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NIOS Class 10 Science and Technology Chapter 6 Periodic Classification of Elements
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Periodic Classification of Elements
Chapter: 6
INTEXT QUESTIONS 6.1
1. Elements A, B and C constitute a Dobereiner’s triad. The atomic mass of A is 20 and that of C is 40. Predict the atomic mass of B.
Ans: Atomic mass of B = 20 + 40/2=30.
2. Which property of atoms was used by Mendeleev to classify the elements?
Ans: Atomic mass.
3. In Mendeleev’s periodic classification, whether chemically similar elements are placed in a group or in a period? *Anomaly means deviation from common rule, irregularity, abnormal, exception.
Ans: Group.
4. Mendeleev’s periodic table had some blank spaces. What did they signify?
Ans: These were the positions of elements which were yet to be discovered.
5. Explain any three defects of Mendeleev’s periodic table.
Ans: Three defects of Mendeleev’s periodic table.
(i) position of hydrogen.
(ii) position of isotopes.
(iii) anomalous pairs of elements.
(iv) grouping of chemically dissimilar element.
(v) separation of chemically similar element.
(vi) no explanation for electronic configuration.
INTEXT QUESTIONS 6.2
1. Give any two defects of Mendeleev’s periodic table which has been removed in the modern periodic table. How were they removed?
Ans: Two defects of Mendeev’s table which has been removed in the modern table.
(i) Fifteen elements are placed in group III A. These elements were not given places in the periodic table.
(ii) Mendeleev’s original periodic table do not have isotopes.
2. Metalloids are present along the diagonal line starting from group 13 and going down to group 16. Do they justify their position in the modern periodic table?
Ans: Position of isotopes. Since all the isotopes of an element have the same atomic number, they all will occupy the same position in the periodic table.
INTEXT QUESTIONS 6.3
1. Fill in the blanks with appropriate words.
(a) The force of attraction between nucleus and valence electrons …………….. in a period from left to right.
Ans: (a) Increases.
(b) Atomic radii of elements …………….. in a period from left to right.
Ans: (b) Decreases.
(c) Atomic radii of elements …………….. in a group from top to bottom.
Ans: (c) increases.
(d) Metallic character of elements …………….. from top to bottom in a group.
Ans: (d) Increases.
2. In the following crossword puzzle, elements are present horizontally, vertically downwards and diagonally downwards. Let us find out how many elements you are able to get within 5 minutes.
Ans: Hydrogen, Carbon, Barium, Sodium, Boron, Chlorine (horizontally) Magnesium, Iodine, Helium, Neon, Silicon, (vertically downwards) Nitrogen, Oxygen(diagonally downwards).
3. Let us find how many riddles you can solve.
(i) I am the only noble gas whose outermost shell has 2 electrons. Who am I?
Ans: Helium.
(ii) I am placed in group 16 of the modern periodic table and essential for your respiration. Who am I?
Ans: Oxygen.
(iii) I combine with chlorine to form your table salt. Who am I?
Ans: Sodium.
| TERMINAL EXERCISE |
1. Which one of the following was the earliest attempt of classification of elements?
(a) Classification of elements into metals and non-metals.
(b) Newlands’ Law of Octaves.
(c) Dobereiner’s Triads.
(d) Mendeleef’s Periodic Table.
Ans: (c) Dobereiner’s Triads.
2. The ‘law of octaves’ was given by.
(a) Mendeleev.
(b) Newlands.
(c) Lother Meyer.
(d) Dobereiner.
Ans: (b) Newlands.
3. According to the periodic law given by Mendeleev, the properties of an element are a periodic function of its
(i) Atomic volume.
(ii) Atomic size.
(iii) Atomic number.
(iv) Atomic mass.
Ans: (iv) atomic mass.
4. The particle which is universally present in the nuclei of all elements is.
(a) Neutron.
(b) Proton.
(c) Electron
(d) A-particle.
Ans: (b) Proton.
