5. PERIODIC CLASSIFICATION OF ELEMENTS
Questions:
1. The atomic number of an element X’ is 16. In the modem periodic table the elements belongs to this block and period.
(a) p – block, 3rd period (b) p – block, 2nd period
(c) s – block, 3rd period (d) s – block, 2nd period
Ans: (a) p – block, 3rd period
2. Write the number of Vertical columns in the modern periodic table. What are these columns called?
Ans: There are 18 vertical columns in the modem periodic table and these are known as groups.
3. What are cations and anions?
Ans: Positively charged ions are called cations and negatively charged ions are called anions.
4. State Mendeleev’s periodic law. Write two achievements of Mendeleev’s periodic table.
Ans: Mendeleev’s Periodic Law “Properties of elements are the periodic function of their atomic masses.” Achievements:
(i) It could classify all the elements discovered at that time.
(ii) It helped in discovery of new elements.
(iii) It helped in correction of atomic mass of some of the elements.
5. Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at “Periodic Law”? [NCERT Exemplar]
Ans: (i) When Mendeleev started his work, 63 elements were known. He studied the compounds of these elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.
(ii) Elements with similar properties were arranged in a group.
(iii) Mendeleev observed that elements were automatically arranged in the order of
increasing atomic masses.
6. (i) What are metalloids? (ii) Name any four metalloids?
Ans: (i) Borderline elements which are intermediate in properties between metals and non-metals are called metalloids. (ii) Boron, silicon, germanium and arsenic.
7. What were the limitations of Newlands’ law of Octaves?
Ans: The limitations of Newlands’ Law of Octaves are:
(i) The law was applicable to elements upto calcium (Ca) only.
(ii) It contained only 56 elements. Further it was assumed by Newland that only 56 elements existed in nature and no more elements would be discovered in the future.
(iii) In order to fit elements into the table, Newland adjusted two elements in the same column as fluorine, chlorine and bromine which have very different properties than these elements. Iron, which resembles cobalt and nickel in properties, has been placed differently away from these elements.
8. (a) Why did Mendeleev have gaps in his periodic table?
(b) State any three limitations of Mendeleev’s classification?
(c) How does electronic configurations of atoms change in a period with increase in atomic number?
Ans: (a) Gaps were left for undiscovered elements in the periodic table.
(b) (i) Position of hydrogen was not justified.
(ii) Increasing order of atomic mass could not be maintained.
(iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.
(c) Number of shells remains the same, number of valence electrons goes on increasing from left to right in a period fill octet is complete. e.g.,
9. A part of s-block in the modern periodic table is given here. The correct arrangement of
these atoms in the increasing order of there atomic size is [SSLC June, 2016]
(a) Na,Mg K (b) K, Na, Mg (c) Mg, Na, K (d) Na, K, Mg
Ans: (c) Mg, Na, K
10. Write any one difference between the electronic configuration of group-i and group-2 elements
. Ans: Group 1 elements have 1 valence electron while Group 2 elements have 2 valence electrons.
11. List any two properties of the elements belonging to the first group of the modern periodic table.
Ans: First group elements are also known as alkali metals. They possess following properties:
(i) These elements exhibit +1 valency.
(ii) These are very reactive as they are not found freely in nature.
12. What is ionisation energy ? How does ionisation energy vary along the period in the modern periodic table? [SSLC June, 2016]
Ans: The energy required to remove the electron from the outermost shell of an isolated gaseous atom is known as ionisation energy. It increases while going from left to right in modern periodic table.
13. What are electropositive atoms? How does electro positivity vary along the period in modern periodic table? [SSLC June 2016]
Ans: Atoms which give up electrons to form cations are called electropositive atoms. Decreases while going from left to right.
14. How does the atomic radius change as we go: (i) From left to right across a period (ii) Down a group in the periodic table?
Ans: On going (i) From left to right across a period, atomic radius decreases as one proton and one electron are added continuously, therefore effective nuclear charge increases.
(ii) Down a group in the periodic table, atomic radius increases because number of shells goes on increasing, therefore distance between nucleus and valence shell increases.
15. (i) Atomic radius of hydrogen is 37 pm. Express it in metres.
(ii) How does atomic size vary in a group and in a period.
Ans: (i) 1pm = 10−12m. Atomic radius = 37 x 10−12 m
(ii) In a group atomic size: increases down the group. In a period atomic size: decreases in moving from left to right.
