Class 9 SELINA Solutions Chemistry Chapter 5: The Periodic Table

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:

i. To study elements better
ii. To correlate the properties of the elements with some fundamental properties which are characteristic of all the elements
iii. To reveal relationships between elements 

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

At. wt. of A = 7, At. wt. of C = 39

At. wt. of B = 23

i.e. Average of weights of A and C.

1. Döbereiner failed to arrange all the known elements in the form of triads.
2. 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. 

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.

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

Yes, Döbereiner'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.

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

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

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

Mendeleev's basis for periodic classification:

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

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.

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

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

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

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).

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

Merits of Mendeleev's classification of elements:

i. Grouping of elements

ii. Gaps for undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.

iii. 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). 

He 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.

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

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

i. Same IA group (Li, Na, K) and IIA group (Be, Ca)
ii. In the second period (Be, Li) and in the fourth period (K, Ca) 

Eka-silicon

Gold and Platinum

Only 63 elements were discovered at the time of Mendeleev's classification of elements.

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

Eighteen groups and seven periods

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.

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

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

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

1. Group 1 is known as the alkali metals.
2. Group 17 is known as the halogens.
3. Group 18 is known as the transition elements. 

1. Group 1: Lithium (Li), Sodium (Na)
2. Group 17: Chlorine (Cl), Iodine (I)
3. Group 18: Helium (He), Neon (Ne) 

There are two elements in the first period.

There are eight elements in the third period.

1. 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.
2. 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. 

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

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

Mendeleev proposed that the chemical properties of the elements are periodic functions of their atomic masses.

Therefore, he arranged the elements according to the increasing order of atomic masses.

As per Mendeleev's classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel.

Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt. Mendeleev could not explain this point.

This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers.

The atomic numbers of cobalt and nickel are 27 and 28 respectively. Therefore, cobalt with a lower atomic number should come before nickel.

Cl³⁵ and Cl³⁷ are two different atoms which are isotopes of chlorine. Modern periodic table is based on atomic number. The atomic number of both these atoms are same, hence they are placed in the same group and period, i.e. group 17 and period 3. 

The element 'X' has 3 electrons in its M shell. That means, 'X' belongs to the period 3.

The number of valence electrons is 3.

Therefore, the element is placed in group 13 with atomic number 13.

Therefore, the element 'X' is 'Aluminium'.

The element with two shells means, the element belongs to period '2'.

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

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

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

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

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.

1. H and P are noble gases.
2. G and O are halogens.
3. A and I are alkali metals.
4. D and L have valency 4. 

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.

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

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

Most reactive

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

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

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).

1. E
2. B 

K L M

Electronic configuration = 2, 8, 7

1. VIIA
2. Third period
3. Seven
4. Valency of T = -1
5. Non-metal
6. Protons = 17, Neutrons = 18

Correct option: (i) - Number of valence electrons

Correct option: (iii) - 32

Correct option: (ii) - Moseley

Correct option: (iii) - 5

Correct option: (iii) - Each element belongs to the same period

Correct option: (iii) - Alkaline earth metals

Correct option: (ii) Atomic number 

Correct option: (iv) - Representative elements

Correct option: (ii) 3^rd period and VIA group

Correct option: (iii) - IInd period

Correct option: (iii) - One

Correct option: (iii) - Fluorine

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

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

a. Helium
b. Argon
c. Neon

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

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

a. Li < Na < K < Rb < Cs
b. F > Cl > Br > I
c. He < Na < Mg
d. Cl < Mg < Na

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.

1. 3
2. +3
3. Metal
4. Aluminium 

a. Ca
b. 1s²2s²2p⁶3s²3p⁶4s²
c. 2
d. Group 2 Period 4
e. Metal
f. Reducing agent

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

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

Be + 2HCl → BeCl₂ + H₂

Mg + 2HCl → MgCl₂ + H₂

Ca + 2HCl → CaCl₂ + H₂

Name of the alkali metals: Lithium, sodium, potassium, rubidium, cesium and francium

Electrons in the outermost orbit: 1 

i. Reaction of alkali metal with oxygen - React rapidly with oxygen

4Na + O₂ → 2Na₂O

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

2M + 2H₂O → 2MOH + H₂

iii. Reaction of alkali metal with acid - React violently with dil. HCl and dil. H₂SO₄ to produce hydrogen

2M + 2HCl → 2MCl + H₂

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

Electronic configuration of the first two alkaline earth metals:

₄Be: 1s²2s²

₁₂Mg: 1s²2s²2p⁶3s²

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

Xenon or krypton from Group 18 can form compounds.