5. Potassium is more metallic than sodium because
(a) Both have 1 electron in their outermost shell.
(b) Both are highly electropositive.
(c) Sodium is larger in size than potassium.
(d) Potassium is larger in size than sodium.
Ans: (c) Sodium is larger in size than potassium.
6. What were the defects in Mendeleev’s periodic classification.
Ans: The defects in Mendeleev’s periodic classification are mentioned below:
(i) Position of Hydrogen The position of hydrogen which is placed in group IA along with alkali metals is ambiguous as it resembles alkali metals as well as halogens (group VII A).
(ii) Position of Isotopes All the isotopes of an element have different atomic masses therefore, each one of them should have been assigned a separate position. On the other hand, they are all chemically similar; hence they should all be placed at the same position. In fact, Mendeleev’s periodic table did not provide any space for different isotopes. For example, two isotopes of carbon are represented as 6C12, 6C14 but placed at the same position.
(iii) Anomalous Pairs of Elements At some places, an element with greater atomic mass had been placed before an element with lower atomic mass due to their properties. For example, cobalt with higher atomic mass (58.9) was placed before nickel with lower atomic mass (58.7).
(iv) Grouping of chemically dissimilar elements Elements such as copper and silver have no resemblance with alkali metals (lithium, sodium etc.), but have been grouped together in the first group.
(v) Separation of chemically similar elements Elements which are chemically similar such as gold and platinum have been placed in separate groups.
7. Which one of the following elements in its chloride does not show the valence equal to its valence electrons?
(a) NaCl.
(b) MgCl2.
(c) AlCl3.
(d) PCl3.
Ans: (d) PCl3.
8. Which one of the following elements has the least tendency to form cation?
(a) Na.
(b) Ca.
(c) B.
(d) Al 9.
Ans: (c) B.
9. Which one of the following does not belong to the family of the alkali metals?
(a) Li.
(b) Na.
(c) Be.
(d) K 10.
Ans: (c) Be.
10. The number of elements in the 5th period of the periodic table is.
(a) 2.
(b) 8.
(c) 32.
(d) 18 ,11.
Ans: (c) 32.
11. The elements with atomic number 9 resembles with the element having atomic number
(a) 35.
(b) 27.
(c) 17.
(d) 8.
Ans: (c) 17.
II. Mark the following statements True (T) or False (F):
1. The properties of the middle element in a Dobereiner’s triads are intermediate between those of the other two.
Ans True.
2. The vertical columns in the periodic table are called periods.
Ans: False.
3. Mendeleev depended only on the atomic mass of elements for his classification.
Ans: True.
4. All elements present in a group are chemically similar.
Ans: True.
5. The modern periodic law is based upon atomic mass.
Ans: False.
6. The importance of atomic number as the fundamental property was released by Henry Mosely.
Ans: True.
7. There are 18 groups in the modern periodic table.
Ans: True.
8. Non-metals are present in the middle portion of the periodic table.
Ans: True.
9. Each period in modern periodic classification begins with filling of electrons in a new shell.
Ans: True.
III. Fill in the blanks:
1. According to the modern periodic law, the properties of elements are periodic function of their …………………
Ans: Atomic numbers.
2. The ………………… number is same as the number of shell which in gradually filled up in the elements of this period.
Ans: Group.
3. In normal elements of a particular period the electrons are gradually filled in ………………… shell.
Ans: Lower.
4. All elements of a particular group have ………………… electronic configurations.
Ans: Same.
5. In the modern periodic table, groups are numbered from ………………… to …………………
Ans: 1 and 8.
6. The second and third periods of the periodic table are called ………………… periods.
Ans: Short.
7. The main group elements are present in group 1 and 2 on the left side and ………………… to ………………… on the right side of the periodic table.
Ans: 17.
8. All the group eighteen elements (except the first one) contain ………………… valence electrons.
Ans: High.
9. All transition elements are metals with ………………… melting and boiling points.
Ans: Relatively.