16. The atomic number of elements A, B, C, D and E are given below
Element A B C D E Atomic No 7 10 12 4 19 From the above table answer the following questions:
(i) Which two elements are chemically similar?
(ii) Which is an inert gas?
(iii) Which element belongs to 3rd period of periodic table?
(iv) Which element among these is a non-metal?
Ans: (i) C and D (ii) B (iii)C (iv)A
17. How many groups and periods are there in the modern periodic table? How do the atomic size and metallic character of elements vary as we move?
(i) Down a group and
(ii) From left to right in a period.
Ans: There are 18 groups and 7 periods in the modern periodic table.
(i) Atomic size generally increases down a group due to the addition of new shell and metallic character also increases down a group due to the increase in tendency to lose electrons.
(ii) Atomic size goes on decreasing along a period from left to right. Metallic character decreases along a period due to decrease in tendency to lose electrons.
18. What is meant by ‘group’ in the modern periodic table? How do the following change on moving from top to bottom in a group?
(i) Number of valence electrons
(ii) Number of occupied shells
(iii) Size of atoms
(iv) Metallic character of element
(v) Effective nuclear charge experienced by valence electrons.
Ans: Vertical columns of the periodic table are known as groups.
(i) The number of valence electrons remains constant when we move down the group.
(ii) The number of occupied shells increases down the group.
(iii) The size of atom increases down the group.
(iv) The metallic character of elements increases down the group.
(v) The effective nuclear charge decreases down the group.
19. Name any two elements of group one and write their electronic configurations. What similarity do you observe in their electronic configurations? Write the formula of oxide of any of the above said element.
Ans: (i) Two elements of group 1 are Na, K / Sodium, potassium. Electronic configurations Na = 2,8,1; K = 2,8,8,1
(ii) Similarity: Both have one valence electron / One electron in outermost shell.
(iii) Oxide: Na2O / K2O.
20. Two elements ‘A’ and ‘B’ belong to the 3 period of Modern periodic table and are in group 2 and 13 respectively. Compare their following characteristics in tabular form:
(i) Number of electrons in their atoms
(ii) Size of their atoms
(iii) Their tendencies to lose electrons
(iv) The formula of their oxides
(v) Their metallic character
(vi) The formula of their chlorides.
Ans:
21. An element ‘X’ belongs to 3rd period and group 16 of the Modern Periodic Table.
(i) Determine the number of valence electrons and the valency of ‘X’.
(ii) Molecular formula of the compound when ‘X’ reacts with hydrogen and write its electron dot structure.
(iii) Name the element ‘X’ and state whether it is metallic or non-metallic.
Ans: (i) Electronic configuration of X = 2,8,6.
Valence electrons = 6,
Valency = 8 – 6 = 2
(ii) Formula with hydrogen − H2X or H2S
22. Na, Mg and Al are the elements of the same period of modern periodic table having one, two and three valence electrons respectively. Which of these elements
(i) has the largest atomic radius,
(ii) is least reactive? Justify your answer stating reason for each case.
Ans: (i) Na will have the largest atomic radius as atomic size goes on decreasing along a period from left to right. It is due to increase in nuclear charge (number of protons in nucleus) which pulls the electrons towards it, i.e., force of attraction between nucleus and valence electrons increases, therefore atomic size decreases.
(ii) Al is least reactive because reactivity of an element depends upon the ability of its atoms to donate or accept electrons. Tendency to lose electrons along a period generally decrease with decrease in atomic size, i.e., the force of attraction between the valence electrons and the nucleus increases, therefore electrons cannot be removed easily.
23. Four elements F, Q, K and S belong to the third period of the Modern Periodic Table and have respectively 1,3,5 and 7 electrons in their outermost shells. Write the electronic configurations of Q and K and determine their valencies. Write the molecular formula of the compound formed when P and S combine.
Ans: Electronic configuration of Q = 2, 8, 3
Valency of Q = 3
Electronic Configuration of R = 2, 8, 5
Valency of R = 8 − 5 = 3
Electronic configuration of P 2, 8, 1
Electronic configuration of S = 2, 8, 7
Formula: PS/NaC1
24. Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the modern periodic table:
On the basis of the above table, answer the following questions:
(i) Name the element which forms only covalent compounds.
(ii) Name the element which is a metal with valency three.
(iii) Name the element which is a non-metal with valency three.
(iv) Out of D and E, which is bigger is size and why?
(v) Write the common name- for the family to which the elements C and F belong.
Ans: (i) Element E will form covalent compounds by sharing its four valence electrons.
