RBSE Class 10 Science Important Questions Chapter 5 Periodic Classification of Elements

Rajasthan Board RBSE Class 10 Science Important Questions Chapter 5 Periodic Classification of Elements Important Questions and Answers.

RBSE Class 10 Science Chapter 5 Important Questions Periodic Classification of Elements

 

Objective Type Questions

‘Question 1.
The basis of Modem Periodic Table is-
(a) Atomic mass
(b) Molecular mass
(c) Atomic number
(d) Atomic volume.
Answer:
(c) Atomic number

Question 2.
The number of electrons in L shell will be –
(a) 2
(b) 18
(c) 8
(d) 32
Answer:
(c) 8

Question 3.
In a group of Periodic Table which one is same for all the elements-
(a) Atomic number
(b) Atomic mass
(c) Number of electrons
(d) Number of valence electrons
Answer:
(d) Number of valence electrons

Question 4.
In a group, on moving top to bottom Atomic size-
(a) Decreases
(b) Increases
(c) Remains constant
(d) Remains uncertain.
Answer:
(b) Increases

Question 5.
Silicon is a-
(a) Metal
(b) Non metal
(c) Metalloids
(d) None of the above.
Answer:
(c) Metalloids

Question 6.
The nature of oxides of metals is generally-
(a) Acidic
(b) Neutral
(c) Amphoteric
(d) Basic
Answer:
(d) Basic

Very Short Answer Type Questions

Question 1.
Who first tried to organise elements into groups ?
Answer:
In 1817, Johann Wolfgang Dobereiner, tried to arrange the elements with similar properties into groups.

Question 2.
Which element was identified by Dobereiner as a catalyst ?
Answer:
Platinum.

Question 3.
Why did Mendeleev leave some blank spaces in the Periodic Table ?
Answer:
Mendeleev boldly predicted the existence of some elements that had not been discovered at that time. For them Mendeleev left spaces in Periodic Table.

Question 4.
Out of sodium, chlorine and silicon which one is metalloid?
Answer:
Silicon.

Question 5.
At the time of hypothesis of Newlands Law of Octaves, how many elements was present in the nature?
Answer:
At the time of hypothesis of Newlands’ Law of Octaves, 56 elements was present in the nature.

Question 6.
What is Newlands’ Law of Octaves?
Answer:
When elements are arranged in the increasing order of their atomic masses then property of every eighth element is similar to first element, this is called Newlands’ Law of Octaves.

Question 7.
What is Period and Group in the Periodic Table? What are their number ?
Answer:
In the Periodic Table horizontal rows are known as Periods and vertical columns are known as Groups. In Modem Periodic Table there are 7 Period and 18 Groups.

Question 8.
What is Modem Periodic Law?
Answer:
According to Moseley, physical and chemical properties of elements are a periodic function of their atomic numbers, this is known as Modern Periodic Law.

Question 9.
Write the Mendeleev’s Periodic Law.
Answer:
According to Mendeleev, physical and chemical properties of elements are a periodic function of their atomic masses.

Question 10.
Write the formula for the maximum number of electrons in a shell.
Answer:
The maximum number of electrons in a shell are 2n² where n is number of shell.

Question 11.
Write the name of metals in third period elements.
Answer:
Sodium (Na), Magnesium (Mg) and Aluminium (Al) are metals in third period elements.

Question 12.
An element Y is present in third group of Periodic Table then what will be the formula of its oxide?
Answer:
The element belongs to third group so its valence is 3 and the valence of oxygen is 2 so formula of its oxide will be Y₂O₃

Question 13.
What will be the valence of elements present in a group.
Answer:
The valence of elements present in a group = number of electrons in the valence shell or 8-number of valence electrons.

Question 14.
Out of chlorine, Bromine and Iodine which one is least reactive towards Hydrogen?
Answer:
Reactivity decreases from F₂ to I₂into halogen family so Iodine is least reactive towards hydrogen.

