by prediction. Mendeléev's gaps and the discovery of gallium (predicted atomic mass 68, found 69.7; oxide E₂O₃ as Ga₂O₃) persuaded chemists where arrangement alone had not.
2. Changing the basis can remove the anomalies. Mass-based tables could not place isotopes, cobalt and nickel, or count the missing elements. Moseley's atomic number fixed all three; hydrogen remains the honest exception.
3. The periodic table as a resource map. Gallium, germanium, silicon, antimony, tellurium and the rare earths are on India's list of 30 critical minerals. A position in the table explains an element's chemistry; its supply and its uses explain why a government lists it.
Exam Strategy
Prelims fact-traps:
- 118 elements known, 94 naturally occurring (NCERT).
- Triads: middle atomic mass ≈ average of the other two; Li 6.9, Na 23.0, K 39.0. N, P, As are not a triad.
- Newlands: hydrogen to thorium, 56 elements, every eighth element alike; holds only up to calcium. NCERT's year is 1866; his own letter naming the law is dated 8 August 1865.
- Mendeléev: 63 elements known; basis = atomic mass plus formulae of hydrides and oxides; Periodic Law on atomic mass. First table 1869; NCERT prints the 1872 version.
- Eka = Sanskrit for one. Eka-boron = scandium, eka-aluminium = gallium, eka-silicon = germanium.
- Cobalt (58.9) was placed before nickel (58.7). By atomic number, cobalt is 27 and nickel 28.
- Moseley, 1913: atomic number is the basis of the Modern Periodic Law.
- 18 groups, 7 periods; periods hold 2, 8, 8, 18, 18, 32, 32 elements.
- Group = same valence electrons; period = same number of shells.
- Size: decreases across, increases down. Metallic character: decreases across, increases down.
- Metalloids (NCERT): B, Si, Ge, As, Sb, Te, Po.
- Hydrogen: group 1 or group 17; no fixed position.
- Metal oxides basic, non-metal oxides acidic (in general).
Mains: use the four attempts as a ready example of how science corrects itself, and the gallium prediction as the evidence. When you add current material (critical minerals, rare earths), name the list and its date.
Practice Questions
Questions 1-2 are the NCERT exercise MCQs. Practice (UPSC-pattern, not past papers): questions 3-10.
1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of the Periodic Table?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Answer: (c). Across a period the effective nuclear charge rises, so atoms lose electrons less easily (NCERT Answers, 2020-21 edition, p. 281).
2. Element X forms a chloride with the formula XCl₂, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na
(b) Mg
(c) Al
(d) Si
Answer: (b). XCl₂ means valency 2, as for magnesium (MgCl₂); a high-melting solid chloride suggests an ionic compound of a metal (NCERT Answers, 2020-21 edition, p. 281).
3. Which of the following are listed by NCERT as metalloids?
1. Boron
2. Germanium
3. Bismuth
4. Polonium
Select the correct answer:
(a) 1 and 2 only
(b) 1, 2 and 4 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4
Answer: (b). NCERT's list is boron, silicon, germanium, arsenic, antimony, tellurium and polonium. Bismuth is not on it.
4. Consider the following statements about Newlands' Law of Octaves:
1. It was found to apply only up to calcium.
2. Newlands assumed that only 56 elements existed.
3. It placed the noble gases in a separate column.
Which of the statements given above are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (b). The discovery of the noble gases made the Law of Octaves irrelevant; they had no place in it.
5. Mendeléev used which of the following as the basis of his Periodic Table?
1. Atomic mass
2. Formulae of the hydrides and oxides of elements
3. Atomic number
Select the correct answer:
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (b). Atomic number became the basis only after Moseley (1913).
6. Mendeléev predicted an element with atomic mass 68, an oxide E₂O₃ and a chloride ECl₃. The element found later was
(a) scandium
(b) gallium
(c) germanium
(d) indium
Answer: (b). Eka-aluminium became gallium (atomic mass 69.7, Ga₂O₃, GaCl₃).
7. Using NCERT's atomic radii, which of the following orders is correct?
(a) O < N < C < Be < Li
(b) Li < Be < C < N < O
(c) Na < Li < K < Rb < Cs
(d) Cs < Rb < K < Na < Li
Answer: (a). O 66, N 74, C 77, Be 111, Li 152 pm. In group 1 the order runs the other way: Li 152 < Na 186 < K 231 < Rb 244 < Cs 262.
8. With reference to rare-earth elements as defined by the Indian Bureau of Mines, consider the following statements:
1. They are a group of 17 elements.
2. They include scandium and yttrium.
3. All of them are lanthanides.
Which of the statements given above are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (b). The 17 are scandium, yttrium and the 15 lanthanides (atomic numbers 57 to 71).
9. Which of the following are on India's list of 30 critical minerals, as given by the Ministry of Mines in the Rajya Sabha on 24 July 2023?
1. Gallium
2. Germanium
3. Arsenic
4. Tellurium
Select the correct answer:
(a) 1 and 2 only
(b) 1, 2 and 4 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4
Answer: (b). Arsenic is not on the list.
