Periodic Classificartion Of Elements
Class: 10 | Chapter: 5
Important Questions & Answer
1. Alkali metals do not form dipositive ions why?Ans. Dipositive ions are formed when an element looses its two electrons. Alkali metals have not two electrons but have only one electron in last shell so, it looses one electron to get stable electronic configuration.
2. Why non-metals are called electro negative elements?
Ans. Electronegative elements are those elements which have tendency to gain electrons to form negative ions. Non - metals like oxygen,nitrogen etc show this behaviour that's why they are called electronegative.
3. How were the positions of different isotopes decided in the modern periodic table?
Ans. Modern periodic table is based on atomic number and Isotopes are the atoms of same element which have different mass number and same atomic number therefore they aquire same position.
4. Hydrogen is regarded as a rogue element in the periodic table. Discuss.
Ans. It is becouse Hydrogen resembles in properties with both alkali metals and halogens. In some properties it shows the properties of alkali metals and in some other properties it is same as the halogens therefore, it's position is controversial.
5. Why do elements in a group show same valency?
Ans. In a group, all elements have same outermost electronic configuration like all alkali metals have one electron in their last shell hence they show same valency.
6. Why are the members of group 1 known as alkali metals?
Ans. Group 1 elements are called as alkali metals becouse they are metals with high reactivity and when they react with water, they form basic (alkaline) solutions.
7. Name other elements which belong to the same family as
(i) Calcium (ii) Carbon
Ans. (a) Berilium, Magnesium, Strotium, Barium
(b) Silicon, Germanium, tin, lead.
8. How could the Modern Periodic table remove various anomalies of Mendeleev’s Periodic Table?
Ans. Modern periodic table removed various anomalies of Mendeleev's periodic table such as :
1. Position of isotopes : In Modern periodic table, all isotopes have same position becouse all have same atomic number.
2. Inversion in the periodic table : This anomaly also removed becouse these elements were arranged in the increasing order of atomic number.
9. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans. Calcium, Strontium show same chemical reactions as magnesium becouse they belong to same group.
10. In the Modern Periodic Table, which are the metals among the first ten elements?
Ans. Lithium and Berilium.
11. (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 common?
Ans. (a) (a) Boron has electronic configuration 2, 3 and have valency 3 . So, all elements in the same column will have same general electronic configuration, valency 3 and will be metallic.
(b) Fluorine has electronic configuration 2, 7. It has valency 8 - 7 = 1 and is non - mettalic. Therefore, all elements in same column will have 7 electrons in last shell, valency 1 and will be non - metallic.
12. 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)
Ans. (a) Atomic no. = No. Of Electrons
So, atomic no. = 2 + 8 + 7 => 17
(b) Chemical nature depends upon electronic configuration. So, it will be similar to F (9) having electronic configuration 2,7.
13. 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?
Ans. N = 2, 5
P = 2, 8, 5
N will be more electronegative becouse electronegativity decreases down the group.
14. In the modern Periodic Table calcium (atomic number 20) is surrounded by elements with atomic number 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Ans. Ca = 2, 8, 8, 2
12 = 2, 8, 2
19 = 2, 8, 8, 1
21 = 2, 8, 8, 1, 2
38 = 2, 8, 18, 2
Elements with atomic no. 12 and 38 will resemble with calcium becouse these elements have same outermost shell configuration.
15. A metal M forms an oxide having the formula M2O3. It belongs to 3 period in the modern periodic table. Write the atomic number and valency of the metal.
Ans. It's oxide gas formula , M2O3 which indicates that M had valency 3 and it belongs to 3 period indicates that last 3 electrons are in 3rd shell So, it should be Aluminium having atomic number 13 and valency 3.
16. The atomic numbers of three elements X, Y and Z are 3, 11 and 17 respectively.
State giving reason which two elements will show similar chemical properties.
Ans. X (2, 1) and Y(2, 8, 1) will show similar chemical properries becouse they both gave same outermost shell electronic configuration.
17. How many vertical columns are there in the modern periodic table and what are they called?
Ans. 18 vertical columns are there in the modern periodic table and these are called as groups.
18. Write two reasons responsible for late discovery of noble gases?
Ans. 1. They are present in very less quantity in air.
