Class 9th Chemistry Unit 3 Question And Answer
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Unit 3
Q1. Define periodic table?
Ans: The arrangement of elements in groups and periods on the basis of their similarities and properties is called periodic table.
Q2. Define Doberiener triads?
Ans: A German chemist doberiener observed relationship between atomic masses of several group of three elements called triads. In these groups the central or middle elements had atomic mass average of the other two elements.
Q3. Give Example of Doberiener Triads?
Ans: For Example, one triad group is that of calcium (40) strontium (88) and barium (137) the atomic, there was a repetition in chemical properties of every eight elements if they were arranged by the increasing atomic masses.
Q4. Describe Drawbacks of Newland of octaves?
Ans: Newland work could not get much recognition because There mass of strontium is the average of the atomic masses of calcium and barium
Q5. Describe Newland of octaves?
Ans: In 1864 British chemist Newlands put forward his observations in the form of law of octaves. Ho noted that was no space considered for undiscovered element. ii. The noble Gases were also not known at that time.
Q6. Who was Mendeleev?
Ans: Mendeleev (1834-1907) was a Russian chemists and inventor. He was the creator of first version of periodic table.
Q7. Describe Mendeleev’s periodic law?
Ans: Properties of elements are the periodic function of their atomic masses.
Q8. Write down the Drawbacks of Mendeleev’s periodic table?
Ans: There was following drawbacks: i. His failure to explain the position of isotopes. ii. There was wrong order of the atomic masses of some elements.
Q9. Describe Modern periodic law?
Ans: In 1913 H. Moseley described modern periodic law. “Properties of the elements are the periodic function of their atomic number”
Q 10. Write down significance of atomic number?
Ans: Atomic number is more fundamental properties then atomic masses because atomic number of every element is fixed and it is increases regularly from element to element. No two elements can have the same atomic number.
Q 11. Why the improvement in Mendeleev’s periodic table was made?
Ans: Because atomic number of element is more fundamental property then atomic mass in two respects. i. In increases regularly from element to element. ii. It is fixed for every element.
Q 12. Who introduced the name of periodic table?
Ans: In 1869 Mendeleev introduced the name of periodic
Q 13. Why and how elements are arrange in a period?
Ans: When the elements are arranged according to i to increasing atomic number from left to right in a horizontal row (period) properties of f elements were found repeating after regular intervals.
Q 14. Define groups and periods?
Ans: Groups: vertical columns present in the periodic table are called groups. It study from top to bottom. Period: Horizontal rows present in the periodic table called period. Is study from left to right.
Q 15. Define group number and period number?
Ans: Group Number: it tells about the number of electrons present in the outermost shell of an atom. Bns: Period Number: it tells about the number of electronic shells present in the atoms.
Q 16. How many groups and periods present in Modern periodic Table?
Ans: There are 18 groups and 7 periods present in the Modern periodic table.
Q 17. How many blocks are present in Modern periodic Table?
Ans: There are four blocks present in Modern periodic table i. S-Block ii. P-Block iii. D-Flock iv. F-flock
Q 18. How elements are arranged into 4-Blocks?
Ans: On the basis of completion of a particular sub shell, elements with similar sub shells, electronic configuration are referred as a block of elements.
Q 19. Define normal or representative elements?
Ans: Elements belong to s and p block are called normal or representative element.
Q 20. Define transition elements.
Ans: Those elements which belong to d and f blocks are called transition elements.
Q 21. Who were Alchemists?
Ans: Alchemists are groups of scientist who were trying to convert inferior metal to superior metals and trying to get secret of eternal life.
Q 22. Write down the names of elements belonging to group-1?
Ans: Group-1 consists of hydrogen, lithium, sodium, potassium, rubidium, cesium and francieum.
Q 23. How the properties of elements repeat after regular intervals?
Ans: Properties of elements repeat after regular intervals because left to right. intervals b of increasing atomic number from
Q 24. In which pattern modern periodic table was arranged?