10. The gaps in Mendeléev's table turned out to be its strongest evidence. Explain, and say why atomic number replaced atomic mass as the basis of classification. (150 words)
NCERT exercises (where the answers are on this page): 3 (a) neon, (b) magnesium (2, 8, 2), (c) silicon (2, 8, 4), (d) boron (2, 3), (e) carbon (2, 4); 4 (a) three valence electrons, (b) seven valence electrons (valency 1); 5 atomic number 17 (chlorine), similar to F(9); 6 A is a non-metal, C is less reactive than A (the pull to gain electrons falls down a group), C is smaller than B (size falls across a period), A forms an anion; 7 N is 2, 5 and P is 2, 8, 5, and nitrogen is more electronegative because its valence electrons are closer to the nucleus; 8 "Position of elements"; 9 12 (magnesium) and 38 (strontium), which are in calcium's group; 10 Tables 1, 2 and 4.
📦 Revision Capsule
Hard Facts
- 118 elements known; 94 natural (NCERT, 2020-21).
- Döbereiner's triads: Li, Na, K; Ca, Sr, Ba; Cl, Br, I (NCERT: 1817).
- Newlands' octaves: H to Th, 56 elements, valid up to Ca (NCERT: 1866; his letter dated 8 August 1865).
- Mendeléev: 63 elements; Periodic Law on atomic mass; first table 1869; NCERT prints the 1872 version.
- Eka-boron = Sc; eka-aluminium = Ga (68 predicted, 69.7 found); eka-silicon = Ge.
- Moseley, 1913: Modern Periodic Law on atomic number.
- 18 groups, 7 periods; 2, 8, 8, 18, 18, 32, 32 elements per period.
- Metalloids (NCERT): B, Si, Ge, As, Sb, Te, Po.
- International Year of the Periodic Table: 2019 (UNGA, 20 December 2017).
Core Concepts
- A group shares an outer-shell configuration; a period shares a number of shells.
- Across a period: more nuclear charge, same shell, smaller atoms, less metallic.
- Down a group: more shells, bigger atoms, more metallic.
- Atomic number removed three of Mendeléev's problems (Co/Ni order, isotopes, unknown gaps); hydrogen's place is still open.
Confused Pairs
- Mendeléev's Periodic Law (atomic mass) vs Modern Periodic Law (atomic number).
- Group (column, same valence electrons) vs period (row, same shells).
- Metalloid (intermediate) vs non-metal.
- 1866 (NCERT, Newlands) vs 1865 (his letter).
- 1869 (Mendeléev's first table) vs 1872 (the version NCERT prints).
- Eka-aluminium (gallium) vs eka-silicon (germanium) vs eka-boron (scandium).
Data Points
- Period II radii (pm): Li 152, Be 111, B 88, C 77, N 74, O 66.
- Group 1 radii (pm): Li 152, Na 186, K 231, Rb 244, Cs 262.
- Hydrogen radius 37 pm; 1 pm = 10⁻¹² m.
- Triads: Ca 40.1, Sr 87.6, Ba 137.3; Cl 35.5, Br 79.9, I 126.9.
- Cobalt 58.9 vs nickel 58.7 (atomic mass); 27 vs 28 (atomic number).
- Rare-earth elements: 17 (Sc, Y and 15 lanthanides, Z = 57-71).
PYQ Pattern
- Prelims: prelims-2026-gs1-060 (rare-earth elements and the National Critical Mineral Mission).
Sources
- NCERT, Science, Textbook for Class X, 2020-21 edition, ch. 5 "Periodic Classification of Elements" (pp. 79-92) and Answers (p. 281), whole-book zip as archived on 9 October 2021 — Wayback Machine.
- NCERT, Science, Textbook for Class X, Reprint 2026-27, front matter (Contents and imprint) — ncert.nic.in PDF.
- NCERT, Science, Textbook for Class IX, ch. 4 "Structure of the Atom", Reprint 2025-26 — ncert.nic.in PDF.
- General Chemistry: Principles, Patterns, and Applications (v1.0), section 7.1 "The History of the Periodic Table", Saylor Academy open textbook — saylordotorg.github.io.
- Purdue University, Chemical Education, "John Newlands" — chemed.chem.purdue.edu.
- Purdue University, Chemical Education, "Johann Wolfgang Dobereiner" — chemed.chem.purdue.edu.
- IUPAC, "The United Nations Proclaims the International Year of the Periodic Table of Chemical Elements", 28 December 2017 — iupac.org.
- PIB (Ministry of Mines), "Thirty Critical Minerals List Released", 24 July 2023, Release ID 1942027 — pib.gov.in.
- PubChem (US National Library of Medicine), Periodic Table of Elements — pubchem.ncbi.nlm.nih.gov.
- Indian Bureau of Mines, Indian Minerals Yearbook 2024 (63rd edition) — ibm.gov.in PDF.
