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Chapter Review:

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• Groups having three elements each. So he called these groups called triads.
• According to Dobereiner's Traids "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".
• According to Newlands’ Law of Octaves "Every eighth element repeat the properties of the first element".
• Newlands’ Law of Octaves worked well with lighter elements only.
• When Mendeleev started work on Periodic Table, oniy 63 elements were known.
• The Modern Periodic Table has 18 vertical columns known as ‘groups’
  and 7 horizontal rows known as ‘periods’.
• The Modern Periodic Law state that ‘Properties of elements are a periodic function of their atomic number.’
• The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
• The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.
• The atomic radius decreases in moving from left to right along a period. 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.
• Boron, silicon, germanium, arsenic, antimony, tellurium and polonium are intermediate in properties and are called metalloids or semi   metals.

Text-book Questions

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Page no. 81

Q1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Answer: Yes, triples are also found in Newlands's octave column. Example:- Li, Na and K.

Q2. What were the limitations of Döbereiner’s classification?

Answer: Dobereiner could identify the triad of only three elements at that time and classification of all the elements could not be done according to their triad.

Q3. What were the limitations of Newlands’ Law of Octaves?

Answer: Newlands's octet theory was able to classify elements with atomic mass up to calcium only.

Page no.  85

Q1.Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:

K, C, AI, Si, Ba.

Answer: K - K₂O , O - O₂ , Al - Al₂O₂ ,Si - SiO₂ , Ba - BaO .

Q2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)

Answer: Scandium and Germanium.

Q3. What were the criteria used by Mendeléev in creating his Periodic Table?Answer: Noble gases are inert, hence kept in separate group.

Answer: Mendeleev separated the elements according to their atomic number. He tried to put elements with similar properties in a group.

Q4. Why do you think the noble gases are placed in a separate group?

Answer: Noble gases are inert, hence kept in separate group.

Page no. 90

Q1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

Answer: (i) Elements are separated according to their atomic numbers in the modern periodic table. Hydrogen was placed in the first group.

(ii) The position of the elements in the periodic table shows their chemical reactivity.

(iii) Elements are placed in different order according to their weight, heavy and light.

Q2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Answer: Both beryllium (Be) and calcium (Ca) are like magnesium.

shows reactivity. Both have 2 valence electrons in their outer shell, Be(2,2) and Ca(2,8,8,2).

Q3. Name

(a) three elements that have a single electron in their outermost shells.

(b) two elements that have two electrons in their outermost shells

(c) three elements with filled outermost shells.

Answer: (a) Lithium (Li), Sodium (Na), Potassium (K).

(b) Magnesium (Mg), Calcium (Ca).

(c) Neon (Ne), Argon (Ar), Krypton (Kr).

Q4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

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

Answer: (a) 'Yes' Atoms of these elements have similarity. They have only 1 electron in their outermost shell. Their valencies are also similar.

(b) 'Yes' in these also there is similarity. Both have valency ' o ' and their outermost shell is complete

Q5. In the Modern Periodic Table, which are the metals among the first ten elements?

Answer: Lithium and Beryllium are metals.

Q6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?

Ga Ge As Se Be

Answer: Be

NCERT Solutions Exercise

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Q1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(c) The atoms lose their electrons more easily.

(d) The oxides become more acidic.

Answer: (c) The atoms lose their electrons more easily.

Q2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as

(a) Na

(b) Mg

(c) AI

(d) Si

Answer: (b) Mg

Q3. Which element has

(a) two shells, both of which are completely filled with electrons?

(b) the electronic configuration 2, 8, 2?

(c) a total of three shells, with four electrons in its valence shell?

(d) a total of two shells, with three electrons in its valence shell?

(e) twice as many electrons in its second shell as in its first shell?

Answer: (a) Neon (Ne)

(b) Magnesium (Mg)

(c) Silicon (Si)

(d) Boron (B)

(e) Carbon (C)

Q4. (a) What property do all elements in the same column of the Periodic Table as boron have in common?

(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?

Answer: (a) Similar properties :-

(i) All elements are metals.

(ii) Coming down from top to bottom, the metallic properties increase.

(iii) All are good conductors of electricity.

(b) Similar properties :-

(i) All elements are non-metals and valency is 7.

(ii) All are bad conductors of electricity.

Q5. An atom has electronic configuration 2, 8, 7.

(a) What is the atomic number of this element?

(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)

N(7) ,F(9), P(15) ,Ar(18) 

Answer: (a) Its atomic number is 17.

(b) There will be similarity in chemical properties with F(9) - 2,7.

Q6. The position of three elements A, B and C in the Periodic Table are shown below –

Group 16      Group 17

   _            _

   _            A

   _            _

   B            C     

(a) State whether A is a metal or non-metal.

(b) State whether C is more reactive or less reactive than A.

(c) Will C be larger or smaller in size than B?

