Chapter 2 — Periodic Classification of Elements
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Can You Recall?
- What are the types of matter?
- What are the types of elements?
- What are the smallest particles of matter called?
- What is the difference between molecules of elements and compounds?
What You Will Learn
Periodic classification helps organize the 118 known elements to reveal patterns in their properties.
1) Elements and Their Classification
Initially, elements were broadly classified as metals and nonmetals. With growing knowledge, a third category, metalloids, was identified. As more elements were discovered (about 30 by 1800, 118 today), scientists searched for patterns in properties to build better classifications.
2) Dobereiner’s Triads (1817)
Dobereiner grouped elements into sets of three (triads) having similar chemical properties and arranged them in increasing atomic mass. He observed:
Examples of Triads
| Triad | Element-1 (a) | Element-2 (b) | Element-3 (c) | Mean \( \dfrac{a+c}{2} \) |
|---|---|---|---|---|
| Li–Na–K | Li 6.9 | Na 23.0 | K 39.1 | \(\dfrac{6.9+39.1}{2}=23.0\) |
| Ca–Sr–Ba | Ca 40.1 | Sr 87.6 | Ba 137.3 | \(\dfrac{40.1+137.3}{2}\approx 88.7\) (close to Sr 87.6) |
| Cl–Br–I | Cl 35.5 | Br 79.9 | I 126.9 | \(\dfrac{35.5+126.9}{2}\approx 81.2\) (close to Br 79.9) |
Limitations
- Not all known elements could be grouped into triads.
- Worked only for a few sets with approximate masses.
3) Newlands’ Law of Octaves (1866)
Newlands arranged elements (H to Th) in increasing atomic mass and found that every eighth element had properties similar to the first—analogous to musical octaves.
Merits
- Early attempt to correlate properties with atomic mass.
- Correctly grouped some similar elements (e.g., Li–Na–K; Be–Mg).
Limitations
- Applicable reliably only up to Calcium.
- Forced fit into 56 slots: placed two elements in one box (e.g., Co & Ni; Ce & La).
- Grouped dissimilar elements together; couldn’t accommodate new discoveries.
4) Mendeleev’s Periodic Table (1869–72)
Mendeleev arranged 63 known elements in increasing order of atomic mass and aligned them so that elements with similar chemical and physical properties fell into the same groups (columns). He used properties such as formulae of oxides/hydrides (e.g., \( \mathrm{R_2O, RO, R_2O_3, RO_2, R_2O_5, RO_3, R_2O_7, RO_4} \) ), melting/boiling points, and densities.
Merits
- Corrections: Revised atomic masses (e.g., Be from 14.09 to 9.4) for better placement.
- Prediction: Left gaps for undiscovered elements (eka-B, eka-Al, eka-Si) and predicted their properties. Later discovered as Sc, Ga, Ge—close match to predictions.
- Noble Gases: On discovery, he created a zero group without disturbing the table—noble gases fit naturally.
Demerits
- Co/Ni anomaly: Same whole-number mass led to sequence ambiguity.
- Isotopes: Same properties but different masses—no clear place.
- Non-uniform mass increase: Couldn’t predict undiscovered element counts between heavy elements.
- Hydrogen’s position: Shows traits of both Group 1 and Group 17; placement unclear.
Prediction vs Reality: Gallium (eka-Al)
| Property | Predicted (E) | Actual (Ga) |
|---|---|---|
| Atomic mass | 68 | 69.7 |
| Density (g/cm\(^{3}\)) | 5.9 | 5.94 |
| Melting point | Low | 30.2\(^{\circ}\)C |
| Chloride | \(\mathrm{ECl_3}\) | \(\mathrm{GaCl_3}\) |
| Oxide | \(\mathrm{E_2O_3}\) (amphoteric) | \(\mathrm{Ga_2O_3}\) (amphoteric) |
5) Modern Periodic Law (Moseley, 1913)
Experiments with X-rays showed that atomic number \(Z\) equals the positive charge on the nucleus (number of protons) and is more fundamental than atomic mass.
6) Modern Periodic Table (Long Form)
- Rows (Periods): 7; Columns (Groups): 18.
- Two detached rows at bottom: Lanthanides and Actinides.
- Blocks: s-block (Groups 1–2), p-block (13–18), d-block (3–12, transition), f-block (lanthanides & actinides).
- Zig-zag line: Separates metals (left) and nonmetals (right); metalloids lie along the border.
- Hydrogen: Still special—shares features with both Group 1 and Group 17.
Groups & Electronic Configuration
Elements in the same group share the same number of valence electrons (similar outer configuration), explaining their similar properties. Down a group, one electron shell is added each step.