Question 15.
What is special name of group 1 elements?
Answer:
The special name of group 1 elements is Alkali metals.

Question 16.
What will be change in the basic property of oxides of third period elements?
Answer:
In the third period on moving left to right the basic property of oxides of elements decreases.

Question 17.
Write the example of Dobereiner’s Triad.
Answer:
Cl, Br and I is the example of Dobereiner’s Triad.

Question 18.
The electron losing tendency of elements decreases in a period. Why?
Answer:
As we move from left to right, the effective nuclear charge acting on the valence shell electrons increases in a period so the electron losing tendency decreases.

Question 19.
Metals are electro-positive, why?
Answer:
Metals have the tendency to lose the electrons so they are electro – positive.

Question 20.
In which part of a group the metallic character and atomic size of elements will be more?
Answer:
The metallic character and atomic size of elements will be more in the lower part of a group.

Question 21.
What is period in the Periodic Table and how the structure of atom changes due to change in atomic number?
Answer:
In the Periodic Table, horizontal rows are known as periods. Number of electrons and protons changes due to change in atomic number, so the structure of atom also changes.

Question 22.
How many elements are known till today?
Answer:
118 elements.

Question 23.
According to Newlands imagi-nation, what was the total number of elements in nature ?
Answer:
According to Newland’s imagi-nation, the total number of elements in nature was 56.

Short Answer Type Questions

Question 1.
What is Dobereiner’s Triads? Explain with examples.
Answer:
In 1817, Dobereiner, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each called ‘triads’. Dobereiner showed that when the three elements of a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
For example, triad consisting of lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
Other examples, Ca (40.1) Cl (35.5)
Sr (87.6) Br (79.9)
Ba (137.3) I (126.9)

Question 2.
What is Newlands’ Law of octaves? Name the elements having similar properties of the following –
1. Nitrogen
2. Lithium
Answer:
Newlands’ Law of octaves: In 1866, Newland arranged the then known i elements in the order of increasing atomic masses. He started with the element having the lowest atomic mass (hydrogen) and ended at thorium (56^th element). He found that every eighth element had properties similar to that of the first element. He compared this with the octaves of music. So it is known as ‘Newlands’ Law of Octaves’. In Newlands’ Octaves, the element having similar properties of nitrogen is phosphorous and sodium has similar properties as of lithium.

Question 3.
How the elements are arranged in the Mendeleev’s Periodic Table and what was the basis of classification by them? Explain.
Answer:
At the time of Mendeleev 63 elements were known. He arranged the elements on the basis of their basic property, that is similarity in atomic mass and chemical property in their table. Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen because hydrogen and oxygen are very reactive and form compounds with most of the elements. The formulae of the hydrides and oxides formed by an element were treated as the basic properties of an element for its classification.

He sorted out the elements with similar properties and observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses. It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On the basis, of this he formulated a Periodic Law, which states that ‘the properties of elejnents are the periodic function of their atomic masses’. This is called Mendeleev’s Periodic Law.
Mendeleev’s Periodic Table contains vertical columns called groups and horizontal rows called periods.

Question 4.
How did Mendeleev name unknown elements at that time ? Explain by giving examples.
Answer:
Mendeleev preducted the existence of some elements that had not been discovered at that time. Mendeleev named them by prefixing a Sanskrit numeral, Eka (one) to the name preceding element, in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka – boron, Eka – aluminium and Eka- silicon respectively.

Question 5.
What is Modern Periodic Law and what was the basis of Modern Periodic Table? Explain.
Answer:
Modem Periodic Law: “Properties of elements are the periodic function of their atomic number.” The Modem Periodic Table, given by Henery Moseley is on the basis of Atomic number, which is more basic property. Atomic number gives us the number of protons in the nucleus of an atom and this number increases by one on going from one element to the next. Elements, when arranged in the increasing order of atomic number (Z) lead us to the classification known as the Modem Periodic Table. Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing order of atomic number.

