Chapter 2 - Periodic Classification of Elements
1. Study the entries in the following table and rewrite them putting the connected items in a single row.
| Column 1 | Column 2 | Column 3 |
|---|---|---|
i. Triad | a. Lightest and negatively charged particle in all the atoms | 1. Mendeleev |
ii. Octave | b. Concentrated mass and positive charge | 2. Thomson |
iii. Atomic number | c. Average of the first and the third atomic mass | 3. Newlands |
iv. Period | d. Properties of the eighth element similar to the first | 4. Rutherford |
v. Nucleus | e. Positive charge on the nucleus | 5. Dobereiner |
vi. Electron | f. Sequential change in molecular formulae | 6. Moseley |
Ans:
| Column 1 | Column 2 | Column 3 |
|---|---|---|
i. Triad | c. Average of the first and the third atomic mass | 5. Dobereiner |
ii. Octave | d. Properties of the eighth element similar to the first | 3. Newlands |
iii. Atomic number | e. Positive charge on the nucleus | 6. Moseley |
iv. Period | f. Sequential change in molecular formulae | 1. Mendeleev |
v. Nucleus | b. Concentrated mass and positive charge | 4. Rutherford |
vi. Electron | a. Lightest and negatively charged particle in all the atoms | 2. Thomson |
2. Choose the correct option and rewrite the statement.
1. The number of electrons in the outermost shell of alkali metals is ______
(i) 1
(ii) 2
(iii) 3
(iv) 7
Ans: (i) 1
2. Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in ______
(i) Group 2
(ii) Group16
(iii) Period 2
(iv) d-block
Ans: (i) Group 2
3. Molecular formula of the chloride of an element X is XCl. This compound is a solid having a high melting point. Which of the following elements will be present in the same group as X.
(i) Na
(ii) Mg
(iii) Al
(iv) Si
Ans: (i) Na
4. In which block of the modern periodic table are the nonmetals found?
(i) s-block
(ii) p-block
(iii) d-block
(iv) f-block
Ans: (ii) p-block
3. An element has its electron configuration as 2, 8, 2. Now answer the following questions.
a. What is the atomic number of this element?
Ans: Atomic number of the element is 12.
b. What is the group of this element?
Ans: The element belongs to group 2.
c. To which period does this element belong?
Ans: The element belongs to period 3.
d. With which of the following elements would this element resemble? (Atomic numbers are given in the brackets)
N (7), Be (4) , Ar (18), Cl (17)
Ans: The element would resemble beryllium (Be).
4. Write down the electronic configuration of the following elements from the given atomic numbers. Answer the following question with explanation.
a. ₃Li, ₁₄Si, ₂He, ₁₁Na, ₁₅P
Which of these elements belong to period 3?
Ans:
| Element | Electronic Configuration |
|---|---|
₃Li | 2, 1 |
₁₄Si | 2, 8, 4 |
₂He | 2 |
₁₁Na | 2, 8, 1 |
₁₅P | 2, 8, 5 |
Explanation:
The elements ₁₄Si, ₁₁Na and ₁₅P belong to period 3 as three shells (K, L and M) are occupied by electrons.
b. ₁H, ₇N, ₂₀Ca, ₁₆S, ₄Be, ₁₈Ar
Which of these elements belong to the second group?
Ans:
| Element | Electronic Configuration |
|---|---|
₁H | 1 |
₇N | 2, 5 |
₂₀Ca | 2, 8, 8, 2 |
₁₆S | 2, 8, 6 |
₄Be | 2, 2 |
₁₈Ar | 2, 8, 8 |
Explanation:
The elements ₂₀Ca and ₄Be belong to second group as they have two valence electrons.
c. ₇N, ₆C, ₈O, ₅B, ₁₃Al
Which is the most electronegative element among these?
