Page 81
Ans: 1
Only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.
Dobereiner’s triads
Newlands’octaves
Ans: 2
Limitation of Dobereiner’s classification:
All known elements could not be classified into groups of triads on the basis of their properties.
Ans: 3
Limitations of Newlands’law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.
(ii) Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.
(iii) The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
Page 85
Ans: 1
K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
Ans: 2
Scandium and germanium
Ans: 3
Mendeleev’s periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.
Ans: 4
Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.
Page 90
Ans: 1
Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the elements is not always regular from one to its next. It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner.
It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers. Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the modern periodic table.
Ans: 2
Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
Ans: 3
(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
Ans: 4
(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.
Ans: 5
Among the first ten elements, lithium (Li) and beryllium (Be) are metals.
Ans: 6
Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.
Page 91
Ans: 1
(c) The atoms lose their electrons more easily.
(On moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)
Ans: 2
(b) X would most likely be in the same group of the Periodic Table as magnesium (Mg).
Ans: 3
(a) Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).
Ans: 4
(a) All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1.
Ans: 5
(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2
Page 92
Ans: 6
(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.
Ans: 7
Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.
Ans: 8
In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.
Ans: 9
The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
Ans: 10
Ans: 1
Only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.
Dobereiner’s triads
| Li | Ca | Cl |
| Na | Sr | Br |
| K | Ba | I |
H
|
Li
|
Be
|
B
|
C
|
N
|
O
|
F
|
Na
|
Mg
|
Al
|
Si
|
P
|
S
|
Cl
|
K
|
Ca
|
Cr
|
Ti
|
Mn
|
Fe
|
Co and Ni
|
Cu
|
Zn
|
Y
|
In
|
As
|
Se
|
Br
|
Rb
|
Sr
|
Ce and La
|
Zr
|
−
|
−
|
Ans: 2
Limitation of Dobereiner’s classification:
All known elements could not be classified into groups of triads on the basis of their properties.
Ans: 3
Limitations of Newlands’law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.
(ii) Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.
(iii) The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
Page 85
Ans: 1
K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
Ans: 2
Scandium and germanium
Ans: 3
Mendeleev’s periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.
Ans: 4
Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.
Page 90
Ans: 1
Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the elements is not always regular from one to its next. It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner.
It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers. Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the modern periodic table.
Ans: 2
Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
Ans: 3
(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
Ans: 4
(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.
Ans: 5
Among the first ten elements, lithium (Li) and beryllium (Be) are metals.
Ans: 6
Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.
Page 91
Ans: 1
(c) The atoms lose their electrons more easily.
(On moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)
Ans: 2
(b) X would most likely be in the same group of the Periodic Table as magnesium (Mg).
Ans: 3
(a) Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).
Ans: 4
(a) All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1.
Ans: 5
(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2
Page 92
Ans: 6
(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.
Ans: 7
| Element | K L M |
| Nitrogen Phosphorus | 2 5 2 8 5 |
Ans: 8
In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.
Ans: 9
The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
Ans: 10
Mendeleev’s periodic table
|
Modern periodic table
| ||
| 1. | Elements are arranged in the increasing order of their atomic masses. | 1. | Elements are arranged in the increasing order of their atomic numbers. |
| 2. | There are a total of 7 groups (columns) and 6 periods (rows). | 2. | There are a total of 18 groups (columns) and 7 periods (rows). |
| 3. | Elements having similar properties were placed directly under one another. | 3. | Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group. |
| 4. | The position of hydrogen could not be explained. | 4. | Hydrogen is placed above alkali metals. |
| 5. | No distinguishing positions for metals and non-metals. | 5. | Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side. |
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