Question 6.
(a) What is the valence of the element with atomic number 17 and how it is determined?
(b) Bromine and Iodine belongs to same group, why?
Answer:
(a) The atomic number of element is 17 so it has 17 protons in the nucleus and 17 electrons in die atom. The electronic configuration of this element is 2, 8, 7 means 7 electrons are present in its valance shell thus one electron is required to complete its shell. So the valence of it is -1.
Valence of an element = number of electrons in the valence shell.

Or

Valence of an element = 8-number of electrons in the valence shell.

(b) Bromine and Iodine belongs to same group because both the elements have 7 elecdons in their valance shell and shows – 1 valance.

Question 7.
(a) How the position of cobalt (Co) and Nickel (Ni) are determined in the Modern Periodic Table.

(b)
(i) Write the names of the elements present in group 1 of Modern Periodic Table.
(ii) Write the electronic configuration of first three elements of group 1.
(iii) What are the similarities in the electronic configuration of these elements.
(iv) How many valance electrons are present in these three elements.
Answer:
(a) The atomic number of cobalt (Co) is 27 and the atomic number of Nickel (Ni) is 28. In the Modem Periodic Table elements are arranged in the increasing order of their atomic number so the cobalt having less atomic number comes first and after that nickel which have comes more atomic number. .

(b)
(i)The elements present in group 1 of Modem Periodic Table are Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr).
(ii)The elecdonic configuration of first three elements of group 1 are as follows.

	K	L	M
Hydrogen (H)	1
Lithium (Li)	2	1
Sodium (Na)	2	8	1

(iii)All these elements have same number of elecdons in the outermost shell (one in each) means number of valence elecdons are same.
(iv)Number of valance elecdons in these three elements is one.

Question 8.
(i) In the long form of the Periodic Table (Modern Periodic Table), Second period elements are Li, Be, B, C, N, O, F and Ne. Write the electronic configuration of these elements.
(ii) Are the number of Valance electrons same in all these elements?
(iii) Are the number of shells ,same in these elements?
Answer:
(i) The elecdonic configuration of second period elements of Modem Periodic Table are as under –

Element	Atomic	Electronic Configuration
		K shell	L shell
Li	3	2	1
Be	4	2	2
B	5	2	3
C	6	2	4
N	7	2	5
0	8	2	6
F	9	2	7
Ne	10	2	8

(ii) No, the valance electrons in these elements are different.
(iii) Yes, the number of shells in these elements are same.

Question 9.
(a) What is valence?
(b) How the valence of an element is determined? What change occurs in valance in.a group and a period?
Answer:
(a) Valence: The number of electrons given or taken or shared by an atom for bond formation with other atom is called valence of an element.

(b) Determination of Valance: Valance of elements is determined as follows –
(i) If the number of electrons in the outermost shells of atoms of elements are 1, 2, 3 or 4 then the valence of these elements will be 1, 2, 3 and 4 respectively.

(ii) If the number of electrons in the outermost shells of atoms of elements are 5, 6 or 7 then the valence of these elements will be 3, 2 and 1 respectively.

(iii) If the number of electrons in the outermost shells of atom of an element is 8 then the valence of that element will be zero.
In a group, valence of all the elements are almost same but in a period on moving left to right, first valence increases from 1 to 4 then decreases upto zero in case of noble gases.

Element	Li	Be	B	C	N	O	F	Ne
Valence	1	2	3	4	3	2	1	0

Question 10.
Why is a zig-zag line drawn in the Modem Periodic Table ?
Answer:
In the Modem Periodic Table, a zig-zag line separates metals from nonmetals. The border line elements—boron, silicon, germanium, arsenic, antimony, tellurium and polonium—are intermediate in properties and are called metalloids or semi metals.