Ans:
| Element | Electronic Configuration |
|---|---|
₇N | 2, 5 |
₆C | 2, 4 |
₈O | 2, 6 |
₅B | 2, 3 |
₁₃Al | 2, 8, 3 |
Explanation:
The most electronegative element is ₈O as electronegativity decreases down a group while it increases moving across a period from left to the right.
d. ₄Be, ₆C, ₈O, ₅B, ₁₃Al
Which is the most electropositive element among these?
Ans:
| Element | Electronic Configuration |
|---|---|
₄Be | 2, 2 |
₆C | 2, 4 |
₈O | 2, 6 |
₅B | 2, 3 |
₁₃Al | 2, 8, 3 |
Explanation:
The most electropositive element is ₁₃Al electropositivity increases down a group while it decreases moving across a period from left to the right.
e. ₁₁Na, ₁₅P, ₁₇Cl, ₁₄Si, ₁₂Mg
Which of these has the largest atoms?
Ans:
| Element | Electronic Configuration |
|---|---|
₁₁Na | 2, 8, 1 |
₁₅P | 2, 8, 5 |
₁₇Cl | 2, 8, 7 |
₁₄Si | 2, 8, 4 |
₁₂Mg | 2, 8, 2 |
Explanation:
The element which has the largest atom is ₁₁Na because atomic size decreases moving across a period from left to the right.
f. ₁₉K, ₃Li, ₁₁Na, ₄Be
Which of these atoms has the smallest atomic radius?
Ans:
| Element | Electronic Configuration |
|---|---|
₁₉K | 2, 8, 8, 1 |
₃Li | 2, 1 |
₁₁Na | 2, 8, 1 |
₄Be | 2, 2 |
Explanation:
The element with the smallest atomic radius is ₄Be because atomic size decreases moving across a period from left to the right while it increases down a group.
g. ₁₃Al, ₁₄Si, ₁₁Na, ₁₂Mg, ₁₆S
Which of the above elements has the highest metallic character?
Ans:
| Element | Electronic Configuration |
|---|---|
₁₃Al | 2, 8, 3 |
₁₄Si | 2, 8, 4 |
₁₁Na | 2, 8, 1 |
₁₂Mg | 2, 8, 2 |
₁₆S | 2, 8, 6 |
Explanation:
The element with the highest metallic character is ₁₁Na as metallic character decreases moving across a period from left to the right.
h. ₆C, ₃Li, ₉F, ₇N, ₈O
Which of the above elements has the highest nonmetallic character?
Ans:
| Element | Electronic Configuration |
|---|---|
₆C | 2, 4 |
₃Li | 2, 1 |
₉F | 2, 7 |
₇N | 2, 5 |
₈O | 2, 6 |
Explanation:
The element with the highest nonmetallic character is ₉F as nonmetallic character increases moving across a period from left to the right.
5. Write the name and symbol of the element from the description.
a. The atom having the smallest size.
Ans: The atom with the smallest size is hydrogen. The symbol of the hydrogen atom is ₁H.
b. The atom having the smallest atomic mass.
Ans: The atom with the smallest atomic mass is hydrogen. The symbol of the hydrogen atom is ₁H.
c. The most electronegative atom.
Ans: The most electronegative atom is fluorine. The symbol of the fluorine atom is ₉F.
d. The noble gas with the smallest atomic radius.
Ans: The noble gas with the smallest atomic radius is helium. The symbol of the helium atom is ₂He.
e. The most reactive nonmetal.
Ans: The most reactive non-metal is fluorine. The symbol of the fluorine atom is ₉F.
6. Write short notes.
a. Mendeleev’s periodic law.
Ans:
(i) Mendeleev’s periodic law states that the physical and chemical properties of elements are a periodic function of their atomic weights.
(ii) According to Mendeleev, the periodic physical and chemical properties of elements depend on their atomic weight, such that when arranging elements in increasing order of atomic weight, elements with similar properties are repeated after regular intervals of increasing atomic weight.
b. Structure of the modern periodic table.