Example: Group 1 (alkali metals) each have 1 valence electron; Group 17 (halogens) have 7.
Periods & Electronic Configuration
Elements across a period have the same number of occupied shells, but valence electrons increase by one moving left → right as atomic number increases.
Period 2: Li, Be, B, C, N, O, F, Ne all have 2 shells (K, L) but different valence electrons.
Electron Capacity of Shells
| Shell | Symbol | n | Max electrons \(2n^{2}\) |
|---|---|---|---|
| 1st | K | 1 | 2 |
| 2nd | L | 2 | 8 |
| 3rd | M | 3 | 18 |
| 4th | N | 4 | 32 |
7) Periodic Trends (Valency, Atomic Size, Metallic/Nonmetallic Character)
Valency
- Across a period: Typically increases from 1 to 4 and then decreases to 0 (noble gas), based on tendency to reach octet.
- Down a group: Generally remains the same (same number of valence electrons).
Atomic Size (Atomic Radius)
Distance from nucleus to the outermost shell; commonly expressed in picometers (pm), where \(1~\text{pm}=10^{-12}\,\text{m}\).
- Across a period (→): radius decreases due to increasing nuclear charge pulling electrons closer while added electrons enter the same shell.
- Down a group (↓): radius increases as new shells are added, despite higher nuclear charge.
Metallic vs Nonmetallic Character
Metallic (Electropositive) Character
Tendency to lose valence electrons and form cations; increases when effective nuclear attraction on valence electrons is low.
- Across a period: decreases (nuclear charge ↑, atomic size ↓).
- Down a group: increases (more shells, valence electrons farther, easier to lose).
Nonmetallic (Electronegative) Character
Tendency to gain electrons and form anions; closely related to electronegativity.
- Across a period: increases (effective nuclear charge ↑).
- Down a group: decreases (larger size, shielding ↑).
Zig-Zag Boundary (Metalloids)
A diagonal boundary in the p-block separates metals (left) and nonmetals (right). Elements along the border (e.g., B, Si, Ge, As, Sb, Te, Po) are metalloids with intermediate properties.
8) Gradations within Families
Halogens (Group 17): Physical State Down the Group
- \(\mathrm{F_2}\), \(\mathrm{Cl_2}\): gases
- \(\mathrm{Br_2}\): liquid
- \(\mathrm{I_2}\): solid
Alkaline Earth Metals (Group 2): Reaction with Water
Reactivity increases down the group: Be (no reaction with water) < Mg (steam) < Ca, Sr, Ba (cold water).
9) Units & Notation (Clean Typesetting)
Examples with clear division and exponents:
Tip: When you want a full-size fraction inside a sentence, use \dfrac{…}{…}. Use \tfrac{…}{…} for a compact one.
Chapter Wrap-Up
From Dobereiner’s triads and Newlands’ octaves to Mendeleev’s atomic-mass-based table and finally the modern atomic-number-based periodic table, our classification of elements has evolved to reveal deep periodicity. The long form table explains group similarities, period variations, and key trends such as valency, atomic size, and metallic/nonmetallic character—forming a powerful map for understanding chemical behavior.
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Chapter 2 — Exercise Answers
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Q1) Rearrange Columns (Match the pairs)
| Column 1 | Correct Match (Column 2/3) | Reason / Hint |
|---|---|---|
| i. Triad | c. Average of the first and the third atomic mass | Dobereiner’s mean: \( \ddfrac{a+c}{2}\approx b \) |
| ii. Octave | d. Properties of the eighth element similar to the first | Newlands’ law of octaves |
| iii. Atomic number | e. Positive charge on the nucleus | Moseley: \(Z\) = nuclear charge (protons) |
| iv. Period | f. Sequential change in molecular formulae | Properties vary progressively across a period |
| v. Nucleus | b. Concentrated mass and positive charge | Rutherford’s nuclear model |
| vi. Electron | a. Lightest and negatively charged particle | Discovered by J. J. Thomson |
Q2) Choose the correct option & rewrite
a) Outermost electrons in alkali metals
(i) 1 — Alkali metals (Group 1) have one valence electron.
b) Alkaline earth metals (valency 2) are in…
(i) Group 2 — Group number equals valence electron count for s-block.
c) If \( \mathrm{XCl} \) is ionic & high m.p., which is in the same group as X?
(i) Na — \( \mathrm{XCl} \) implies X has valency 1 (Group 1 like Na).
d) Nonmetals are mainly found in which block?
(ii) p-block — Nonmetals (and metalloids) dominate the p-block.
Q3) Element with configuration \(2,8,2\)
- a) Atomic number: 12 (total electrons \(=2+8+2\)).
- b) Group: Group 2 (two valence electrons).
