Selina Concise Chapter 5 The Periodic Table ICSE Solutions Class 9 Chemistry

ICSE Solutions for Chapter 5 The Periodic Table Class 9 Selina Chemistry

Exercise 5 A

1. What is the need for the classification of elements?
Answer

It is impossible for a chemist to study all the elements and their compounds. Hence, classification is a must.

Following are the reasons for the classification of elements:

• To study elements better
• To correlate the properties of the elements with some fundamental properties which are characteristic of all the elements
• To reveal relationships between elements

2. What was the basis of the earliest attempts made for the classification and grouping of elements?

Answer

The first classification of elements was into 2 groups-metals and non-metals.

3. A, B and C are the elements of a Dobereiner’s triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

Why was Dobereiner’s triad discarded?

Answer

(a) At. wt. of A = 7, At. wt. of C = 39

At. wt. of B = 23 i.e. Average of weights of A and C.

(b) Dobereiner failed to arrange all the known elements in the form of triads.

In the triad of fluorine (19), chlorine (35.5) and bromine (80), it is observed that the mean of the atomic masses of fluorine and bromine is ½(19 + 80) = 49.5, not 35.5.

4. Explain ‘Newland’s Law of Octaves.’ Why was the law discarded?

Answer

Elements when arranged in the increasing order of their atomic weights are similar to the eighth and the first note of the musical scale. For example, the eighth element from lithium is sodium. Similarly, the eighth element from sodium is potassium. Thus, lithium and sodium provide any specific place for hydrogen.

• This classification did not work with heavier elements.
• Newland adjusted two elements Cobalt (Co) and Nickel (Ni) in the same slot.
• Fe, which resembles Co and Ni in properties, has been placed far away.

5. Did Dobereiners triads also exist in the columns of Newland’s Octaves? Compare and find out.

Answer

Yes, Dobereiner’s triads also exist in the columns of Newland’s octaves. For example, the second column of Newlands classification has the elements Lithium (Li), Sodium (Na) and Potassium (K), which constitute a Döbereiner’s triad.

6. (a) Lithium, sodium and potassium elements were put in one group on the basis of their similar properties. What are those similar properties?

(b) The elements calcium, strontium and barium were put in one group or family on the basis of their similar properties.

What were those similar properties?

Answer

(a) Elements of lithium, sodium and potassium have the following similar properties:

• All these have one electron in the outermost shell.
• They form unipositive ions.
• These are good reducing agents are soft metals.
• They impart colour to the flame.
• Common name of the group is alkali metals [Group 1A].

(b)

• All of them are metals.
• Oxide of each of them is alkaline in nature.
• Each has valency 2.

7. (a) What was Mendeleev’s basis for classification of elements?

(b) Mendeleev’s contributions to the concept of periodic table laid the foundation for the Modern Periodic Table. Give reasons.

Answer

(a) Mendeleev’s basis for periodic classification:

• Similarities in the chemical properties of elements.
• Increasing order of atomic weights of elements.

(b) Mendeleev’s contributions to the concept of periodic table

Mendeleev laid the foundation for the modern periodic table by showing periodicity of the properties of the elements by arranging the elements (63) then known into 8 groups, by leaving gaps for undiscovered elements and predicting their properties. He made separate groups for metals and non-metals. He also created periods in which the element gradually changes from metallic to non-metallic character. He was also able to show that the element in the same sub-group had the same valency.

8. State Mendeleev’s periodic law.

Answer

Mendeleev’s periodic law: The physical and chemical properties of all the elements are a periodic function of their atomic masses.

9. Use Mendeleev’s Periodic Table to predict the formula of

(a) hydrides of carbon and silicon.

(b) oxides of potassium, aluminium and barium.

Answer

(a) C is in Group 4. So, the hydride will be CH₄ (Methane).

Si is in Group 4. So, the hydride will be SiH₄ (Silane)

(b) K is in Group 1. So, the oxide will be K₂O (Potassium oxide).

Al is in Group 3. So, the oxide will be Al₂O₃ (Aluminium oxide).

Ba is in Group 2. So, the oxide will be BaO (Barium oxide).

10. Which group of elements was missing from Mendeleev’s original periodic table?

Answer

Anomalous pairs of elements were missing from Mendeleev’s periodic table.

11. State the merits of Mendeleev’s classification of elements.

Answer

Merits of Mendeleev’s classification of elements:

• Grouping of elements
• Gaps for undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
• He predicted the properties of the then unknown elements on the basis of properties of elements lying adjacent to the vacant slots (eka-aluminium and eka-silicon).

