Class 10 · Science & Technology
Unit 14 – Classification of Elements (Solved)
Modern Periodic Table & Properties of Elements
A. Multiple Choice Questions
1
Which of the following is true about the modern periodic table?
iii. It is based on the atomic number
2
Who is regarded as the "father of the modern periodic table"?
iii. Moseley
3
Mendeleev arranged elements in his periodic table on the basis of:
ii. Atomic mass
4
In the modern periodic table, elements are arranged according to:
i. Increasing atomic number
5
Which of the following is correct about group of the periodic table?
ii. Same number of valence electrons
6
Which law states that "properties of elements are a periodic function of their atomic number"?
iii. Modern periodic law
7
Which group in the modern periodic table contains the noble gases?
iii. Group 18
8
Which of the following is a d-block element?
iii. Iron (Fe)
9
What are called the elements of group 17?
iii. Halogens
10
The periodic table is divided into how many blocks based on electronic configuration?
iii. 4
11
Which of the following elements is a metalloid?
ii. Boron
12
Matching — Elements, Characteristics, and Valency (Ca=metal/2, B=metalloid/3, Na=metal/1, O=non-metal/2):
iii. 3-d-i, 4-a-ii, 1-b-ii, 2-c-iii
13
Which of the following is correct about atomic size and electronegativity?
iv. In group electronegativity decreases; in period atomic size decreases
14
Possible molecular formula from: Element 1 (1s²,2s¹ = Li), Element 2 (3s¹ = Na), Element 3 (2p⁴ = O):
iv. Na₂O
15
Which of the following metals is most reactive?
iv. Rb
B. Very Short Answer Questions
B1. State modern periodic law.
The physical and chemical properties of elements are periodic functions of their atomic numbers.
B2. What are groups?
Groups are the 18 vertical columns of the modern periodic table. Elements in the same group have the same number of valence electrons and similar chemical properties.
B3. What are periods?
Periods are the 7 horizontal rows of the modern periodic table. Elements in the same period have the same number of electron shells.
B4. How many groups and periods are there in the modern periodic table?
18 groups (vertical columns) and 7 periods (horizontal rows).
B5. How many groups are there in s-block?
2 groups — Group 1 (alkali metals) and Group 2 (alkaline earth metals).
B6. How many groups are there in d-block?
10 groups — Groups 3 to 12 (transition elements).
B7. Molecular formula of compound from 1s², 2s², 2p⁶, 3s² and 1s², 2s², 2p⁴.
Element 1 (1s², 2s², 2p⁶, 3s²) = Mg, valency 2 | Element 2 (1s², 2s², 2p⁴) = O, valency 2
Molecular formula: MgO
Molecular formula: MgO
B8. Which would have higher atomic size between F and Na?
Na has a higher atomic size. Na (Period 3) has 3 electron shells and lower nuclear charge per shell compared to F (Period 2, high nuclear charge in a smaller shell), making Na's atomic radius much larger.
C. Short Answer Questions
C1. Write any two properties of s-block elements.
Property 1They are soft, silvery, lustrous metals (except H) and are good conductors of heat and electricity.
Property 2They are highly electropositive; they easily lose 1 or 2 valence electrons, making them very reactive, especially with water.
C2. Most reactive metal and most reactive non-metal among elements up to atomic number 20?
Most reactive metal: Potassium (K, Z=19) — in Group 1, Period 4; loses its valence electron most easily among metals up to Z=20.
Most reactive non-metal: Fluorine (F, Z=9) — most electronegative element; gains electrons most readily.
Most reactive non-metal: Fluorine (F, Z=9) — most electronegative element; gains electrons most readily.
C3. Write any two differences between electropositivity and electronegativity.
| Electropositivity | Electronegativity |
|---|---|
| Tendency of an atom to lose electrons and form positive ions | Tendency of an atom to attract shared electrons towards itself |
| Decreases across a period (→) and increases down a group (↓) | Increases across a period (→) and decreases down a group (↓) |
C4. Write any two differences between s-block and p-block elements.
| s-block | p-block |
|---|---|
| Last electron enters s-orbital; found in Groups 1 & 2 | Last electron enters p-orbital; found in Groups 13–18 |
| All are metals except Hydrogen | Includes metals, non-metals, and metalloids |
C5. Why is oxygen larger in size than neon? Explain.
Both O (Z=8) and Ne (Z=10) are in Period 2. Across a period, atomic size decreases as nuclear charge increases while electrons are added to the same shell. Oxygen has fewer protons (8) than Neon (10), so the nucleus pulls the outer electrons less strongly in O than in Ne. Therefore, oxygen has a larger atomic radius than neon.
C6. Write any one electropositive and one electronegative element.
Electropositive element: Sodium (Na) — readily loses its valence electron to form Na⁺.
Electronegative element: Fluorine (F) — most electronegative element; strongly attracts shared electrons.
Electronegative element: Fluorine (F) — most electronegative element; strongly attracts shared electrons.
C7. What would happen if there is no periodic table? Explain in any two points.
Point 1It would be extremely difficult to study the individual properties of all 118 elements; memorizing and comparing them without a systematic arrangement would be nearly impossible.
Point 2Predicting properties of undiscovered elements and understanding trends in chemical behavior (reactivity, atomic size, electronegativity) would be impossible without a structured reference.
C8. Write any two differences between period and group.
| Period | Group |
|---|---|
| A horizontal row in the periodic table (7 total) | A vertical column in the periodic table (18 total) |
| Elements have the same number of electron shells | Elements have the same number of valence electrons |
C9. Write any two advantages of modern periodic table.
Advantage 1It corrected the anomalous positions in Mendeleev's table (e.g., Argon before Potassium) by using atomic number instead of atomic mass.
