Q1. Rearrange columns 2 and 3 so they match column 1.
Answer: Make table from the textbook.
Explanation:
- Johann Dobereiner, a German chemist, was the first to observe repeating patterns in element properties. He grouped elements in sets of three with similar behaviour, known as Dobereiner's triads. He showed that the atomic mass of the middle element was nearly the average of the other two.
- John Newlands, an English chemist, proposed the Law of Octaves. Arranging elements in increasing atomic mass, he noticed that every eighth element resembled the first, similar to musical octaves.
- Henry Moseley, an English scientist, discovered that an element's atomic number equals the positive charge in its nucleus, i.e., the number of protons. The atomic number is represented as Z.
- Mendeleev arranged elements according to their physical and chemical properties. Horizontal rows were called periods, and he placed elements sequentially according to their changing molecular formulae.
- Rutherford bombarded thin gold foil with alpha particles. Based on his observations, he concluded:
- The nucleus lies at the centre of the atom.
- It carries a positive charge.
- Most of the atom's mass is concentrated in this central nucleus.
- J.J. Thomson proposed that atoms are spherical in shape. He compared an atom to a watermelon, where electrons (negatively charged) are embedded like seeds, and the rest of the atom holds positive charge.
Q2. Choose the correct option and rewrite the statement.
a. The number of electrons in the outermost shell of alkali metals is… (i) 1
Explanation:
Alkali metals occupy Group 1 of the periodic table. Every element in this group has one valence electron, as shown by their configurations.
b. Alkaline earth metals have valency 2. Their position in the modern periodic table is… (i) Group 2
Explanation:
Group placement depends on valence electrons. Alkaline earth metals have two electrons in their outermost shell; therefore, they are placed in Group 2.
c. In XCl, X forms a high-melting solid. Which element is in the same group? (i) Na
Explanation:
Since one atom of X combines with one atom of chlorine, X must have valency 1. Among the options, sodium (Na) has one valence electron and forms NaCl. Thus, X belongs to sodium's group.
d. In which block are non-metals found? (ii) p-block
Explanation:
Most non-metals lie in the p-block. This block contains metals, non-metals, and metalloids, separated by a zig-zag boundary.
Q3. An element has electronic configuration 2,8,2. Answer the following:
a. What is the atomic number? 12
Explanation:
Total electrons = 2 + 8 + 2 = 12 → atomic number = 12.
b. What is its group? Group 2
Explanation:
There are two electrons in the outer shell → valency 2 → Group 2.
c. Which period does it belong to? Period 3
Explanation:
It has three shells; hence it is placed in the third period.
d. Which element does it resemble? (Given atomic numbers) Be
Explanation:
Be = 2,2 → also has two valence electrons → belongs to Group 2, like the given element.
Q4. Answer the following sub-questions
A. Write electronic configurations of the given elements and identify which belong to Period 3.
3Li → 2,1 | 14Si → 2,8,4 | 2He → 2 | 11Na → 2,8,1 | 15P → 2,8,5
Answer:
Na (2,8,1), Si (2,8,4) and P (2,8,5) all have three shells → they lie in Period 3.
B. Write electronic configurations and identify which belong to Group 2.
1H → 1 | 7N → 2,5 | 20Ca → 2,8,8,2 | 16S → 2,8,6 | 4Be → 2,2 | 18Ar → 2,8,8
Answer:
Ca (2,8,8,2) and Be (2,2) have two valence electrons → Group 2 elements.
C. Write electronic configurations and identify the most electronegative element.
7N → 2,5 | 6C → 2,4 | 8O → 2,6 | 5B → 2,3 | 13Al → 2,8,3
Answer: Oxygen
Electronegativity increases left→right across a period and decreases down a group. Among the listed elements, oxygen attracts electrons the most strongly.
D. Write configurations and identify the most electropositive element.
Be → 2,2 | C → 2,4 | O → 2,6 | B → 2,3 | Al → 2,8,3
Answer: Aluminium
Electropositivity increases downward and decreases across a period. Aluminium loses electrons more easily than the others; hence it is most electropositive.