12. Why did Mendeleev’s leave some gaps in his periodic table os elements? Explain your answer with an example.

Answer

Mendeleev’s left gaps in the table for the undiscovered elements. He discovered the properties of such elements with the help of neighboring elements. He discovered eka-silicon with atomic mass of 72 which was later named Germanium with atomic mass 72.6.

13. The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

Answer

Henry Moseley found that when cathode rays struck anodes of different metals, the wavelength of these metals was found to decrease in a regular manner of changing the metal of anode in the order of its position in the periodic table. By this, he concluded that the number of positive charges present in the nucleus due to protons (atomic number) is the most fundamental property of the element.

So, Henry Moseley found that the atomic number is a better fundamental property of an element compared to its atomic mass. This lead to the modern periodic law.

This law gave explanations for anomalies in Mendeleev’s classification of elements such as:

• Position of isotopes with the same atomic number can be put in one place in the same group.
• Position of argon and potassium: Potassium with higher atomic number should come later, and argon with lower atomic number should come first.

14. Consider the following elements: Be, Li, Na, Ca, K. Name the elements of (a) same group (b) same period.

Answer

Element	At. No.	Electronic distribution
Be	4	2, 2
Li	3	2, 1
Na	11	2, 8, 1
Ca	20	2, 8, 8,2
K	19	2, 8, 8, 1

Same IA group (Li, Na, K) and IIA group (Be, Ca)

In the second period (Be, Li) and in the fourth period (K, Ca)

15. (a) Name an element whose properties were predicted on the basis of its position in Mendeleev’s periodic table.

(b) Name two elements whose atomic weights were corrected on the basis of their positions in Mendeleev’s periodic table.

(c) How many elements were known at the time of Mendeleev’s classification of elements?

Answer

(a) Eka-silicon

(b) Gold and Platinum

(c) Only 63 elements were discovered at the time of Mendeleev’s classification of elements

Exercise 5 B

1. (a) State the modern periodic law.

(b) How many periods and groups are there in the modern periodic table? 

Answer

(a) Modern periodic law: The physical and chemical properties of all elements are a periodic function of their atomic numbers.

(b) Eighteen groups and seven periods

2. What is the main characteristic of the last elements in the periods of a periodic table? What is the general name of such elements?

Answer

Last elements of each period have their outermost shell complete, i.e. 2 or 8 electrons.

The general name is inert gases or noble gases.

3. What is meant in the periodic table by? :

(a) a group, and

(b) a period ?

Answer

(a) Vertical columns in a periodic table which have the same number of valence electrons and similar chemical properties are called a group.

(b) In a periodic table, elements are arranged in the order of increasing atomic numbers in horizontal rows called periods.

4. From the standpoint of atomic structure, what determines which elements will be the first and which the last in a period of the periodic table?

Answer

Atomic number determines which element will be the first and which will be the last in a period of the periodic table.

5. (a)What are the following groups known as?

1. Group 1
2. Group 17
3. Group 18

(b) Name two elements of each group.

Answer

(a) Group 1 is known as the alkali metals.

Group 17 is known as the halogens.

Group 18 is known as the transition elements.

(b) Group 1: Lithium (Li), Sodium (Na)

Group 17: Chlorine (Cl), Iodine (I)

Group 18: Helium (He), Neon (Ne)

6. What is the number of elements in the:

(a) 1^st period, and

(b) 3^rd period, of the modern periodic table?

Answer

(a) There are two elements in the first period.

(b) There are eight elements in the third period.

7. How does number of (i) valence electrons (ii) valency; vary on moving from left to right in the second period of a periodic table?

(a) In the second period of a periodic table?

(b) In the third period of a periodic table?

Answer

(a) The valence electrons in the same shell (outermost shell) increase progressively by one across the period. The first element hydrogen has one valence electron and helium has two valence electrons.

On moving from left to right in a period, valency increases from 1 to 4, then falls to one and ultimately to zero in the last group.

(b) Valence electrons in the same shell (outermost shell) increase progressively by one across the period. The first element sodium has one valence electron and magnesium has two valence electrons.

On moving from left to right in the third period, valency increases from 1 to 7 and ultimately to zero in the last group.

8. How do atomic structures (electron arrangements) change in a period with increase in atomic numbers moving left to right?

Answer

The size of atoms decreases when moving from left to right in a period. Thus, in a particular period, the alkali metal atoms are the largest and the halogen atoms are the smallest.