10. The group of 14 rare-earth elements belonging to the group 3 and 7th period are called …………………
Ans: Lanthanides.
11. All elements present in a given ………………… have the same valency.
Ans: Group.
12. Atomic size ………………… in a period from left to right.
Ans: Decrease.
13. Magnesium is ………………… metallic than calcium.
Ans: Less.
14. Carbon belongs to group ………………… of the Periodic table.
Ans: 14.
15. All the elements of group 15 have ………………… valence electrons.
Ans: 3.
B. Subjective Questions.
I. Very short Answer Questions (Answer in one word or one sentence)
1. What was the earliest classification of elements?
Ans: The first classification of elements was done by Dobereiner.In 1829, J.W. Dobereiner, a German chemist made groups of three elements each and called them triads. All three elements of a triad were similar in their physical and chemical properties. He proposed a law known as Dobereiner’s law of triads. According to this law, when elements are arranged in order of increasing atomic mass, the atomic mass of the middle element was nearly equal to the arithmetic.
2. State Newlands’ law of octaves.
Ans: The Newland law states the eight elements subsequently show the same property.
3. Which classification of elements failed after the discovery of noble gases?
Ans: Mendeleev classification failed after the discovery of noble gases.
4. State Mendeleev’s Periodic Law.
Ans: Mendeleev periodic law states that the elements are kept in the periodic table according to their atomic masses.
5. How were the groups numbered in the Mendeleev’s periodic table?
Ans: There are 8 group in Mendeleev periodic table.
6. Name the fundamental properties of element on which the modern periodic law is based.
Ans: Atomic number.
7. How many groups are there in the modern periodic table?
Ans: There are 18 group in modern periodic table.
8. How have groups been numbered in the modern periodic table?
Ans: 18 group been number in modern periodic table.
9. What are normal elements?
Ans: In group 1, 2 and from 13 to 17 are normal elements.
10. What are the elements present in the middle portion of the modern periodic table called?
Ans: Transition elements.
11. What is atomic size?
Ans: The size of elements including its shell.
12. How does atomic size vary in a period and in a group?
Ans: Atomic size increases down a group and decreases from left to right a period.
13. Where would the element with largest atomic size be placed in any group?
Ans: At the lowest point under a group.
14. Give the number of a group in which metallic, metalloid and non-metallic, all three types of elements, are present.
Ans: Group 14.
II. Short Answer Questions (Answer in 30-40 words).
1. State Dobereiner’s law of triads.
Ans: Dobereiner law of triads states that the average of the atomic masses of the first and third element in a triad will be roughly equal to the atomic mass of the second element in that triad.
2. Show that chlorine, bromine and iodine (atomic masses 35·5, 80 and 127 respectively) constitute a triad.
Ans: The mass of Bromine is =(35.5 + 127/2)=81.25. So, here we clearly see that this three-element chlorine, bromine, and iodine shows similar properties and forms a triad.
3. What were the reasons for the failure of Newlands’ law of octaves ?
Ans: Newland octave law states that eight consecutive elements in a periodic table show the similar characteristics but sometimes it fails to explain the condition where we see that although the atomic numbers are same this has no similar properties.
4. Describe Mendeleev’s periodic table briefly in terms of rows and columns and their raw being.
Ans: The Mendeleev periodic table has arranged the different elements according to their atomic masses. In this table the all the elements are arranged in 7 periods and 8 groups.
5. Give any two achievements of the Mendeleev’s Periodic classification.
Ans: Tow achievements of the Mendeleev ‘s periodic classification are mentioned below:
(i) Prediction of New Elements: Mendeleev left gaps in his table for undiscovered elements, predicting their properties based on the elements around them. He named these elements with the prefix “eka-” followed by the element it resembled. Notably, he predicted the properties of elements like Gallium and Germanium with remarkable accuracy, which were later discovered and confirmed his predictions.
