Text-book Questions

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