Li > Be > B > C > N > O > F

9. (a) This question refers to elements of the periodic table with atomic numbers from 3 to 18. In the table below, some elements are shown by letters, even though the letters are not the usual symbols of the elements.

3	4	5	6	7	8	9	10
A	B	C	D	E	F	G	H
11	12	13	14	15	16	17	18
I	J	K	L	M	N	O	P

Which of these is:

(a) a noble gas?

(b) a halogen?

(c) an alkali metal?

(d) an element with valency 4?

(b) If A combines with F, what would be the formula of the resulting compound?

(c) What is the electronic configuration of G?

Answer

(a) H and P are noble gases.

G and O are halogens.

A and I are alkali metals.

D and L have valency 4.

(b) Li₂O. A stands for lithium and F stands for oxygen. The valence of lithium is +1 and the valence of O is -2, i.e. A₂F.

(c) G has atomic number 9; therefore, its electronic arrangement is 2, 7.

10. Sodium and aluminium have atomic numbers 11 and 13, respectively. They are separated by one element in the periodic table, and have valencies 1 and 3 respectively. Chlorine and potassium are also separated by one element in the periodic table (their atomic numbers being 17 and 19, respectively) and yet both have valency 1. Explain.

Answer

Na and Al have the capacity to donate an electron due to which the valency is positive, whereas Cl and K can only gain or lose one electron due to which their valency is -1 and +1, respectively. This is the only difference between these two.

11. Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common.

Answer

These elements have a full outermost subshell, which results in high stability. They only react with other elements in extreme circumstances.

12. In which part of a group would you separately expect the elements to have:

(a) the greatest metallic character?

(b) the largest atomic size?

Answer

(a)The greatest metallic character can be expected at the bottom of the group.

(b)The largest atomic size can be expected at the lower part of the group.

13. What happens to the number of valence electrons in atoms of elements as we go down a group of the periodic table?

Answer

The number of valence electrons remains the same as we go down a group.

14. The position of elements A, B, C, D and E in the periodic table are shown below:

Group 1	Group 2	Group 17	Group 18
			D
	B	C
A			E

(a) State which are metals, non-metals and noble gas in this table.

(b) State which is most reactive (i) metal (ii) non-metal

(c) Which type of ion will be formed by element A, B and C.?

(d) Which is larger in size (i) D or E (ii) B or C

Answer

(a) Metals: A and B; Non-metals: C; Noble gases: D and E

(b) Most reactive

(i) Metals: Alkali metals (Group I); Caesium

(ii) Non-metals: Halogens (Group 17); Fluorine

(c) Element A will form a positive ion 1⁺ (cation).

Element B will form a positive ion 2⁺ (cation).

Element C will form a negative ion 1^– (anion).

(d) (i) E

(ii) B

15. Write the electronic configuration of element ₁₇T³⁵.

(a) What is the group number of T?

(b) What is the period number of T?

(c) How many valence electrons are there in an atom of T?

(d) What is the valency of T?

(e) Is it a metal or non-metal?

(f) State number of protons and neutrons in T.

Answer

Electronic configuration of ₁₇T³⁵

K	L	M
2	8	7

(a) VIIA

(b) Third period

(c) Seven

(d) Valency of T = -1

(e) Non-metal

(f) Protons = 17, Neutrons = 18

Exercise 5 C

1. Element P has atomic number 19. To which group and period, does P belong? Is it a metal or a non-metal? Why?

Answer

Atomic number of P = 19

Its electronic configuration = 2, 8, 8, 1

Group no. of the element = 1A

Period no. of the element = 4

P is a metal.

2. An element belongs to the third period and Group IIIA (13) of the periodic table. State:

(a) the number of valence electrons,

(b) the valency,

(c) if it is a metal or non-metal?

(d) the name of the element.

Answer

(a) 3

(b) +3

(c) Metal

(d) Aluminium

3. Name and state the following with reference to the elements of the first three periods of the periodic table.

(a) Noble gas with duplet arrangement of electrons.

(b) Metalloid in Period 3.

(c) Valency of elements in Group 14 and 15.

(d) Noble gas having electronic configuration: 2, 8, 8.

(e) Group whose elements have zero valency.

(f) A covalent compound formed by an element in period 2 and a halogen.

(g) Non-metallic element present in Period 3 of Groups 15 and 16

(h) An electrovalent compound formed by an alkaline earth metal and a halogen.

(i) Bridge elements of Period 3 of Group 1,2 and 3.