Ans: Modern periodic table was arranged depends upon atomic number. The arrangement of elements based on their electronic configuration created a long form of periodic table.
Q 25. How many elements are in first period and what are their names and symbols?
Ans: First period contains only two elements these elements are i. Hydrogen ii. Helium
Q 26. How many elements are placed in 4 ^ (th) period?
Ans: There are 18 elements present in 4 ^ (th) period.
Q 27. From which element lanthanide series starts?
Ans: Lanthanide series is started from after lanthanum (Z = 57)
Q 28. From which period actinide series is start?
Ans: Actinide series is started from actinium (Z = 89)
Q 29. Define lanthanides. In which period it belongs?
Ans: Elements atomic no 58 to 71 are called lanthanides. It belongs to 6 ^ (th) period.
Q 30. Define actinides in which periods in belongs?
Ans: Elements atomic number 90 to 103 are called actinides. It belongs to 7th period.
Q 31. How many elements present in 3rd period? Write their name and symbols?
Ans: There are 8-elements present in 3rd period. Their names and symbols are as: i. Sodium Na ii. Magnesium Mg iii. Aluminum Al iv. Silicon Si V. Phosphorous P vi. Sulphur S vii. Chlorine CI viii. Argon Ar
Q 32. How many periods are considered to be normal periods? Ans: First three periods considered as normal period.
Q 33. What is the reason of arranging elements in group?
Ans: Depending upon outermost electrons and arranged in groups. electronic configuration elements in period table are
Q 34. What do you mean by periodic function?
Ans: Periodic Function means properties of elements repeating after regular intervals such that elements of similar properties and similar configuration are placed in same group.
Q 35. Why the elements are called s, p-Block elements?
Ans: Because, the electronic configurations of elements are ends in s and p sub shells.
Q 36. How many members are in group 17? Is there any liquid, what is its name?
Ans: There are six elements present in 17th group. Yes, Bromine in an element which exists in liquid state.
Q 37. Define atomic size and atomic radius?
Ans: The half of the distance between the nuclei of the two bounded atoms is called atomic radius.
Q 38. Write down the trends of atomic size in group and period in the periodic table?
Ans: Group: in group atomic size of elements increases from top to bottom. Period: in any period, atomic size of elements decreases from left to right in a period.
Q 39. Why atomic radius of elements increase down the group?
Ans: The atomic radius increases from top to bottom in a group. Reason: because the increase of one more electronic shell in atoms of successive element which decreases the effective nuclear charge.
Q 40. What is the reason of atomic size decreases from left to right in a period?
Ans: In period, the atomic radii gradually decrease from left to right in periods.
Q 41. Define ionization energy. Write down its unit?
Ans: The ionization energy is the amount of energy required to remove the most loosely bounded electron from the valence shell of an isolate gaseous atom. Its unit is klmol¹.
Q 42. What is the trend of ionization energy in periodic table?
Ans: lonization energy decreasing while moves from top to bottom in any group. It increased from left to right in any period.
Q 43. Why ionization energy decreasing from top to bottom in any group.
Ans: lonization energy decreasing in a group due to increase in atomic size of an atom. Q 44. Why ionization energy increasing in a period? Ans: Because when we move from left to right in a period atomic size of atom decreases.
Q 44. Define electron affinity. Write down its unit.
Ans: The amount of energy released when an electron is added up in the outermost shell of an isolated gaseous atom. Its unit is kimol¹
Q 45. What is the trend of electron affinity in affinity in group and period in the periodic table?
Ans: Electron affinity decreases in the group and increases in period.
Q 46. Why electron affinity decreases in a group?
Ans: Electron affinity decreases in a group because of increasing atomic size of atom in a group.
Q 47. Why electron affinity increases in a period?
Ans: Electron affinity increases in a period because of decrease in atomic size of atom from left to right in a period.
Q 48. What are S.I units of atomic radius?
