1.The basis of classification of elements changed from the atomic mass to atomic number due to

The properties of elements depend upon the number of electrons present in the valence shell which are related to atomic number.Thus the properties of different elements can be compared if we know their atomic numbers.On the other hand, atomic mass can in no way determine the chemical proper-ties of elements, because it does not vary regularly with gradation in the chemical properties of elements.❗

2.a)an element (e.g. arsenic, antimony, or tin) whose properties are intermediate between those of metals and solid non-metals or semiconductors.❗

b)The metalloids; boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), polonium (Po) and astatine (At) are the elements found along the step like line between metals and non-metals of the periodic table. Metalloids have properties of both metals and non-metals❗

c)The main group elements include groups 1 and 2 (excluding Hydrogen) on the left of the periodic table and groups 13 to 18 on the right of the table. The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table❗

3.*Anomalies of Mendeleev’s periodic table
*Correct position of hydrogen was not assigned.
*Position of isotopes was not defined.
*Placement of heavier element before lighter element.❗

4.The major limitations of Newlands' law of octaves were :  (i) It was applicable to only lighter elements having atomic masses upto 40 u, i.e., upto calcium. After cal properties. For example chromium (Cr) and yttrium (Y) are the first and the eighth element placed in the same column but they have entirely different properties.(ii) It was assumed that only 63 elements existed in nature and no new elements would be discovered in the future. But later on several new elements were discovered whose properties did not fit into the law of octaves. (iii) Some similar elements have been separated from one another while some dissimilar elements have been placed in the same column. For example, iron (Fe) resembles cobalt (Co) and nickel (Ni) but has been placed far away from these elements. Similarly cobalt and nickel do not resemble halogens (F, CI, Br) but have been placed in the same column. (iv) When noble gases were discovered, the properties of the eighth element were no longer similar to the first one. Actually now it was the first and the ninth element which has similar properties..❗

Q1. Basis of classification from atomic mass to atomic number

Reason for changing the basis of classification

- Atomic mass was initially used as the basis of classification of elements in the periodic table.
- However, with the discovery of isotopes, it was found that the atomic mass of an element could vary due to the presence of different isotopes.
- This led to difficulties in arranging the elements in a systematic manner.
- The solution to this problem came with the discovery of the relationship between the atomic number of an element and its electronic configuration.
- It was found that the electronic configuration of an element was directly related to its position in the periodic table.
- Thus, the basis of classification was changed from atomic mass to atomic number.

Advantages of using atomic number as the basis of classification

- The atomic number is a unique identifier for each element, and it does not change due to the presence of isotopes.
- The electronic configuration of an element is directly related to its atomic number, which makes it easier to arrange the elements in a systematic manner.
- The modern periodic table, which is based on atomic number, provides a better understanding of the properties and behavior of the elements.

Q2. (a) Metalloid (b) Names and atomic numbers of metalloids (c) Location in the Periodic Table

What is a metalloid?

- A metalloid is an element that has properties of both metals and non-metals.
- They have physical and chemical properties that are intermediate between those of metals and non-metals.

Names and atomic numbers of metalloids

- There are six metalloids in the periodic table: Boron (5), Silicon (14), Germanium (32), Arsenic (33), Antimony (51), and Tellurium (52).
- These elements are located in the p-block of the periodic table.

Location in the Periodic Table

- Metalloids are located along the zigzag line that separates the metals from the non-metals in the periodic table.
- They are found in groups 13 to 16 of the periodic table.

Q3. Anomalies of Mendeleev’s periodic table renamed by modern periodic law

Anomalies in Mendeleev’s periodic table

- Mendeleev’s periodic table had certain anomalies, such as the placement of certain elements in the wrong group or the wrong order.
- For example, iodine was placed in the same group as chlorine and fluorine, even though it had properties that were more similar to those of the elements in group 17.
- Similarly, elements such as cobalt and nickel were placed in the wrong order based on their atomic masses.

Renaming by modern periodic law

- The modern periodic law corrected these anomalies by organizing the elements based on their atomic number rather than their atomic mass.
- This led to a more accurate placement of the elements in the periodic table.
- Iodine was moved to group 17, and cobalt and nickel were placed in the correct order based on their atomic numbers.

Q4. Limitations of Newlands’ Law of Octaves

Newlands’ Law of Octaves

- Newlands’ Law of Octaves states that every eighth element in the periodic table has properties that are similar to the first element.
- He arranged