PERIODIC CLASSIFICATION OF ELEMENTS
Page 81
Q.1 Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Sol.
Yes, Dobereiner’s traiads exists in the columns of Newland’s Octaves.
Example – In the Dobereiner’s triads Lithium, Sodium and Potassium make a triad, and these elements also present in the same column of Newland’s Octaves as these have same properties.
Q.2 What were the limitations of Döbereiner’s classification?
Sol.
Limitations of Dobereiner’s classification:
(i) Dobereiner could find only three triads; .i.e total of 9 elements only.
(ii) However the total number of elements were more than that of those encompassed in Dobereiner’s Triad.
(iii) Thus, Dobereiner’s could not be classify most of the elements known at that time.
Q.3 What were the limitations of Newlands’ Law of Octaves?
Sol.
Limitations of Newland’s Law of Octaves:
(i) Newland’s Law of Octaves was applicable up to calcium only.
(ii) Elements after Calcium did not obey the Newland’s Law of Octaves.
(iii) Newland adjusted two elements under the same slot inspite of their different properties, such as cobalt and chlorine were put in the same note while there properties were different.
(iv) It appears that Newland assumed that only 56 elements existed in nature; after discovery of more elements his Law could no longer hold true.
Page 85
Q.1 Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.
Sol.
(i) The general formula for oxides of 1st, 2nd, third and 4th group are R2O, RO, R2O3 and RO2 respectively in the Mendeleev’s Periodic Table, where R denotes the element.
(ii) Since, Potassium (K) belongs to 1st group, thus, formula of its oxide would be K2O.
(iii) Barium (Ba) belongs to 2nd group, thus formula of its oxide would be BaO.
(iv) Aluminium (Al) belongs to the 3rd group, thus, formula of its oxide would be Al2O3.
(v) Carbon (C) and Silicon (Si) belong to the 4th group, thus, formula of its oxide would be CO2 and SiO2 respectively.
(vi) Thus, formula of oxides of the given elements would be K2O, CO2, Al2O3, SiO2 and BaO; respectively.
Q.2 Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)
Sol. Scandium and Germanium
Q.3 What were the criteria used by Mendeléev in creating his Periodic Table?
Sol.
Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.
Q.4 Why do you think the noble gases are placed in a separate group?
Sol.
Since, noble gases are inert and all of them show similar properties, thus they have unique properties compare to other elements, thus they are placed in a separate group in Periodic Table.
Page 90
Q.1 How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Sol.
Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:
(i) Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
(ii) In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.
(iii) Elements having same valence electron are kept in same group.
(iv) Elements having same number of shells were put under the same period.
(v) Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.
Q.2 Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Sol.
Beryllium and Calcium. These elements show chemical reactions similar to magnesium because magnesium, calcium and beryllium belong to same group, i.e. Group 2nd in the Modern Periodic Table.
Q.3 Name
(a) Three elements that have a single electron in their outermost shells.
(b) Two elements that have two electrons in their outermost shells.
(c) Three elements with filled outermost shells.
Sol.
(a) Hydrogen, Lithium and Sodium
(b) Answer: Calcium and Magnesium
(c) Neon, Argon and Krypton
Q.4 Explain the following:
(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?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Sol.
(a) Lithium, Sodium and Potassium have following similarities:
(i) They all have valence electron equal to 1.
(ii) They all belong to same group, i.e. Group 1st in Periodic Table.
(iii) They all form ionic bond with non-metals.
(iv) They all are highly electropositive.
(v) They all are highly reactive.
(b) Following properties are common in the atoms of Helium and Neon:
(i) Both have completely filled outermost orbit.
(ii) Both have stable electronic configuration.
(iii) Both belong to same group, i.e. zero group in Modern Periodic Table.
Q5. In the Modern Periodic Table, which are the metals among the first ten elements?
Sol. Lithium and Beryllium
Q.6 By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? Ga, Ge, As, Se, Be
Sol.
Metallic character of elements depends upon the position in periodic table. Group numbers of given elements are as follows:
(i) Beryllium (Be) – Group 2nd
(ii) Ga (Gallium), Ge(Germanium), As (Arsenic) and Se (Sellenium) belongs to 13th, 14th, 15th and 16th groups respectively.
(iii) Since, metallic character decreases by moving left to right in a period, thus, Beryllium is the most metallic as it is at the left of 2nd period and Selenium is a non-metal as it is right most in the third period.
(iv) Thus, Beryllium is most metallic and selenium has least metallic character as it is a non-metal.
Back Exercise Questions
Q.1 Which of the following statements is not a correct statement about the trends when going from left to right across the periods of 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.
Sol.
(c) The atoms lose their electrons more easily
(c) The atoms lose their electrons more easily
Q.2 Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na (b) Mg (c) AI (d) Si
Sol. (b) Mg
Q.3 Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Sol.
(a) Ne (Neon)
(b) Mg (Magnesium)
(c) Si (Silicon) (14): 2, 8, 4
(d) B (Boron) (5): 2, 3
(e) C (Carbon) (6): 2, 4
Q.4 Answer the following:
(a) What property do all elements in the same column of the Periodic Table as boron have in common?
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Sol.
(a) Boron is metalloid while all other elements in the same column of the Periodic Table are metals, thus metallic character is common in the same column of the periodic table as boron have.
(b) All elements belong to same column as fluorine, are non-metals and are highly electronegative in character. All of them have valence electrons equal to seven, and thus can accept one electron to complete their octet.
Q.5 An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(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)
Sol.
(a) 17
(b) F (9) Fluorine (As fluorine has valence electron equal to seven)
Q.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 a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Sol. (a) Non-metal
(b) Less reactive than A
(c) Smaller
(d) Answer: Anion
Q.7 Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Sol.
Nitrogen (7): 2, 5
Phosphorus (15): 2, 8, 5
Since, electronegativity decreases with moving from top to bottom in a group, thus Nitrogen will be more electronegative.
Q.8 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Sol.
Electronic configuration of an element gives the information of valence electrons and number of shell present in the element.
We get the information of group number after knowing valence electrons.
Number shell present in an element is equal to period number.
Thus, by knowing electronic configuration we know the group number and period number of an element, which is the position of element in periodic table.
Q.9 In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Sol. Elements with atomic numbers 12 and 38 have the similar chemical and physical properties of calcium.
Q.10 Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.
Sol.
Comparison of Mendeleev’s Periodic Table and Modern Periodic Table
| Mendeleev's periodic table | Modern periodic table |
|---|---|
| 1. Elements were arranged in increasing order of atomic masses. | 1. Elements are arranged in increasing order of atomic numbers. |
| 2. There are 8 groups. | 2. There are 18 groups. |
| 3. Each group is divided into subgroups a and b. | 3. Groups are not divided into subgroups. |
| 4. The group for noble gases was not present, as noble gases were not discovered at that time. | 4. A separate group, i.e. group 18 is present for noble gases. |
| 5. There was no place for isotopes. | 5. This problem was rectified, as slots are determined on the basis of atomic number. |
Tags
Class 10