2. They are unreactive so, ut was difficult to isolate them.
19. An element ‘X’ has atomic number 13:
(a) Write its electron configuration.
(b) State the group to which ‘X’ belongs?
(c) Is ‘X’ a metal or a non-metal?
(d) Write the formula of its bromide.
Ans. (a) 2, 8, 3.
(b) X belongs to group 13.
(c) X is a metal.
(d) AlBr3
20. An element ‘M’ has atomic number 11:
(a) Write its electron configuration.
(b) State the group to which ‘M’ belongs?
(c) Is ‘M’ a metal or a non-metal?
(d) Write the formula of its chloride.
Ans. (a) 2, 8, 1
(b) M belongs to group 1.
(c) M is a metal.
(d) MCl.
21. Choose from the following:
20Ca, 3Li, 11Na, 10Ne
(i) An element having two shells completely filled with electrons.
(ii) Two elements belonging to the same group of the periodic table.
Ans. (a) 10 Ne with electronic configuration 2,8.
(b) 3 Li, 11 Na.
22. The atomic number of an element is 16. Predict (i) the number of valence electrons in its atom
(ii) its valencies
(iii) its group number
(iv) whether it is a metal or a non-metal
(v) the nature of oxide formed by it.
Ans. (i) The atomic no. is 16 . So, electronic configuration is 2,8,6 and gence valence electrons will be 6 .
(ii) Valency = 8 - 6 => 2
(iii) Group 16.
(iv) It is a non - metal.
(v) Oxides will be acidic.
23. Explain the variation of the following properties of the element in the periodic table.
(i) Atomic radius in a period.
(ii) Metallic character in a period.
(iii) Valency in a group.
Ans. (i) Atomic radius or size decreases in a period becouse of increase in no. of protons and electrons simultaneously. Nuclear attraction increases and hence electronic shells will shrink.
(ii) Metallic character in a period decrease becouse of decrease in atomic radius. When atomic size decreases, it is very difficult to loose electron.
(iii) Valency remains same through out the group becouse in a particular group electronic configuration is same.
24. The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.
Ans.It is called Dobereiner's triad arrangement. Li, Na and K follow the same arrangement.
25. Elements have been arranged in the following sequence on the basis of their increasing atomic masses.
F, Na, Mg, Al, Si, P, S, Cl, Ar, K
(a) Pick two sets of elements which have similar properties.
(b) The given sequence represents which law of classification of elements?
Ans. (a) F, Cl and Na, K have similar properties.
(b) Newland's law of octaves.
26. Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendele′ev.
Ans. Eka - silicon is Germanium and it's chloride formula is GeCl4. Eka - aluminium is Gallium and it's chloride formula is GaCl3.
27. Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.
Ans. A belongs to group 13, B belongs to group 14 and C belongs to group 2.
Valency of A = 3
Valency of B = 4
Valency of C = 2
28. Arrange the following elements in increasing order of their atomic radii.
(a) Li, Be, F, N
(b) Cl, At, Br, I
Ans. (a) F < N < Be < Li
(b) Cl < Br < I < At
29. Identify and name the metals out of the following elements whose electronic configurations are given below.
(a) 2, 8, 2
(b) 2, 8, 1
(c) 2, 8, 7
(d) 2, 1
Ans. (a) Magnesium
(b) Sodium
(c) Chlorine
(d) Lithium
30. Identify the elements with the following property and arrange them in increasing order of their reactivity
(a) An element which is a soft and reactive metal
(b) The metal which is an important constituent of limestone
(c) The metal which exists in liquid state at room temperature.
Ans. (a) Sodium (Na)
(b) Calcium (Ca)
(c) Mercury (Hg)
Reactivity order : Hg < Ca< Na
31. What is the similarity in the electronic configuration of Mg, Ca and Sr ?
Ans. Mg, Ca and Sr have same outermost shell electronic configuration. They all have 2 electrons in the last shell.
32. Name the members of alkaline earth family. Which out of them is radioactive in nature ?
Ans. Berilium, Magnesium, Calcium, Strotium, Barium and Radium. Radium is radioactive.
33. The two isotopes of chlorine have atomic mass 35 u and 37 u. Should they be placed in separate slots in the periodic table ?