(ii) Element D is a metal with valency three.
(iii) Element B is a non-metal with valency three.
(iv) We know that the size of elements decreases on moving left to right in a period. Therefore, element D is greater than element E.
(v) The name of the family to which elements C and F belong is the noble gas family (group 18).
25. Four elements A, B, C and D along with their electronic configuration are given below:
Now answer the following questions: (i) Which two elements belong to the same period?
(ii) Which two elements belong to the same group?
(iii) Which element out of A and C is more reactive and why?
Ans: (i) A and B, C and D belong to the same period.
(ii) A and C have one valence electron, so they belong to same group.
(iii) C is more reactive because C is placed below A in the periodic table and reactivity increases down the group.
26. Write the number of period the modern periodic table has. How do the valency and metallic character of elements vary on moving from left to right in a period? How do the valency and atomic size of elements vary down a group?
Ans: There are 7 horizontal rows known as periods, that the modern periodic table has. In periods, the number of valence electrons increases from left to right and due to decrease in atomic size, the force of attraction between the valence electrons and the nucleus increases. Metallic character decreases along the period due to decrease in tendency to lose electrons. Atomic size generally increases from top to bottom in a group due to the addition of a new shell i.e., number of shells goes on increasing down the group.
27. Examine the elements of the third period and classify them as metals or non-metals. Comment on the metallic and non-metallic character of the third period. Name the most metallic element and non-metallic element of this period.
Ans: Na (Sodium), Mg (Magnesium) and Al(Aluminium) are metals. Si (Silicon) is a metalloid. P (Phosphorus), S (Sulphur) and Cl (Chlorine) are non-metals. As we go from left to right across the third period, metallic character decreases and non-metallic character increases. The most metallic element in the third period is Na and the most non-metallic element is Cl.
28. An element X (2, 8, 2) combines separately with SO24−, PO34− radicals.
(i) Write the formulae of the two compounds so formed.
(ii) To which group of the modern periodic table does the element X belong.?
(iii) Will it form covalent or ionic compound? Give reasons.
Ans: (i) Formulae of these compounds are XSO4, X3(PO4)2.
(ii) It belongs to group 2nd and 3rd period.
(iii) It will form ionic compounds as it can lose two electrons to acquire stable electronic configuration.
29. What were the limitations of Newlands’ Law of Octaves?
Ans: (i) The law was applicable to elements upto calcium (Ca) only.
(ii) It contained only 56 elements. Further it was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future.
(iii) In order to fit elements into the table, Newland adjusted two elements in the same column as fluorine, chlorine and bromine which have very different properties than these elements. Iron, which resemble cobalt and nickel in properties, has been placed differently away from these elements.
30. Name (a) Three elements that have a single electron in their outermost shell.
(b) Two elements that have two electrons in their outer most shell.
(c) Three elements with filled outermost shell.
Ans: (a) Li, Na, K (b)Be,Mg (c) He, Ne, Ar
Textbook Exercises
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.
Ans: (c) The atoms lose their electrons more easily.
2. Element X forms a chloride with the formula XCl2, which is a solid with high melting point. X would most like be in the same group of the periodic table as
(a) Na (b) Mg (c) Al (d) Si
Ans: (b) Mg
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 in its valence shell?
(e) Twice as many electrons in its second shell as in its first shell.
Ans: (a) Ne (b) Mg (c) Si (d) B (e) C
4. (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?
Ans: (a) All the elements have 3 valency and their oxides are generally amphoteric. (b) All the elements have valency 1, high non metallic character and from ionic bonds.
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? N (7), F (9), P (15), Ar (18)
Ans: (a) 17 (b) F (9)
6. The position of three elements A, B and C in the periodic Table is shown below:
(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 C?
Ans: (a) Element A will be a non-metal
(b) C will be less reactive than A
(c) C will be smaller than B
(d) C will form anion
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?
Ans: N = 2, 5 ; P = 2, 8, 5 Due to smaller size, nitrogen atom will be more electronegative.
8. How does the electronic configuration of an atom related to its position in the modern periodic table?
Ans: The position of elements depends upon number of valence electrons which depend upon electronic configuration. Those elements which have same valence electrons, occupy same group. Those elements which have one valence electron belong to group Elements which have two valence electrons belong to group 2. Period number is equal to the number of shells.
9. 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: The element with atomic no. 12 and 38 will have same physical and chemical properties.
10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and Modern Periodic Table
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