E. Write configurations and find the element with the largest atom.
Na → 2,8,1 | P → 2,8,5 | Cl → 2,8,7 | Si → 2,8,4 | Mg → 2,8,2
Answer: Sodium (Na)
All are in Period 3. Atomic size decreases from left to right. Na is farthest left → largest atomic radius.
F. Write configurations and find the smallest atomic radius.
K → 2,8,8,1 | Li → 2,1 | Na → 2,8,1 | Be → 2,2
Answer: Beryllium (Be)
Atomic size decreases across a period and increases down a group. Be lies high and to the right → smallest radius.
G. Identify which element has highest metallic character.
Al → 2,8,3 | Si → 2,8,4 | Na → 2,8,1 | Mg → 2,8,2 | S → 2,8,6
Answer: Sodium (Na)
All are in Period 3. Metallic character decreases left→right; Na lies far left → highest metallic nature.
H. Identify which element has highest non-metallic character.
C → 2,4 | Li → 2,1 | F → 2,7 | N → 2,5 | O → 2,6
Answer: Fluorine (F)
All lie in Period 2. Electronegativity rises left→right; fluorine is the strongest electron-acceptor.
Q5. Write name and symbol of elements described.
| Description | Element Name | Symbol |
|---|---|---|
| Smallest atom | Hydrogen | H |
| Lowest atomic mass | Hydrogen | H |
| Most electronegative atom | Fluorine | F |
| Noble gas with smallest size | Helium | He |
| Most reactive non-metal | Fluorine | F |
Q6. Short Notes
A. Mendeleev's Periodic Law
Mendeleev proposed that "The properties of elements recur periodically when they are arranged in increasing atomic mass." He organised elements in rows called periods and columns called groups. Some atomic masses were re-evaluated to fit the pattern, and he even left empty spaces predicting undiscovered elements.
B. Structure of the Modern Periodic Table
The modern periodic table consists of 7 periods and 18 groups. Atomic numbers appear above each element. Two additional rows—lanthanides and actinides—are placed separately. A total of 118 element boxes exist. The table is divided into s-, p-, d- and f-blocks. s-block has alkali and alkaline earth metals; p-block contains metals, non-metals and metalloids; d-block contains transition metals.
C. Position of Isotopes
Isotopes are atoms of the same element having equal atomic number but different mass number. Since isotopes display identical chemical behaviour, they are placed in the same position as their parent element in the modern periodic table. Mendeleev's table could not properly account for isotopes because it was based on atomic masses.
Q7. Scientific Reasons
A. Atomic radius decreases across a period
As we move left to right, the atomic number increases, raising nuclear charge. Electrons enter the same shell, so shielding does not increase much. The nucleus pulls the outer electrons more strongly, reducing atomic size.
B. Metallic character decreases across a period
Metals lose valence electrons to form positive ions. Across a period, increasing nuclear charge holds electrons more tightly. Thus, the tendency to lose electrons reduces, lowering metallic character.
C. Atomic radius increases down a group
Down a group, electrons occupy additional shells. Although nuclear charge increases, the distance between nucleus and valence electrons grows. Hence, the atomic radius increases down the group.
D. Same group → Same valency
Group number depends on valence electrons. Elements in the same group have identical outer-shell electron counts. Thus, their valency is the same. Example: alkaline earth metals all have valency 2.
E. Third period has only 8 elements
Although the third shell can hold 18 electrons, only the 3s and 3p subshells (2 + 6 electrons) fill for Period 3. Electrons start filling 4s after 3p, so only 8 elements appear in this period.
Q8. Identify the following
A. Identify the period with electrons in K, L, and M shells
Answer:
Third period
B. Group with valency zero
Answer:
Group 18 — Noble gases
C. Non-metal family with valency one
Answer:
Fluorine, Chlorine, Bromine, Iodine (Halogens)
D. Metals with valency one
Answer:
Alkali metals — Li, Na, K, Rb, Cs, Fr
E. Metals with valency two
Answer:
Alkaline earth metals — Be, Mg, Ca, Sr, Ba, Ra
F. Metalloids in Periods 2 and 3
Answer:
Boron (Period 2), Silicon (Period 3)
G. Non-metals in the third period
Answer:
Phosphorus, Sulfur, Chlorine
H. Two elements with valency 4
Answer:
Carbon (C), Silicon (Si)