Question 11.
(i) Study the change in the Atomic radius (size) of first group elements and arrange these in the increasing order of size.
Element : Na Li Rb Cs K
Atomic radius : 186 152 244 262 231
(ii) Atom of which element is smallest and which one is largest.
(iii) What change occurs in the atomic radius on moving top to bottom in a group.
Answer:
(i) The increasing order of Atomic radius of first group elements are as follows :
Li < Na < K < Rb < Cs

(ii) The atom of Lithium (Li) is smallest and the atom of caesium (Cs) is largest.
(iii) The Atomic size increases in the group because when we move lower side in the group a new shell is added so the distance between nucleus and outermost shell increases. So Atomic size increases although nuclear charge also increases.

Question 12.
(i) The atomic radius of second period elements are given below
Element : B Be O N Li C
Atomic
radius (Pm) : 88 111 66 74 152 77
(i) Arrange these in the decreasing order of Atomic radius.
(ii) Are these elements arranged like period of Periodic Table.
(iii) The atom of which element is largest and which one is smallest.
(iv) How the Atomic radius changes on moving left to right in a period.
Answer:
(i) The decreasing order of atomic radius of second period elements are as follows –
Li>Be>B>C>N>0

(ii) Yes these elements are arranged like period of periodic table.
(iii) The atom of Lithium (Li) is largest and the atom of oxygen (O) is smallest.
(iv) The atomic radius decreases on moving left to right in a period.

Question 13.
(a) What is the meaning of periodicity in the property of elements on the basis of Periodic Table.
(b) The electronic configuration of an atom is 2, 8, 8, 2.
(i) What is the atomic number of this element?
(ii) This element shows chemical similarities with which of the following elements.
(Atomic numbers are given in parentheses)
Na(ll), 0(8), Mg(12), Ne(10), F(9)
Answer:
(a) When the elements are arranged in the increasing order of their atomic number then a type of periodicity is observed in physical and chemical properties means a regular change in the property occurs form one element to another element and after a definite interval there occurs a periodic recurrence of elements in physical and chemical properties. The phenomenon of expression of periodic recurrence is called periodicity.

(b) (i) Atomic number of element = number of protons = number of electrons = 2 + 8 + 8+ 2 = 20.
(ii) This elements shows chemical similarities with Mg(12) because the electronic configuration of magnesium (Mg) is 2, 8, 2 which is similar to the element given above.

Question 14.
(a) An element X have electronic configuration 2, 8, 7 then –
(i) What is the atomic number of this elements?
(ii) This element belongs to which period?
(iii) This element belongs to which group?
(b) How many valence electrons are present in this element?
Answer:
(a)(i) The atomic number of this element is 17.
(ii) This element belongs to third period.
(iii) This element belongs to 17th group.

(b) Seven valence electrons are present in this element.

Question 15.
The atomic numbers of two elements X and Y are 11 and 17 then—
(a) These elements belongs to which group of Periodic Table.
(b) Out of these elements which one metal and which one is non metal.
(c) These elements belongs to which period of periodic table.
Answer:
(a) The element X belongs to first group and element Y belongs to 17th group of Periodic Table.
(b) Element X is metal and element Y is non metal.
(c) Both the two elements belongs to third period of Periodic Table.

Question 16.
(a) Give the name of elements which are present in the third period of periodic table and classify them into metals and non metals.
(b) In which side of the Periodic Table, metals are present?
Answer:
(a) Following elements are present in third period of the Periodic Table, Na, Mg, Al, Si, P, S and Cl. Out of these elements Na, Mg and Al are metals, P, S and Cl are non metals while Si is metalloid.
(b) Metals are present towards the left hand side of the Periodic Table.

Question 17.
An element Y is present in second period and 16th group of the Periodic Table then –
(a) Is is metal or non-metal?
(b) What is valence of it?
Answer:
(a) Element Y is non-metal, because this is present towards the right hand side of the Periodic Table (16^th group).
(b) Element Y is oxygen having atomic number 8 so the valance of it is 2 because the electronic configuration of it is 2, 6.