(j) Alkali metal in period 3 that dissolves in water giving a strong alkali.

(k) Typical elements of Groups 14 and 15.

(l) Alkaline earth metal in period 3.

Answer

(a) Helium

(b) Silicon

(c) 4, 3

(d) Argon

(e) Noble gases

(f) Carbon tetrachloride (CCl₄)

(g) Silicon, Phosphorus

(h) Sodium chloride (Na⁺Cl^–)

(i) Li and Mg; Be and Al; B and Si

(j) Sodium

(k) Typical elements of Period 2 belonging to Group 14 and 15 are carbon and nitrogen. Typical elements of Period 3 belonging to Group 14 to 15 are silicon and phosphorus.

(l) Beryllium

4. Match column A with column B

Column A	Column B
(a) Element short by 1 electron in octet	(i) Transition elements
(b) Highly reactive metals	(ii) Noble gases
(c) Non-reactive elements	(iii) Alkali metals
(d) Elements of groups 3 to 12	(iv) Alkaline earth metals
(e) Radioactive elements	(v) Halogens
(f) Elements with 2 electrons in the outermost orbit	(vi)Actinides

Answer

Column A	Answers
(a) Element short by 1 electron in octet	(v) Halogens
(b) Highly reactive metals	(iii) Alkali metals
(c) Non-reactive elements	(ii) Noble gases
(d) Elements of Groups 3 to 12	(i) Transition elements
(e) Radioactive elements	(vi) Actinides
(f) Elements with 2 electrons in the outermost orbit	(iv) Alkali earth metals

5. Complete the table:

Atomic number	Element	Electronic configuration	Select element of the same group
11	Sodium		(Ca/N/K)…
15	Phosphorus		(Ba/N/Rb)…
16	Sulphur		(F/Cl/O)…
9	Fluorine		(Ca/Cl/K)…

Answer

Atomic number	Element	Electronic configuration	Select element of the same group
11	Sodium	2, 8, 1	K
15	Phosphorus	2, 8, 5	N
16	Sulphur	2, 8, 6	O
9	Fluorine	2, 7	Cl

6. Write down the word that will correctly complete the following sentences:

(a) Relative atomic mass of a light element up to calcium is approximately_____ its atomic number.

(b) The horizontal rows in a periodic table are called _____ .

(c) Going across a period left to right, atomic size _____ .

(d) Moving left to right in the second period, number of valence electrons _____ .

(e) Moving down in the second group, number of valence electrons___________.

Answer

(a) Relative atomic mass of a light element up to calcium is approximately 20its atomic number.

(b) The horizontal rows in a periodic table are called periods.

(c) Going across a period left to right, atomic size increases.

(d) Moving left to right in the second period, number of valence electrons increases from 1 to 8.

(e) Moving down in the second group, number of valence electrons remain same.

7. (a) Name the alkali metals, How many electron(s) they have in their outermost orbit.

(b) Take any one alkali metal and write its reaction with (i)oxygen (ii)water (iii)acid.

Answer

(a) Name of the alkali metals: Lithium, sodium, potassium, rubidium, cesium and francium electrons in the outermost orbit:

(b) Reaction of alkali metal with oxygen – React rapidly with oxygen. 4Na + O₂ → 2Na₂O

Reaction of alkali metal with water – React with water violently and produce hydrogen.

2M + 2H₂O → 2MOH + H₂

Reaction of alkali metal with acid – React violently with dil. HCl and dil. H₂SO₄to produce hydrogen

2M + 2HCl → 2MCl + H₂

8. (a) Name the method by which alkali metals can be extracted.

(b) What is the colour of the flame of sodium and potassium?

 Answer

(a) Alkali metals can be extracted by the electrolysis of their molten salts.

(b) The colour of the flame of sodium is golden yellow, and the colour of the flame of potassium is pale violet.

9. An element A has 2 electrons in its fourth shell. State:

(a) its atomic number

(b) its electronic configuration

(c) its valency

(d) position in the periodic table

(e) is it a metal or non metal

(f) is it an oxidising or reducing agent

Answer

(a) Ca

(b) 1s²2s²2p⁶3s²3p⁶4s²

(c) 2

(d) Group 2 Period 4

(e) Metal

(f) Reducing agent

10. (a) Name the first three alkaline earth metals.

(b) Write the reactions of first three alkaline earth metals with dilute hydrochloric acid.

Answer

(a) The first three alkaline earth metals are Beryllium, Magnesium and Calcium.