Ans: S.I. units of atomic radius is Pico meter and Nano meter.
Q 49. Why the 2nd ionization energy of an element is higher than first one?
Ans: When an electron is remove from an atom mono positive ion is formed. Mono positive ion has one more proton then electrons. Its nucleus has more influence to electrons and more energy is needed to remove second electron. This is why 2nd I.E has greater value.
Q 50. Why the ionization energy of sodium is less than that of magnesium?
Ans: Because both sodium and magnesium belong to same period when we move left to right in period atomic size decreases and ionization energy increases that is why sodium has low ionization energy because of greater size then magnesium.
Q 51. Why it is difficult to remove an electron from halogens?
Ans: Because halogen has smaller atomic size and high electron affinity and elect negative values so it is difficult to remove the electron from halogens.
Q 52. What is shielding effect?
Ans: The decrease in the attractive force exerted by the nucleus on the valence shell electrons due to presence of the electrons laying between the nucleus and valence shell is called shielding effect.
Q 53. How does shielding effect decrease the forces of electrostatic attractions between and ectrosta outermost elections?
Ans: The electron present between the nucleus and the outmost shell on an atom, reduce the nuclear charge felt by the electrons present in the outmost shell. The attraction of outmost electrons toward nucleus is partially reduced because of presence of inner electron. As a result, an atom experience less nuclear charge than that of the actual charge which is the actual c charges (Zeff).
Q 54. Why does the bigger size of atoms have more shielding effect?
Ans: The bigger size of atoms has more shielding effect because in bigger size atoms the number of inner shell and inner shell electrons increases hence the shielding effect of bigger size atoms also increases.
Q 55. Why the trend of electron affinity and electronegativity does same in a period?
Ans: The trend of electronegativity same because from left to right in period the size of an atom decreases and the attraction of the nucleus for electron also increases. It means the more nuclear force on electron and more energy is required. Therefore, electron affinity and electronegativity value also increases from left to right in a period.
Q 56. Define electronegativity?
Ans: The ability of an atom to attract the sheared pair of electrons toward itself in a molecule is called electronegativity.
Q 57. Which element has the highest electronegativity?
Ans: Fluorine has highest value of electronegativity i.e. 4.0
Q 58. Why Noble Gases are not very reactive?
Ans: Because, noble gases do have 2 or 8 electrons in their valence shells. It means that all the noble gases have their valance shells completely filled. Their atoms do not have vacant spaces in their valance shell to accommodate extra electrons. Therefore, noble gases do not gain, lose or share electrons. That is why noble gases are not very reactive.
Q 59. Why cesium atomic no 55 requires little energy to release its one electron present in the outmost shell.
Ans: Cesium requires little energy to release its outer most electrons because it has greater atomic size and low ionization energy so in cesium atom distance between nucleus and outermost shell is very large.
Q 60. How is periodicity of properties depending upon the number of protons in an atom?
Ans: In periodic table the atomic number increases from left to right and nuclear charge will also increase. When nuclear charge changes the properties of elements will also change.
Q 61. What is the difference between Mendeleev’s Periodic Law and Modern Periodic Law?
Ans: Mendeleev’s Periodic Law: Properties of elements are the periodic functions of their atomic masses.Modern Periodic Law: Properties of elements are the periodic functions of their atomic number.
Q 62. Why and how elements are arranged in 4th period?
Ans: The elements are arranged in 4th period because they are all having same electronic shells and elements are arranged in 4th period by increasing atomic number from left to right in a period.
Q 63. Why shielding effect of electrons makes cation formation easy?
Ans: The shielding effect of electron makes the cation formation easy because the electron presents between the nucleus and the outermost shell of an atom, reduce the nuclear charge felt by the electrons present in the outermost shell. The attraction of outer electron towards nucleus is partially reduced because of presence of inner electrons. As a result, as atom experiences less nuclear charge than that of the actual charge so it makes the cation formation easy.
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