- c) Period: Period 3 (three occupied shells: K, L, M).
- d) Resembles: Be (4) — same group → similar properties.
Q4) Electronic configurations & concepts
a) Which are in Period 3?
- \(3\,\mathrm{Li}: 2,1\) (P-2), \(14\,\mathrm{Si}: 2,8,4\) (P-3), \(2\,\mathrm{He}: 2\) (P-1), \(11\,\mathrm{Na}: 2,8,1\) (P-3), \(15\,\mathrm{P}: 2,8,5\) (P-3)
- Ans: Na, Si, P
b) Which belong to Group 2?
- \(1\,\mathrm{H}: 1\) (G-1), \(7\,\mathrm{N}: 2,5\) (G-15), \(20\,\mathrm{Ca}: 2,8,8,2\) (G-2), \(16\,\mathrm{S}: 2,8,6\) (G-16), \(4\,\mathrm{Be}: 2,2\) (G-2), \(18\,\mathrm{Ar}: 2,8,8\) (G-18)
- Ans: Be, Ca
c) Most electronegative among \( \mathrm{N, C, O, B, Al} \)
Ans: Oxygen (O)
d) Most electropositive among \( \mathrm{Be, C, O, B, Al} \)
Ans: Aluminium (Al)
e) Largest atoms among \( \mathrm{Na, P, Cl, Si, Mg} \)
Ans: Sodium (Na) — farthest left in Period 3.
f) Smallest atomic radius among \( \mathrm{K, Li, Na, Be} \)
Ans: Beryllium (Be) — right of Li in Period 2.
g) Highest metallic character among \( \mathrm{Al, Si, Na, Mg, S} \)
Ans: Sodium (Na)
h) Highest nonmetallic character among \( \mathrm{C, Li, F, N, O} \)
Ans: Fluorine (F)
Q5) Name & symbol from description
- a) Smallest atomic size: Helium — He
- b) Smallest atomic mass: Hydrogen — H
- c) Most electronegative: Fluorine — F
- d) Noble gas with smallest radius: Helium — He
- e) Most reactive nonmetal: Fluorine — F
Q6) Short notes
a) Mendeleev’s periodic law
Mendeleev arranged 63 elements in increasing atomic mass and observed periodic recurrence of properties. He stated: “Properties of elements are periodic functions of their atomic masses.” He corrected atomic masses, left gaps (eka-elements) and successfully predicted properties (e.g., Ga, Ge). Later, noble gases were added in a zero group without disturbing the table.
b) Structure of the modern periodic table
- 7 periods (rows), 18 groups (columns); long form arranged by atomic number \(Z\).
- Blocks: s (Groups 1–2), p (13–18), d (3–12, transition), f (lanthanides & actinides).
- Zig-zag boundary: metals (left), nonmetals (right), metalloids along the border.
- Group similarity from same valence electron count; period variation from increasing \(Z\) across same shell.
c) Position of isotopes in Mendeleev’s vs Modern table
- Mendeleev: Based on atomic mass → isotopes (same properties, different masses) posed a placement problem.
- Modern: Based on atomic number \(Z\) (protons) → all isotopes of an element have same \(Z\) and occupy the same position.
Q7) Scientific reasons
- Atomic radius decreases left → right: Across a period, electrons add to the same shell while nuclear charge increases; higher effective nuclear charge pulls electrons closer, so radius decreases.
- Metallic character decreases left → right: Increased effective nuclear charge and smaller radius make losing electrons harder (ionization energy ↑), so metallic character falls.
- Atomic radius increases down a group: A new shell is added each step; increased shielding outweighs nuclear charge increase → larger radius.
- Same group → same valency: Equal number of valence electrons gives same combining capacity (valency).
- Only 8 elements in Period 3 though shell M holds 18: In Period 3, only \(3s\) and \(3p\) subshells fill (\(2+6=8\)). The \(3d\) subshell starts filling from the fourth period (after \(4s\)), limiting Period 3 to eight elements.
Q8) Write the names from the description
- a) Period with electrons in K, L, M shells: Third period
- b) Group with valency zero: Group 18 — Noble gases
- c) Family of nonmetals with valency one: Halogens — Group 17
- d) Family of metals with valency one: Alkali metals — Group 1
- e) Family of metals with valency two: Alkaline earth metals — Group 2
- f) Metalloids in 2nd & 3rd periods: Boron (B), Silicon (Si)
- g) Nonmetals in the third period: Phosphorus (P), Sulphur (S), Chlorine (Cl), Argon (Ar)
- h) Two elements with valency 4: Carbon (C), Silicon (Si)
Neat Math Notes (for clarity on this chapter)
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