(d) Which type of ion, cation or anion, will be formed by element A?

Answer: (a) A- is a non-metal.

(b) C is less active.

(c) the size of C is smaller than that of B

(d) Element A will form anion.

Q7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Answer: The atomic number of nitrogen is 7 (2,5) so it will be more electronegative. Phosphorus and Nitrogen are both non-metals but Phosphorus 15(2,8,5) comes down from nitrogen and thus becomes negative.

Q8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Answer: Elements are classified according to their electronic configuration. The electron present in the outermost shell of an element gives its valency and the total number of shells is its periodic number.

Q9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Answer: C(20) is surrounded by elements with atomic numbers 12, 19, 21 and 38 in the periodic table. Out of these, the elements with atomic number 21 (2,8,9,2) and atomic number 38 (2,8,18,2) will have the same chemical and physical properties.

Q10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Answer:Modern Periodic Table :- 

(i) It is ordered according to atomic number.

(ii) It has 18 squares.

(iii) In these inert gases are kept in the 18th class.

Mendeleev's periodic table:-

(i) It is based on atomic mass.

(ii) It has 8 groups.

(iii) There is no place for inert gases in them.

Question 1 : Write the common name of CaO(s).

Answer: Limestone.

Question 2 : Why is sodium hydrogen carbonate a main ingredient of antacids?

Answer: Since sodium hydrogen carbonate is an alkaline substance it neutralizes acids.

Question 3 : In what physical states are the reactants and products represented in a chemical reaction?

Answer: In the form of solid (s), liquid (l), gas (g) and aqueous solution (aq).

Question 4 : What happens when a magnesium ribbon is lit in the presence of air?

Answer: It forms white colored magnesium oxide.

Question 5 : Why is respiration called an exothermic reaction? Give reason.

Answer: The process of respiration which takes place continuously in our cells is a type of exothermic reaction. After breaking down the carbohydrates obtained from food, it is converted into glucose, which together with oxygen in the respiration process gives us energy. Since energy is also released in exothermic reaction, therefore respiration is also called exothermic reaction.

Question 6 : Name the reaction in which two or more reactants combine to form a single product. Write a balanced equation for this reaction.

Answer: Combination reaction.

          C + O₂ - CO₂

Question 7 : Which gas is used in the packing bag to keep the fatty or oily food material for a long time? Why ?

Answer: Oxygen is removed from the bag and made with nitrogen, this eliminates the possibility of oxidation and the substances kept in the bag do not discolor.

Question 8 : What properties do they get from the oxidation of fatty or oily food ingredients?

Answer: They become foul smelling.

Question 9 : Write two important measures to slow down the rate of oxidation of fatty or oily food ingredients.

Answer:

Keeping it in an airtight container speeds up the oxidation.
Removing oxygen from the pouch and containing it with nitrogen eliminates the possibility of oxidation.
Question: Name the reaction in which exchange of ions takes place between the reactants.

Question 10 : What is a chemical reaction called?

Answer: Such processes in which substances with new properties are formed are called chemical reactions.

For example, curd formation from milk, rusting of iron etc.

Question 11 : What are chemical equations called?

Answer: When a chemical reaction is expressed through symbols or formulas, it is called a chemical equation.

Additional Questions With Solutions

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Question 1. Write the main features of Mendeleev's periodic table.

Answer: Mendeleev arranged the elements in a horizontal row in the order of their increasing atomic mass. Elements with similar properties were placed in all the vertical pillars. Following are the main features of this table:-

(a) Groups and Subgroups:- There are 8 groups in the periodic table. Groups I to VII are divided into two subgroups.

(b) The number of periods in the periodic table is 7 which are known as 1,2,3,4,5,6 and 7.

1st period - 2 elements

Second Period - 8 Elements

Third Period - 8 Elements

Fourth Period - 18 Elements

Fifth Period - 18 Elements

Sixth Period - 32 Elements

seventh period - incomplete period

Question 2: Write some characteristic features of the periodic table.

Answer: Some of the distinctive features of the periodic table are as follows:-

(i) Elements are placed in a special way in groups and periods than before.

(ii) The properties of many elements were inferred from this type of classification.

(iii) The atomic masses of many elements were modified.

(iv) The possibility of discovery of new elements increased and new elements were also discovered.

Question 3 - Differentiate between modern periodic table and Mendeleev's periodic table.
Answer -
Modern periodic table
1. Elements are arranged in increasing atomic number.
2. This periodic table has 18 vertical columns and 7 horizontal rows.
3. Isotopes of elements are placed at the same place as their corresponding elements because they have the same atomic number.
4. Chemically dissimilar elements are kept in separate groups.
Mendeleev's periodic table
1. Elements are arranged in increasing atomic masses.
2. This periodic table has 8 vertical columns and 6 horizontal rows.
3. The isotopes of the elements are not found in proper positions.
4. Chemically dissimilar elements held together.