Ncert solutions for class 10 science Periodic Classification of Elements
Answer: Yes, Dobereiner’s triads also existed in the columns of Newland’s Octaves. For example, Li, Na, K.
Q2. What were the limitations of Dobereiner’s classification?
Ans: Dobereiner could find only three triads i.e total of 9 elements only. It failed to arrange all the then-known elements in the form of triads of elements having similar chemical properties. Thus, Dobereiner’s classification was not able to classify most of the elements known at that time.
Q3. What were the limitations of Newlands’ law of octaves?
Ans: (i) Newlands law of octaves was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess properties similar to that of the first element.
(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands’ law of Octaves.
(iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.
Q4. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba
Answer: K2O, CO2, Al2O3, SiO2, BaO.
Q5. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table? (any two)
Answer: Scandium and Germanium.
Q6. What were the criteria used by Mendeleev in creating his Periodic Table?
Ans: Mendeleev used the relationship between the atomic masses of the elements and their physical and chemical properties. He used similarity in physical properties and similarity in the formation of hydrides and oxides of elements.
Q7. Why do you think the noble gases are placed in a separate group?
Answer: Noble gases are chemically inert and are present in the atmosphere in extremely low concentrations. Therefore, owing to their similar inert behaviour and similar electronic configuration, they are justified to be placed in a separate group.
Q8. How could the modern periodic table remove various anomalies of Mendeleev’s periodic Table?
Ans: (i) The modern periodic table is based on atomic number, while Mendeleev’s periodic table was based on atomic mass.
(ii) The isotopes of an element have same number of protons (or atomic number). So they are alloted the same position in modern periodic table.
(iii) Cobalt and nickel are placed at 9th and 10th position respectively.
(iv) Hydrogen has been alloted special position, i.e., it is placed at the top of alkali metals in the first group.
Q9. Name two elements you would expect to show chemical reactions similar to magnesium.
Ans: Beryllium (Be) and Calcium (Ca).
(a) three elements that have a single electron in their outermost shells.
Ans: Lithium, Sodium, and Potassium
(b) two elements that have two electrons in their outermost shells.
Ans: Beryllium, Calcium
(c) three elements with filled outermost shells.
Answer: Helium, Neon, Argon
Q11. (a) Lithium, sodium, and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Ans: (a) Lithium, sodium, and potassium all belong to the same group. The atoms of lithium, sodium, and potassium all have only one electron in their outermost shells and all of these are metals. All of these react with water to form alkalies.
(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: (b) The atoms of helium and neon have their outermost shells completely filled. Helium has its first shell completely filled, while neon has its first and second shells (K and L) completely filled.
Q12. In the modern periodic table, which are the metals among the first ten elements?
Ans: Lithium, beryllium and boron
Q13. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics?
Ga, Ge, As, Se, Be
Answer: Beryllium (Be)