Advantage 2It provides a systematic arrangement that reveals periodic trends (atomic size, reactivity, electronegativity), making it easy to predict properties of elements.
C10. X = 1s², 2s², 2p⁶ | Y = 1s², 2s², 2p⁶, 3s², 3p¹ | Z = 1s², 2s², 2p⁶, 3s², 3p⁶
Identification: X = Neon (Ne, Z=10) — noble gas | Y = Aluminum (Al, Z=13) — metal | Z = Argon (Ar, Z=18) — noble gas
a. Compound formed: X (Ne) and Z (Ar) are noble gases with complete valence shells — they do not form compounds. No compound is formed between these three elements.
b. Largest size: Y (Aluminum) has the largest size. Al has 3 electron shells like Ar, but fewer protons (Z=13 vs Z=18), so nuclear pull is weaker, making Al's atomic radius larger. X (Ne) has only 2 shells, so it is the smallest.
a. Compound formed: X (Ne) and Z (Ar) are noble gases with complete valence shells — they do not form compounds. No compound is formed between these three elements.
b. Largest size: Y (Aluminum) has the largest size. Al has 3 electron shells like Ar, but fewer protons (Z=13 vs Z=18), so nuclear pull is weaker, making Al's atomic radius larger. X (Ne) has only 2 shells, so it is the smallest.
D. Long Answer Questions
D1. Write any four characteristics of modern periodic table.
1. BasisArranged in order of increasing atomic number (not atomic mass like Mendeleev's table).
2. StructureContains 7 periods (horizontal rows) and 18 groups (vertical columns).
3. BlocksDivided into 4 blocks (s, p, d, f) based on which orbital the last electron enters.
4. ArrangementMetals on the left, non-metals on the right, metalloids along the stair-step line; Lanthanides and Actinides placed separately at the bottom.
D2. How are electropositive and electronegative elements different? Write any four points.
| S.N. | Electropositive Elements | Electronegative Elements |
|---|---|---|
| 1 | Tend to lose electrons and form positive ions (cations) | Tend to gain or attract electrons and form negative ions (anions) |
| 2 | Electropositivity decreases across a period (→) | Electronegativity increases across a period (→) |
| 3 | Electropositivity increases down a group (↓) | Electronegativity decreases down a group (↓) |
| 4 | Mainly shown by metals (e.g., Na, K, Ca) | Mainly shown by non-metals (e.g., F, Cl, O) |
D3. Write the group and period of Magnesium, Oxygen, Bromine, and Potassium.
| Element | Atomic Number | Electronic Config (last part) | Group | Period |
|---|---|---|---|---|
| Magnesium (Mg) | 12 | …3s² | Group 2 | Period 3 |
| Oxygen (O) | 8 | …2p⁴ | Group 16 | Period 2 |
| Bromine (Br) | 35 | …4p⁵ | Group 17 | Period 4 |
| Potassium (K) | 19 | …4s¹ | Group 1 | Period 4 |
D4. What are the things to be improved in the modern periodic table? Explain with valid logic.
1. Position of HHydrogen is placed in Group 1 (alkali metals) but is a non-metal. It shows properties of both Group 1 and Group 17 (halogens), so its placement is ambiguous.
2. Lanthanides & ActinidesThey are placed separately at the bottom, breaking the continuity of the table. Proper incorporation within the main table would make it more logically consistent.
3. IsotopesIsotopes of the same element (same atomic number, different mass) have different physical properties but occupy the same position, which can cause confusion.
4. Synthetic elementsNewly synthesized heavy elements (beyond Z=118) are difficult to accommodate, and the table may need future restructuring to include them systematically.
D5. Elements denoted by proto names: A = Na (Group 1, Period 3), B = F (Group 17, Period 2), C = Li (Group 1, Period 2), D = Cl (Group 17, Period 3).
a. Electronic configuration of element C (Li, Z=3):
1s², 2s¹ — 2 electrons in the first shell, 1 electron in the second shell. Last electron in s-orbital → s-block element.
b. Compound formed by C (Li) and D (Cl):
Li has valency 1 (loses 1 electron); Cl has valency 1 (gains 1 electron).Molecular formula: LiCl (Lithium Chloride)
c. Which element has the largest size among A, B, C, D?
A (Sodium, Na) has the largest atomic size.Na (Period 3, Group 1) has 3 electron shells and only 1 loosely held valence electron. Compared to C (Li, 2 shells), B (F, tiny — high nuclear charge in Period 2), and D (Cl, 3 shells but higher nuclear charge Z=17 vs Z=11), Na has the largest atomic radius due to the extra shell and lowest effective nuclear charge per shell.
D6. Graph: P, Q, R, S are Period 3 elements plotted against number of valence electrons vs atomic radius. (P=1 valence e, Q=3, R=6, S=7)
Identification: P = Na (1 valence e, s-block) | Q = Al (3 valence e, p-block) | R = S/Sulfur (6 valence e) | S-label = Cl (7 valence e)
Molecular formula: NaCl (Sodium Chloride / Common Salt)
a. Block of element Q (Al):
Q belongs to the p-block. Al (Z=13) is in Group 13 and its last electron enters the 3p orbital.
b. Why is P (Na) more metallic than Q (Al)?
Both are in Period 3. Across a period, metallic character decreases from left to right. Na (Group 1) has a lower nuclear charge and loses its single valence electron more easily than Al (Group 13), which requires more energy to lose 3 electrons. Therefore, Na is more electropositive and more metallic.
c. Molecular formula of compound formed by P (Na) and S-label (Cl):
Na has valency 1; Cl has valency 1.Molecular formula: NaCl (Sodium Chloride / Common Salt)