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.

i. Group 17 elements react with metals to form metal halides which are neutral in nature.
ii. Group 17 elements react with non-metals to form acidic compounds such as hydrogen halides. 

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.

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'.

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

Uses of Modern periodic table:

1. Periodic table has been useful in predicting the existence of new elements.

2. It has been useful in the past in correcting the position of elements in relation to their properties.

3. Study of elements and their compounds has become systematic and easier to remember.

4. Position of an element in the periodic table reveals its

(i) Atomic number

(ii) Electronic configuration

(iii) Number of valence electrons

(iv) Properties

5. Nature of chemical bond, formula of compound formed and properties of that compound can all be predicted from the periodic table.

For example: Elements present in groups 1, 2 and 3 (metals) lose electrons to form positive ions, while elements of groups 15, 16 and 17 (non-metals) gain electrons to form negative ions. When these positive and negative ions combine, they form a compound that is electrovalent in nature.

If elements of group 14, 15, 16 and 17 combine with those of 15, 16 and 17, i.e. non-metals with non-metals, they form covalent compounds.

6. Position of an element in the periodic table reveals

(i) Valency of the element

(ii) Whether the element is a metal or a non-metal -metal occupy the extreme left positions of the periodic table while non-metals are at the extreme right of the periodic table.

(1) Grouping of elements. He generalized the study of the elements then known to a study of mere eight groups.

(2) Gaps for undiscovered elements. In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time. Mendeleev correctly thought that such elements would be discovered later.

(3) Prediction of properties of undiscovered elements. He predicted the properties of the then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots. He actually predicted the properties of some undiscovered elements in 1871.

For example :

(i) Eka aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later on in 1876.

(ii) Properties of eka-silicon are the properties of germanium.

(4) Incorrect atomic mass corrected. He was able to correct the values of atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties. 

Defects of the table proposed by Mendeleev:

(1) Anomalous pairs:

The following pairs of elements did not follow Mendeleev's principles:

(i) Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1.

(ii) Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6.

(iii) Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9.

(2) Position of isotopes:

Isotopes of an element are atoms of that element having similar chemical properties but different atomic masses. According to Mendeleev's periodic law, isotopes of an element must be given separate places.  

(3) Grouping of chemically dissimilar elements:

Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium, etc.) but they have been placed together in the first group.

(4) Separation of chemically similar elements: Elements that are chemically similar, such as gold and platinum have been placed in separate groups.

(5) Electron arrangement: It does not explain the electron arrangement of elements.

(6) Position of hydrogen: Hydrogen was not given a fixed position. It was considered in Group IA as well as in Group VIIA because it forms both a positive ion, viz. in HCl, and a negative ion, viz. in NaH.

The main characteristics of representative elements:

(a) They include both metals and non-metals. There is a regular gradation from metallic to non-metallic character as one moves from left to right across the period.

(b) They form electrovalent as well as covalent compounds with non-metals.

(c) Metallic nature increases on moving down any of these seven groups.

(d) Metals, which are good conductors of heat and electricity, are present in groups 1 and 2. Non-metals, which are present in groups 16 and 17, are poor conductors of heat and electricity.

(e) Some heavier elements, like tin and lead, exhibit variable valencies.

The characteristics of halogens:

(a) reactivity: 

Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example, Fluorine is the most reactive and iodine is solid.

(b) intensity of colours:

The intensity of the colour of the element also increases from pale to dark.

Fluorine is a pale yellow gas, chlorine is a greenish yellow gas, bromine is a reddish brown liquid and iodine is a violet solid.

(c) formation of ions:

Halogens form negative ions carrying a single charge [Fluorine ions F⁻, Chloride ions Cl⁻, Bromide ions Br⁻, iodide ions I⁻]

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.

(a) The name of scientist who gave this table: John Newland (A scientist and lover pf music).

(b) Group 18 (Noble gases) were not known at this time.

(c) This system worked quite well for the lighter elements.

(d) The table shown above relates the properties of the elements to their atomic mass.

(e) Reasons for discarding this table (Newland's Octaves):

• This classification did not work with heavier elements, i.e. those lying been calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves.

• Newland adjusted two elements cobalt (Co) and nickel (Ni) in the same slot and these were placed 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 far away from these elements.

Metals : Sodium, Calcium, Potassium, Aluminium

Non-metals - Nitrogen, Carbon, Fluorine, Chlorine, Oxygen

Valence number of electrons as per electronic configuration of sodium is one, Hence, sodium belongs to group 1.

Hydrogen was not given a fixed position. It was considered in Group IA as well as in Group VIIA because it forms both a positive ion, viz. in HCl, and a negative ion, viz. in NaH.

Thus, the properties of hydrogen relate to both Group 1 and Group 17.