Selina Concise Chemistry Class 9 ICSE Solutions The Periodic Table
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Selina ICSE Solutions for Class 9 Chemistry Chapter 8 The Periodic Table
Page No. 79
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
The first classification of elements was into 2 groups-metals and non-metals.
(b) Döbereiner 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.
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.
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.
(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.
- They are good reducing agents.
- They are soft metals.
- They impart colour to the flame.
- Common name of the group is alkali metals [Group 1A].
- All of them are metals.
- Oxide of each of them is alkaline in nature.
- Each has valency 2.
(a) Mendeleev’s basis for periodic classification:
- Similarities in the chemical properties of elements.
- Increasing order of atomic weights of elements.
(b) 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.
(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).
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:
- 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).
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
- 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.
|Element||At. No.||Electronic distribution|
|Na||11||2, 8, 1|
|Ca||20||2, 8, 8|
|K||19||2, 8, 8, 2|
- Same IA group (Li, Na, K) and IIA group (Be, Ca)
- In the second period (Be, Li) and in the fourth period (K, Ca)
b. Gold and Platinum
c. Only 63 elements were discovered at the time of Mendeleev’s classification of elements.
Page No. 86
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
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.
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.
Atomic number determines which element will be the first and which will be the last in a period of the periodic table.
- Group 1 is known as the alkali metals.
- Group 17 is known as the halogens.
- Group 18 is known as the transition elements.
- Group 1: Lithium (Li), Sodium (Na)
- Group 17: Chlorine (Cl), Iodine (I)
- Group 18: Helium (He), Neon (Ne)
(a) There are two elements in the first period.
(b) There are eight elements in the third period.
- 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.
- 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.
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
- 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.
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.
These elements have a full outermost subshell, which results in high stability. They only react with other elements in extreme circumstances.
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.
The number of valence electrons remains the same as we go down a group.
(a) Metals: A and B; Non-metals: C; Noble gases: D and E
(b) Most reactive
- Metals: Alkali metals (Group I); Caesium
- 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).
K L M
Electronic configuration = 2, 8, 7
b. Third period
d. Valency of T = -1
f. Protons = 17, Neutrons = 18
Page No. 91
Atomic number of P = 19
Electronic configuration = 2, 8, 8, 1
Group number of the element = 1
A Period number of the element = 4
P is a metal.
(c) 4, 3
(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
(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.
|Column A||Column B|
|(a) Elements short of 1 electron in octet||(v) Halogens|
|(b) Highly reactive metals||(iii) Alkali metals|
|(c) Unreactive 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 outermost orbit||(iv) Alkaline earth metals|
|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|
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.
Name of the alkali metals: Lithium, sodium, potassium, rubidium, cesium and francium
Electrons in the outermost orbit: 1
- 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. H2SO4 to produce hydrogen
2M + 2HCl → 2MCl + H2
Page No. 92
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.
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
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:
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:
- 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.
- 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.
- Physical state: Gaseous
- 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.
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.
- Helium gas is used in airships and balloons.
- Neon gas is used in neon lights. The brightly coloured advertising light works when an electric discharge is passed in a tube containing a little of a noble gas such as neon.
d. Group 2 Period 4
f. Reducing agent
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