(ii) Correction of Atomic Weights: In some cases, the atomic weights of certain elements were not fully accurate at the time. By placing elements based on their properties within the periodic table, Mendeleev was able to identify inconsistencies and propose corrections to the accepted atomic weights of certain elements, such as gold and platinum.
6. What were the defects in Mendeleev’s periodic classification.
Ans: Although the Mendeleev periodic table describes the different elements it has some defects like it doesn’t explain about the position of hydrogen and keep the different characteristics showing elements in the same place.
7. State modern periodic law.
Ans: The modern periodic law states that: The physical and chemical properties of the elements are the periodic function of their atomic numbers.
8. Briefly describe the modern periodic table in terms of groups and periods.
Ans: The modern periodic law the different elements are arranged according to their atomic number. This atomic number are increasing from lower to higher. In a modern periodic table, there are 7 periods and 18 groups.
9. Give names of four classes into which the elements have been classified and mention to which groups of the modern period table they belong.
Ans: In a modern periodic table we see the elements are arranged to its atomic number and the number of groups and periods in the modern periodic table is 18 and 7.
10. List the merits of the long form of the modern periodic table and explain any two of them.
Ans: The merits of the form modern periodic table are mentioned below:
(i) Position of isotopes.
(ii) Anomalous pairs.
(iii) Electronic configuration.
(iv) Separation of metals and non-metals
(v) Position of transition metals: It makes the position of the transition elements quite clear.
(vi) Properties of elements: It reflects the differences, the trends and the variations in the properties of the elements in the periodic table.
11. How are the electronic configurations of all the elements belonging to a particular group related? Explain with the help of group 17 elements.
Ans: The group 17 element in a periodic table is known as the halogen element. This element is fluorine, chlorine, bromine, iodine etc.
12. How does the electronic configuration of elements belonging to a particular period vary? Explain with the example of second period elements.
Ans: The electronic configuration of an element describes the different position of different element. For example, the electronic configuration of element Carbon (2,4) tells us about its position in table and group and periods number.
13. Define atomic radius.
Ans: Atomic radius or Atomic Radii is the total distance from the nucleus of an atom to the outermost orbital of its electron.
14. How and why does metallic character vary in a group from top to bottom?
Ans: As moving down the group, the metallic character increases in the periodic table. Since the distance increases between the valence electrons and nucleus, the attraction between them is also less.
III. Long Answer Questions (Answer in 60–70 words).
1. State Mendeleev’s Periodic Law and describe the periodic table constructed on this basis.
Ans: The chemical and physical properties of elements are a periodic function of their atomic masses.A periodic function is the one which repeats itself after a certain interval. Mendeleev arranged the elements in the form of a table which is known as the Mendeleev’s Periodic Table.
Mendeleev’s Periodic Table Mendeleev arranged the elements in the increasing order of their atomic masses in horizontal rows till he came across an element whose properties were similar to those of the first element. Then he placed this element below the first element and thus started the second row of elements. The success of Mendeleev’s classification was due to the fact that he laid more emphasis on the properties of elements rather than on atomic masses. Occasionally, he could not find an element that would fit in a particular position. He left such positions vacant for the elements that were yet to be discovered. He even predicted the properties of such elements and of some of their compounds fairly accurately. In some cases, he even reversed the order of some elements, if it better matched their properties. Proceeding in this manner, he could arrange all the known elements in his periodic table. When more elements were discovered, this periodic table was modified and updated to include them. One more group (zero group) had to be added when noble gases were discovered.
2. What are the merits and demerits of the Mendeleev’s Periodic classification?
Ans: The merits and demerits of the mendeleev’s periodic classification are mentioned below:
Merits
(i) Classification of all elements: Mendeleev’s classification included all the 63 elements known at that time on the basis of their atomic mass and facilitated systematic study of elements.
(ii) Correction of atomic masses: Atomic masses of some elements like Be (beryllium), Au (gold), In (indium) were corrected based on their positions in the table.
