Sunday, June 19, 2022

Class 10 Science | Intext Questions & Answers | Chapter 5 | Periodic Classification of Elements

Page: 81


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

Answer:

Yes.

E.g., Li, Na, K.

If we consider lithium (Li) as the first element, then sodium (Na) is 8th element. If we consider sodium as the first element, then potassium is the 8th element.


2. What were the limitations of Döbereiner classification?


Answer:

It failed to arrange all the known elements in the form of triads having similar chemical properties. Döbereiner identified only three triads from the known elements.


3. What were the limitations of Newlands’ law of octaves?


Answer:

·  It is applicable only up to calcium. After this, every 8th element had no properties similar to that of the first.

· Newlands assumed that only 56 elements existed in nature. But new elements were discovered which did not obey Law of Octaves. To fit elements into his Table, he adjusted two elements in the same slot, and put some unlike elements under the same note. E.g. Cobalt & nickel are in the same slot and are placed in the column of F, Cl & Br. But their properties are very different. Iron resembles Co & Ni. But it was placed far away from them.

· Discovery of noble gases made Law of Octave irrelevant. Thus, this law worked well with lighter elements only.


Page: 85


1. 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.


2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table? (any two)


Answer:

Scandium and Germanium.


3. What were the criteria used by Mendeleev in creating his Periodic Table?


Answer:

Mendeleev used the relationship between the atomic masses of the elements and their similarities in physical and chemical properties (similarity in the formation of hydrides and oxides of element).


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


Answer:

Noble gases are chemically inert. Due to their similar inert behaviour and electronic configuration, they are placed in a separate group.


Page: 90


1. How could the modern periodic table remove various anomalies of Mendeleev’s periodic Table?


Answer:

Mendeleev’s periodic Table was based on the atomic masses of elements. But the Modern Periodic Table is based on the atomic numbers. So it rectified the following anomalies of Mendeleev’s periodic Table.

·  Positions of Co & Ni resolved based on atomic number.

·  Isotopes have the same atomic number so they are placed in the same group.

· Atom numbers are a whole number. So, there is no confusion about the presence of an element between two elements. The modern periodic table is based on atomic number, while Mendeleev’s periodic table was based on atomic mass.


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


Answer:

Beryllium (Be) and Calcium (Ca).

Be and Ca have similar electronic configuration, i.e. two electrons in outermost shells. (Be= 2,2. Ca= 2, 8, 8, 2).

Be and Ca react with oxygen to give basic oxides, BeO and MgO.


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 outer most shells.


Answer:

a.  Lithium, Sodium & Potassium.

b.  Beryllium & Calcium.

c.  Helium, Neon & Argon.


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?


Answer:

a. The atoms of lithium, sodium and potassium (Alkali metals) have only one electron in their outermost shells. They react with water to form alkalis.

b. The atoms of helium and neon have completely filled outermost shells. So they are chemically inert.


5. In the modern periodic table, which are the metals among the first ten elements?


Answer:

The first ten elements are hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. Out of these, lithium, beryllium and boron are metals, because they have 1, 2 and 3 electrons respectively in their outermost shells.


6.  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). In the periodic table, the elements placed on the left show maximum metallic characteristics. Among these elements, Beryllium occupies the most left position. Hence it shows maximum metallic characteristics.


No comments:

Post a Comment