1: Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves. Compare and find out.
Ans: Only one triad of Dobereiner’s triads existed in the columns of Newlands’ Octaves. The triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.
Dobereiner’s triads Newlands’ octaves
L,i Ca, Cl H, Li, Be, B, C, N, O
Na, Sr, Br F, Na, Mg, Al, Si, P, S
K, Ba, I Ni, Cu, Zn, Y, In, As, Se
Br, Rb, Sr, Ce ,La
2: What were the limitations of Dobereiner’s classification.
Ans: All known elements could not be classified into groups of triads on the basis of their properties. He could identify only 3 triads from the elements known at that time.
3: What were the limitations of Newlands’ Law of Octaves.
Ans:
(a) It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblanceto the properties of the elements above them.
(b) Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.
(c) The position of cobalt and nickel in the group of the elements (F, Cl) and Br) had different properties that could not be explained.
(d) Placing of iron far away from cobalt and nickel, which had similar properties as iron, could also not be explained.
INTEXT QUESTIONS (PAGE 85)
1: Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.
Ans:
K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
2: Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table. (Any two)
Ans: Scandium, Germanium
3: What were the criteria used by Mendeleev in creating his Periodic Table.
Ans.
(i) Mendeleev’s periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses.
(ii) So he placed elements with similar chemical properties in the same group.
(iii) Elements were arranged according to the atomic mass in increasing order.
4: Why do you think the noble gases are placed in a separate group.
Ans: Noble gases are inert elements. Their properties are different from all other elements. Therefore, the noble gases are placed in a separate group.
(IN TEXT QUESTIONS) PAGE 90
1: How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table.
Ans.
(i) In the modern periodic table the elements are arranged in the increasing order of their atomic no: thus removing the anomaly of certain pairs of elements in Mendeleev’s Periodic .Table.
(ii) All isotopes of the same elements have different atomic masses but same at number so they are placed in the same position in the Modern Periodic Table.
2: Name two elements you would expect to show chemical reactions similar to Magnesium. What is the basis for your choice.
Ans.
Calcium (Ca) and Barium (Ba) are expected to show chemical reactions similar to Magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
3: 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.
Ans: (a) Lithium (Li), (2,1) Sodium (Na), (2,8,1) and Potassium (K) (2,8,8,1) have a single electron in their outermost shells.
(b) Magnesium (2,8,2) (Mg) and Calcium (Ca) (2,8,8,2) have two electrons in their outermost shell. He(2)
(c) Neon (Ne) (2,8), Argon (Ar)(2,8,8), and have filled outermost shells.
4 :(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.
Ans: (a) Yes. The atoms of all the three elements Lithium, Sodium, and Potassium have one valence electron each. So they readily lose electrons to form positive ions.
(b) Both Helium and Neon have completely filled outermost shell.
5: In the Modern Periodic Table, which are the metals among the first ten elements.
Ans: Among the first ten elements, Lithium (Li) and Beryllium (Be) are metals.
6: 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
Ans: Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.