(iv) Prediction of new elements: Mendeleev arranged the elements in the periodic table in increasing order of atomic mass but whenever he could not find out an element with expected properties, he left a blank space. He left this space blank for an element yet to be discovered.
Demerits
(i) Position of Hydrogen: The position of hydrogen which is placed in group IA along with alkali metals is ambiguous as it resembles alkali metals as well as halogens (group VII A).
(ii) Position of Isotopes: All the isotopes of an element have different atomic masses therefore, each one of them should have been assigned a separate position. On the other hand, they are all chemically similar; hence they should all be placed at the same position. In fact, Mendeleev’s periodic table did not provide any space for different isotopes. For example, two isotopes of carbon are represented as 6C12, 6C14 but placed at the same position.
(iii) Anomalous Pairs of Elements: At some places, an element with greater atomic mass had been placed before an element with lower atomic mass due to their properties. For example, cobalt with higher atomic mass (58.9) was placed before nickel with lower atomic mass (58.7).
Other such pairs are:
(i) Tellurium (127.6) is placed before iodine (126.9).
(ii) Argon (39.9) is placed before potassium (39.1).
(iv) Grouping of chemically dissimilar elements Elements such as copper and silver have no resemblance with alkali metals (lithium, sodium etc.), but have been grouped together in the first group.
(v) Separation of chemically similar elements Elements which are chemically similar such as gold and platinum have been placed in separate groups.
3. Describe the modern periodic table in terms of groups and periods.
Ans: The periodic table based on the modern periodic law is called the Modern Periodic Table. Presently, the accepted modern periodic table is the Long Form of Periodic Table. It may be regarded as an extended form of Mendeleev’s table in which the subgroups A and B have been separated.
4. What are the following types of elements and where are they located in the periodic table?
(a) Main group elements.
(b) Noble gases.
(c) Transition elements.
(d) Inner transition elements.
Ans: The main element of the modern periodic table is kept in the right side of the table. The Noble gas element are kept in the group 18 of the modern periodic table which is called zero group. The transition element is kept in the middle of the modern periodic table. This element is called the d block elements.
5. Discuss the merits of the modern periodic table.
Ans: The merits of the modern periodic table are:
(i) Position of isotopes: All isotopes of an element have the same atomic number and therefore, occupy the same position in the modern periodic table.
(ii) Anomalous pairs: The anomaly regarding all these pairs disappears when atomic number is taken as the basis for classification. For example, cobalt (at. no. 27) would naturally come before nickel (at. no. 28) even though its atomic mass is little more than that of nickel.
(iii) Electronic configuration: This classification is according to the electronic configuration of elements, i.e., the elements having a certain pattern of electronic configuration are placed in the same group of the periodic table. It relates the properties of elements to their electronic configurations. This point will be further elaborated in the next section.
(iv) Separation of metals and non-metals: The position of metals, nonmetals and metalloids are clearly established in the modern periodic table.
(v) Position of transition metals: It makes the position of the transition elements quite clear.
(vi) Properties of elements: It reflects the differences, the trends and the variations in the properties of the elements in the periodic table.
(vii) This table is a simple, systematic and easy way of remembering the properties of different metals.
6. What is the relationship between the electronic configuration and the modern periodic table?
Ans: The electronic configuration of an element describes the different position of different elements. For example, the electronic configuration of element sodium (2,8,1) tells us about its position in table and group and period number.
7. Explain the variation of atomic size in a group and in a period.
Ans: The atomic radius of an element is depending on the group and period as the radius is increasing from going downward to the group whereas if decreasing from left to right of a period. As a result of that we see magnesium has more size than phosphorus.
8. How is metallic character related to ionisation energy ? Explain the variation of metallic character in the periodic table.
Ans: The metallic character and first ionisation energy of an element are inversely related. Elements with high metallic character possess low ionisation energies, which means they can easily lose their valence electrons. Metallic character increases down the group and decreases from left to right in a period. This is due to the Fazan’s Rule as the size of the cation increases, its polarisability decreases and it forms ionic compounds.