(b)  Reactions of the first three alkaline earth metals with dilute hydrochloric acid:

• Be + 2HCl → BeCl₂ + H₂
• Mg + 2HCl → MgCl₂ + H₂
• Ca + 2HCl → CaCl₂ + H₂

11. (a) How do alkaline earth metals occur in nature?

(b) Write the electronic configuration of the first two alkaline earth metals.

Answer

(a) Alkaline earth metals occur in nature in the combined state and not in the free state as they are very reactive.

(b) Electronic configuration of the first two alkaline earth metals:

• ₄Be: 1s²2s²
• ₁₂Mg: 1s²2s²2p⁶3s²

12. Give reasons:

(a) Alkali metals are kept in inert solvent.

(b) Alkali metals and halogens do not occur free in nature.

(c) Alkali and alkaline earth metal compounds usually form electrovalent compound.

(d) Inert gases do not form compounds.

Answer

(a) Due to the reactive nature of alkali metals, they are kept in inert solvents.

(b) Alkali metals and halogens are very reactive; hence, they do not occur in the free state in nature.

(c) Alkali metals and alkaline earth metals have 1 and 2 valence electrons, respectively, in their outermost shell.
They can lose electrons to atoms of non-metals to form an electrovalent compound.

(d) Inert gases have 2 or 8 electrons (duplet/octet) in their outermost orbit. That is their electronic arrangement is very stable, so they are unreactive and do not form compounds.

13. Arrange the following:

(a) Elements of group 1, in increasing order of reactivity

(b) Elements of group 17, in decreasing order of reactivity

(c) He, Na, Mg (increasing order of melting point)

(d) Chlorine, sodium, magnesium (increasing reducing character)

Answer

(a) Li < Na < K < Rb < Cs

(b) F > Cl > Br > I

(c) He < Na < Mg

(d) Cl < Mg < Na

14. (a) State the nature of compounds formed when group 17 elements combine with (i) metals (ii) non-metals.

(b) Why group 17 elements are highly reactive?

Answer

(a) Group 17 elements react with metals to form metal halides which are neutral in nature.

Group 17 elements react with non-metals to form acidic compounds such as hydrogen halides.

(b) Group 17 elements are highly reactive because of their closeness to the noble or stable gas configuration. They can easily achieve a noble gas electron structure.

15. (a) How many electrons do inert gases have in their valence shells?

(b) Name an element of group 18 which can form compounds.

Answer

(a) All the noble or inert gases have 8 electrons in their valence shell except helium which has two electrons in its valence shell.

(b) Xenon or krypton from Group 18 can form compounds.

16. Name the gas used in:

(a) Filling balloons

(b) Light bulbs

(c) Bright colour advertising light works

Answer

(a) Helium

(b) Argon

(c) Neon

17. (a) What is the name given to group 17 elements? Why are they called so?

(b) Comment on the (i) reactivity (ii) colour (iii) physical state of group 17 elements.

Answer

(a) Group 17 elements are called halogens. The name halogens is from Greek halo (sea salt) and gens (producing, forming) and thus means ‘sea salt former’.

(b) Group 17 elements or halogens:

(i) Reactivity: Halogens are the most reactive non-metals, their reactivity decreases down the group. Fluorine is the most reactive halogen and iodine is the least reactive halogen.

(ii) Colour: Fluorine is a pale yellow gas, chlorine is a greenish yellow gas, bromine is a reddish brown liquid and iodine is a violet solid.

(iii) Physical state: Gaseous

18. Two elements P and Q belong to the same period of the modern periodic table and are in the group 1 and group 2, respectively. Compare the following characteristics in the tabular form.

(a) Number of electrons in their atoms

(b) Their tendency to lose electrons

(c) Formation of their oxides

(d) Formulae of their chlorides

Answer

As elements P and Q belong to the same period of the modern periodic table and are in group 1 and group 2, they belong to alkali metals and alkaline earth metals, respectively.

Characteristic	Alkali metals (Element P)	Alkaline earth metals (Element Q)
(a) Number of electrons in their atoms	Valence electron = 1	Valence electrons = 2
(b) Their tendency to lose electrons	Can easily lose electrons	They will lose electrons easily but not as easily as alkali metals can.
(c) Formation of their oxides	React rapidly with oxygen in air 4Na + O2 → 2Na2O	Less reactive than alkali metals
(d) Formulae of their chlorides	NaCl, KCl, CsCl	MgCl2, CaCl2, BaCl2