Ans:
(i) The modern periodic table consists of seven horizontal rows called the periods and eighteen vertical columns called the groups.
(ii) The arrangement of the periods and groups results in the formation of boxes, where each box corresponds to the position of an element.
(iii) In addition to these seven rows, lanthanide and actinide series are shown separately at the bottom of the periodic table. The first row is the lanthanide series and the second row is actinide series.
(iv) The entire periodic table is divided into four blocks: s-block, p-block, d-block and f-block.
a) s-block: contains group 1 and 2 elements
b) p-block: contains group 13 to 18 elements
c) d-block: contains group 3 to 12 elements (transition elements)
d) f-block: contains lanthanide and actinide series
(v) A zig-zag line is drawn in the p-block of the periodic table. Metals lie on the left side while nonmetals lie on the right side of this line. Metalloids lie along the border of this line.
c. Position of isotopes in the Mendeleev’s and the modern periodic table.
Ans:
(i) Isotopes were discovered long time after Mendeleev developed the periodic table.
(ii) All isotopes of the same element have different atomic masses but the same atomic number. They also have the same chemical properties.
(iii) In Mendeleev’s periodic table, elements are arranged in increasing order of their atomic masses such that chemically similar elements are placed together in a group. So, it was difficult to place them in Mendeleev’s periodic table.
(iv) In the modern periodic table, elements are arranged in increasing order of their atomic numbers. Hence, all the isotopes of an element occupy the same position in the modern periodic table.
7. Write scientific reasons.
a. Atomic radius goes on decreasing while going from left to right in a period.
Ans:
(i) The number of elements in the first three periods depends on the electron capacity of the shells and the law of electron octet.
(ii) The electron capacity of the third shell is 18. So the third period should actually contain 18 elements. But it contains only 8 elements due to the law of electron octet.
Hence, atomic radius goes on decreasing while going from left to right in a period.
b. Metallic character goes on decreasing while going from left to right in a period.
Ans:
(i) The electronic configuration of the outermost shell is the same for all the elements belonging to the same group.
(ii) So, the number of valence electrons for all the elements in a group is the same.
(iii) The valency of an element is determined by the valence electrons.
Hence, elements belonging to the same group have the same valency.
c. Atomic radius goes on increasing down a group.
Ans:
(i) While going from left to right in a period, the atomic number increases. So, positive charge on the nucleus increases. At the same time, electrons get added in the same outermost shell.
(ii) This increases effective nuclear charge. As a result, electrons are attracted closer to the nucleus decreasing the size of the atom.
Thus, atomic radius goes on decreasing while going from left to right in a period.
d. Elements belonging to the same group have the same valency.
Ans:
(i) As we move from top to bottom in a group, the number of shells increases.
(ii) The outermost electrons go farther and farther from the nucleus, extending the radius and ultimately increasing the size of the atom even though the nuclear charge increases.
Hence, atomic radius goes on increasing down a group.
e. The third period contains only eight elements even though the electron capacity of the third shell is 18.
Ans:
(i) While going from left to right in a period, electrons are added to the same outermost shell. At the same time, protons get added to the nucleus increasing the nuclear charge. Hence, these electrons experience greater pull from the nucleus due to increased effective nuclear charge.
(ii) As a result, it becomes difficult to remove a valence electron from the atom. Hence, metallic character goes on decreasing while going from left to right in a period.
8. Write the names from the description.
a. The period with electrons in the shells K, L and M.
Ans: Period 3
b. The group with valency zero.
Ans: Group 18
c. The family of nonmetals having a valency one.
Ans: Halogens
d. The family of metals having a valency one.
Ans: Alkali metals
e. The family of metals having valency two.
Ans: Alkaline earth metals
f. The metalloids in the second and third periods.
Ans: Boron and silicon
g. Nonmetals in the third period.
Ans: Phosphorus, sulphur and chlorine
h. Two elements having valency 4.
Ans: Carbon and silicon