Question 18.
How the following properties are changed in a period and a group.
(a) Atomic size
(b) Electronegativity.
Answer:
(a) Atomic size: The atomic radius (Atomic size) decreases on moving from left to right in a period (except zero group). This is due to an increase in nuclear charge which tends to pull the electrons Closer to the nucleus and reduces the size of the atom. While the atomic size increases on moving top to,bottom in a group, because new shells are added on moving down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increases in nuclear charge.

(b) Electronegativity-Electronegativity increases on moving from left to right in a period (except zero group) and value of it decreases on moving top to bottom in a group. .

Question 19.
(a) Define Isotope?
(b) Write the names of two Noble gases.
(c) Electronic configuration of an element is 2, 8, 2. Write its group number and valency.
Answer:
(a) The different atoms of an element which have same atomic number but different atomic masses are called Isotopes. Example Cl³⁵ and Cl3³⁷.

(b) Helium (He) and Neon (Ne).
(c) The element with electronic configuration 2, 8, 2 is magnesium (Mg) and it belongs to group 2 and the valence of it is also, 2.

Question 20.
What changes occur in the following on going from left to right in a period of the periodic table?
(i) Atomic size
(ii) Metallic character
(iii) Number of valence electrons.
Answer:
(i) Atomic size: The Atomic size decreases on going from left to right in a period of the Periodic Table. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

(ii) Metallic character: As the effective nuclear effective charge on the valence shell electrons increases across, a period, the tendency to lose electrons will decrease. So metallic character decreases.

(iii) Number of Valence Electrons: Number of valence electrons first increases and then decreases on going from left to right in a period.

Essay Type Questions

Question 1.
What were the achievements of Mendeleev’s Periodic Table? Give the limitations of Mendeleev’s classification also.
Ans. Mendeleev’s Periodic Law –
According to this law the physical and chemical properties of elements are the periodic function of their atomic masses.
(i) Mendeleev left some gaps in his Periodic’ Table and boldly predicted the existence of some elements that had not been discovered at that time. Mendeleev named them by prefixing Eka (one) to the name of preceding element in the same group. For example, scandium, gallium and germanium, discovered later, have pro¬perties similar to Eka-boron, Eka-aluminium and Eka-silicon, respectively. The properties of Eka-Aluminium predicted by Mendeleev and those of gallium which was discovered later are given in the following table –
Table – Properties of eka-alunlinium and gallium

Property	Eka- aluminium	Gallium
Atomic Mass	68	69.7
Formula of Oxide	E2O3	Ga2O3
Formula of Chloride	ECI3	GaCl3

This proves correctness and usefulness of Mendeleev’s Periodic Table.
(ii) Nobel gases like helium (He), neon (Ne) and argon (Ar) were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. When these gases were disco-vered, they could be placed in a new group without disturbing the existing order.

(iii) Thus in the Mendeleev’s Periodic Table, elements are arranged in the increasing order of their atomic masses and the properties of all the elements of a group are same.

Limitations of Mendeleev’s Classification:
In the Mendeleev’s Periodic Table so many demerits were present the main demerits are as under-

(i) Position of Hydrogen: In the Men-deleev’s Periodic Table the position of hydrogen is uncertain. Hydrogen have similarities in properties with Alkali metals of first group and halogens of seventh group. So it is not decided that the hydrogen should be placed in first group or seventh group.

(ii) Position of isotopes – No place was given for isotopes in the Mendeleev’s Periodic Table.

(iii) Separate place for some elements of similar properties: Mendeleev placed some elements of similar properties in different groups instead of placing in same group.
Example: Copper and Mercury, Silver and Thallium, Barium and Lead.

(iv) Same place for some elements of different properties: In this Periodic Table, some elements of different properties are placed in the same group. Example- elements of IB and IB groups are palced together. ‘

(v) Palcement of three elements together in the eighth group: In this Periodic Table three elements were placed together in eighth group. Example – Fe Co and Ni, Ru Rd and Pd, Os Ir and Pt.