Ans. No, they will be placed in same slot in modern periodic table becouse they have same atomic number which is 17.
34. An element “X” has mass number 35 and number of neutrons is 18.
Identify group number and period of the element “X”.
Ans. We know that, Mass Number = No. of protons + No. of neutrons
=> 35 = no. of protons + 18
=> no. of protons / atomic no. = 17
So, it is chlorine. It lies in third period and group 17.
35. Name three elements which behave as metalloids.
Ans. Silicon, Boron and Germanium.
36. Which common property do all the elements present in the same group have as the element fluorine ?
Ans. All the elements have 7 electrons in last shell. They have valency 1 and are non - metals.
37. Write the electronic configuration of the element present in the third period and seventeenth group of the periodic table.
Ans. Cl = 2, 8, 7
38. What is the basic difference in the electronic configuration of the elements belonging to group 1 and group 2 ?
Ans. Group 1 elements gave only one electron in their outermost shell and group 2 elements have two electrons in last shell.
39. Three elements X, Y and Z belong to 17th group but to 2nd, 3rd and 4th period respectively. Number of valence electrons in Y is 7.
Find the number of valence electrons in X and Z.
Ans. Since they belong to same group no. 17th So, X and Z will be having 7 valence electrons.
40. Element ‘Y’ with atomic number 3 combines with element A’ with atomic number 17. What would be the formula of the compound ?
Ans. Y has electronic configuration 2, 1 and A has 2, 8 ,7 . Both have valency 1 . Hence the formula will be YA.
41. The electronic configuration of an element is 2, 8, 6. Identify the element and name of the family to which it belongs.
Ans. It is Sulphur and it belongs to chalcogens family.
42. How would tendency to gain electrons change as you go down a group ?
Ans. Tendency to gain electrons in a group decreses becouse atomic size increases and attraction for new incoming electron decreases.
43. Identify the elements X and Y from the following information
(a) X has 17 protons and 18 neutrons
(b) Y has 17 protons and 20 neutrons.
Ans. X and Y are isotopes of Chlorine becouse they have same atomic no. ( No. Of Protons) but different no. of neutrons.
44. Identify the elements from the following characteristics and arrange them in increasing order of metallic character.
(a) An element which imparts golden yellow colour to the flame.
(b) An element whose oxide is used as a white wash.
(c) An element which is constituent of chlorophyll i.e. green colouring matter in plants.
Ans. (a) Na
(b) Ca
(c) Mg
45. An element ‘X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of ‘X’. Also write group number, period number and valency of ‘X’.
Ans. Mass no . = P + N
35 = P + 18
=> P = 17
So, atomic no. = 17 and electronic configuration = 2, 8,7
Group no. = 17, Period no. = 3rd Valency = 1
46. Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic configuration for these elements. To which period of the modern periodic table do these two elements belong ? What type of bond will be formed between them and why ?
Ans. X = 2, 8, 2 and Y = 2, 8, 6
These elements belong to 3rd period of the modern periodic table. They form ionic bond between themthem becouse X is metal and Y is non - metal.
47. An element has electronic configuration 2, 8, 3. What is the atomic number of the element ? To which
(i) group and
(ii) period does this element belong ?
Ans. Atomic no. = 2 + 8 + 3 = 13
(i) group = 13
(ii) period = 3rd
48. Write two reasons responsible for the late discovery of noble gases.
Ans. 1. They are present in very less quantity in air.
2. They are unreactive so, ut was difficult to isolate them.
49. Three elements A, B and C have atomic numbers 7, 8 and 9 respectively.
(a) What would be their positions in the modern periodic table ? (Mention group and period both)
(b) Arrange A, B and C in decreasing order of their size.
(c) Which one of the three elements is most reactive ?
Ans. (a) All are placed in 2nd period. A belongs to 15th group, B belongs to 16th and C belongs to 17th group.
(b) A > B > C
(c) C
50. The elements with atomic number 3 to 10 belong to the second period. Taking into account the trends in the general periodic properties, predict.
(a) The most electronegative element
(b) The most electropositive element
(c) The element belonging to noble gas family
(d) The element which constitutes large number of organic compounds.
Ans. (a) Fluorine
(b) Lithium
(c) Neon
(d) Carbon