NCERT Solutions Class 10 Science
The NCERT Solutions in English Language for Class 10 Science Chapter – 5 (Periodic Classification of Elements) has been provided here to help the students in solving the questions from this exercise.
Chapter – 5 (Periodic Classification of Elements)
Questions
1. Did Doberiener’s triads also exist in the columns of Newlands’ octaves ? Compare and find out.
Answer – Yes, some of the Doberiener’s triads did exist in the columns of Newlands’ octaves.
Dobereiner’s triads
| Li | Ca | Cl |
| Na | Sr | Br |
| K | Ba | I |
Newlands’ octaves
|
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 |
− |
− |
2. What were the limitations of Doberiener’s triads ?
Answer – Doberiener was in a position to identify three triads. It could not apply to all the elements known at that time. Therefore, the classification was not so useful.
3. What were the limitations of Nawlands’ Law of Octaves ?
Answer – 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.
Questions
1. Use Mendeleev’s periodic table to predict the formulae for the oxides of the elements :
K, C, Al, Si, Ba.
Answer –
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.
2. Besides gallium which two other elements have since been discovered that fill the gaps left by Mendeleev in creating his periodic table ?
Answer – Two other elements are scandium (Sc) and germanium (Ge). In their gaps, the elements with names Eka-boron and Eka-silicon were placed.
3. What was the criteria used by Mendeleev in creating his periodic table ?
Answer – Mendeleev used atomic masses of the elements as the criteria for creating his periodic table. In this table, the elements were arranged in order of increasing atomic masses.
4. Why do you think that the noble gases should be placed in a separate group ?
Answer – Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.
Questions
1. How could the Modern Periodic table remove various anomalies of Mendeleev’s periodic table ?
Answer – The Modern Periodic Table could remove various anomalies of Mendeleev’s Periodic Table:
1. In the Modern Periodic Table atomic number of an elements is a more fundamental property than its atomic mass.
2. The anomalous position of hydrogen can be discussed after we see what are the basis on which the position of an elements in the Modern Periodic Table depends.
3. The elements present in any one group have the same number of valence electrons.
4. Atoms of different elements with the same number of occupied shells are placed in the same period.
5. In the Modern Periodic Table, a zig-zag line separated metals from non-metals.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice ?
Answer – Magnesium (Mg) belongs to group 2 known as Alkaline Earth Family. The two other elements belonging to the same group are calcium (Ca) and strontium (Sr). The basis of choice is the electronic distribution in the valence shell of these elements. All of them have two electrons each.
| K | L | M | N | O | |
| Mg (Z =12) | 2 | 8 | 2 | — | — |
| Ca (Z = 20) | 2 | 8 | 8 | 2 | — |
| Sr (Z = 38) | 2 | 8 | 18 | 8 | 2 |
3. Name :
(a) three elements that have a single electron in their outermost shells.
Answer – Lithium, sodium, potassium (Alkali metals present in group 1)
(b) three elements that have two electrons in their outermost shells.
Answer – Beryllium, magnesium, calcium (Alkaline earth metals present in group 2)
(c) three elements with filled outermost shells.
Answer – Helium, neon, argon (Noble gases present in group 18).
4.
(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements ?
Answer – Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common ?
Answer – 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.
5. In the modern periodic table, which are the metals among the first ten elements ?
Answer – Metals among the first ten elements are lithium (Li) and beryllium (Be) . These are placed towards the left of the table.
| Period | Group 1 | Group 2 | Group 13 | Group 14 | Group 15 | Group 16 |
| 1
2 |
—
— |
—
Be |
—
— |
—
— |
—
— |
—
— |
| 3
4 |
—
— |
—
— |
—
Ga |
—
Ge |
—
As |
—
Se |
6. By considering their position in the periodic table, which one of the following elements would you expect to have maximum metallic characteristics ?
Answer – Since Be lies to the extreme left hand side of the periodic table, Be (Beryllium) is the most metallic among the given elements.
Exercise
1. Which of the following statements is not correct about the trends while going from left to right across the periodic table ?
(a) The elements become less metallic in nature
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Answer – (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.)
2. Element X forms a chloride with the formula XCI2 which is a solid with high melting point. X would most likely to be in the same group of the periodic table as :
(a) Na
(b) Mg
(c) Al
(d) Si.
Answer – (b) Mg
3. Which element has :
(a) two shells, both of which are completely filled with electrons ?
Answer – Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
(b) the electronic configuration 2, 8, 2 ?
Answer – Magnesium has the electronic configuration 2, 8, 2.
(c) a total of three shells with four electrons in the valence shell ?
Answer – 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) a total of two shells with three electrons in the valence shell ?
Answer – 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) twice as many electrons in the second shell as in the first shell ?
Answer – 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).
4.
(a) Which property do all elements in the same column of the periodic table as boron have in common ?
Answer – 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) Which property do all elements in the same column of the periodic table as fluorine have in common ?
Answer – 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.
5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element ?
Answer – The atomic number of the element is 17 (2 + 8 + 7 = 17).
(b) To which of the following elements would it be chemically similar ? (Atomic numbers are given in parentheses).
N(7), F(9), P(15), Ar(18).
Answer – It would be chemically similar with fluorine (F) which has also 7 electrons in valence shell (2, 7)
6. The position of three elements A, B and C in the periodic table are shown below :
| Group 16 | Group 17 |
| – | – |
| – | A |
| – | – |
| B | C |
(a) State whether A is metal or non-metal.
Answer – Group 17 represents halogen family. All the elements included in the family are Therefore, element A is a non-metal.
(b) State whether C is more reactive or less reactive than A.
Answer – C is less reactive than A.
(c) Will C be larger or smaller in size than B ?
Answer – C will be smaller than B.
(d) Which type of ion, cation or anion will be formed by the element A ?
Answer – A will form anion.
7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write their electronic configuration. Which of these will be more electronegative and why ?
Answer – The electronic configurations of the two elements are :
Nitrogen (Z = 7) 2, 5 ;
Phosphorus (Z = 15) 2, 8, 5
Since the size of nitrogen is small as compared to phosphorus, it has a greater tendency to take up electrons. It is therefore, more electronegative than phosphorus.
8. How does the electronic configuration of an atom relate to its position in the modern periodic table ?
Answer – 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.
9. In the modern periodic table, calcium (Z = 20) is surrounded by the elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium ?
Answer – 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).
10. Compare and contrast the arrangement of elements in Mendeleevs periodic table and the modern periodic table.
Answer – The main points of distinction between Mendeleevs periodic table and Modern periodic table are as follows :
|
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. |