Question 2.
Explain the following –
(a) Why does the size of the atom increase even when the change of the nucleus increases from top to bottom in the group.
(b) The metallic characteristics decreases in the period and increases as the group moves down ?
Answer:
(a) The atomic size increase down the group. This is because new shells are being added as we go down the group. This increases the distance between the outer¬most electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.

(b) As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease. Down the group, the effective nucleas charge experienced by valence electrons is decreasing because the outermost electrons are further away from the nucleus. Therefore, these can be lost easily. Hence, metallic character decreases across a period and increases down a group.

Question 3.
Explain the position of elements in the Modern Periodic Table (long form of the Periodic Table) in detail.
Ans. Modem Periodic Table was given by Henry Mosely. According to him “Properties of elements are a periodic function of their atomic number” modem periodic table have 18 groups (Vertical columns) and 7 periods (Horizontal rows).

The elements – present in any one group have same number of valence electrons for example in group 1 the number of valence electron is 1 while in group 17 (F, Cl) the number of valence electron is 7 so groups in the modem Periodic Table represents an identical outermost shell electronic configuration.

The elements present in a period do not have the same number of valence electrons, but they contain the same number of shells. The number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period. So the atoms of different elements with the same number of shells are placed in the same period. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of the Modem Periodic Table, because the electrons in the atoms of these elements are filled in K, L and M shells. Every period shows that a new shell getting filled with electrons.

The number of electrons in these periods can be explained on the basis of number of electrons filled in to various shells.
The maximum number of electrons in a shell depends on the formula 2n² where n, represents the definite number of shell from the nucleus.
For example,
K shell (n = 1) – 2 x (1)2 = 2, hence the first period has 2 elements.
L shell (n = 2) – 2 x (2)2 = 8, hence the second period has 8 elements.
M shell (n = 3) – 2 x (3)2 = 18, but the outermost shell can have only 8 electrons, so the ,third period also has only 8 elemetns.

The position of an element in the Periodic Table decides its chemical reactivity and the valence electrons determine the kind and number of bonds formed by an element. In the Modem Periodic Table, all the isotopes of an element are placed on one place because the atomic numbers of these are same.

Question 4.
Describe the metallic and non- metallic properties of elements in detail and explain the change in these properties in period and group also.
Answer:
Metalic and Non-metallic character of elements:
In the Modem Periodic Table, a zig-zag line separates metals from non-metals. The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals. In the third period the metals like Na, Mg and Al are present towards the left-hand side of the Periodic Table while the nonmetals like phosphorous (P) sulphur (S) and chlorine (Cl) are present towards the right- hand side. In the middle, silicon is present which is classified as a semi-metal or metalloid metals tend to lose electrons during bond formation means they are electropositive in nature.

As the effective nuclear charge acting on the valence shell electrons increases in a period, the tendency of elements to lose electrons will decrease so the metallic character decreases in a period. Down the group, the effective nuclear charge experienced by valence electrons decreases because the outermost electrons are farther away from the nucleus. Therefore, these electrons can be lost easily. Hence metallic character increases in a group.

Non-metals are electronegative in nature because they tend to form bonds by gaining electrons. Non-metallic character increases in a period and decreases in a group. So the non-metals are found on the right hand side of the periodic table towards the top. These trends also help us to predict the acidic or basic nature of oxides formed by the elements and the oxides of metals are basic and that of non-metals are acidic in general.

In the Periodic Table the reactivity of metals increases in a group like first group (Li, Na, K, Rb and Cs) but the reactivity of non-metals decreases in a group like halogens (F, Cl, Br and I) acidic or basic nature of oxides formed by the elements and the oxides of metals are basic and that of non-metals are acidic in general.

In the Periodic Table the reactivity of metals increases in a group like first group (Li, Na, K, Rb and Cs) but the reactivity of non-metals decreases in a group like halogens (F, Cl, Br and I).
Table-1 Mendeleev’s Periodic Table