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1. The process of grouping the elements on the basis of the similarities and differences for the easy recognition and study of the element and their properties and characteristics is called the classification of elements.

2. The contribution of fohn Dalton in the classification of elements is that he profounded the atomic theory and said that the atoms of different elements have different atomic weights.

The contribution of Prout is that he said the atomic weight of an element is a simple multiple of the atomic weight of hydrogen.

3. The table with 7 periods and 18 groups that are designed in such a way that it can arrange all elements according to their properties, similarities die dissimilarities, etc which helps in the study of elements is called a periodic table.

4. Dobereiner arranged elements in increasing order of their atomic weight in well-marked groups of three-element called trides.

So, the pattern of arranging the dements in each triads, that the atomic weight of middle elements was found to be the arithmetic mean of the atomic weights of other two elements, called Dobereiner's law of trials.

For example

Elements:            Li        Na        K
Atomic weight:   7         23        39

So, 23 = (7+39) / 2

5. Newlands law of octaves stated, 'Elements when arranged increasing order of their atomic weights show resemblance in physical and chemical properties between the eight and first element is similar to the eight and first notes on a musical scale.

For example, potassium is the eight element after sodium.
These elements have similar physical and chemical properties.

6. Increasing order of atomic weight was the basis of Mendeleev's periodic table.

7. Mendeleev's periodic table states that 'The physical & chemical properties of elements are the periodic function of their atomic weight'.

8. Any three characteristics of Mendeleev's periodic table are:

1. It was the first scientist and systematic study of known elements.
2. It contains many groups for undiscovered elements.
3. Properties of several elements can be studied.

9. Any 4 defects in Mendeleev's periodic table are:

1. Inert gas or noble gases were absent.
2. It couldn't arrange lanthanides and actinides.
3. Its eight groups weren't divided into sub-group.
4. It couldn't arrange isotopes.

10. The increasing order of an atomic number of the elements was the basis of the modern periodic table.

11. Modern periodic rule states that, 'The physical and chemical properties of the elements are the periodic function of their atomic number'.

12. In the modern periodic table, the physical & chemical properties of the elements are changed to the periodic function of their atomic numbers instead of atomic weight which solves the problem with isotopes. All isotopes of some element represented by the same atomic number. All the gaps of Mendeleev's periodic table are filled by discovering new elements. The column for inert gases was developed and all groups have sub-groups. In this way, the modern periodic table removes all the demerits of Mendeleev's periodic table.

13. The 14 elements from cerium te lawrencium after lutetium are called lanthanides.
The elements from thorium to lawrencium after actinium are called actinides.

These elements are placed at the bottom of the periodic table because of their similar properties. Lanthanides are rare metals whereas actinides are artificial radioactive metals. They show similar properties to the transitional elements. So, they are placed separately below the transitional elements in blocks of the modem periodic table.

14. The horizontal rows of the periodic table where the elements are arranged according to their similarities and differences for their easy study are called periods.

The vertical columns where elements are arranged for the recognization of the properties and other studies are called groups.

15. The elements where the last electron enters in s-orbital are called s-block elements.
The elements where the last electron enters the p-orbital of the outermost shell are called p-block elements.

The position of s-block elements is IA and IIA groups.
The position of p-block elements are from IIIB to IIB group in the periodic table

16. The electrons present in the outermost shell or valence shell of an atom are called valence electrons.

In a period, valency increases up to the 4th group and decreases up to the 8th group and again increases.

17. The element that shows the characteristics or properties of both metals & non-metals is called metalloids. Any 3 examples are:

1. Silicon (Si)
2. Germanium (Ge)
3. Arsenic (As)

18. a. Mendeleev's and the modern periodic table.

 Mendeleev's Modern It supported the first scientific study of elements. It is the latest and standard table of elements. Elements were arranged with the increasing order of their atomic weight. Elements were arranged with the increasing order of their atomic number. It had several gaps for undiscovered elements. Most of the elements are discovered and arranged. It was unable to arrange isotopes. It is able to arrange isotopes. It didn't have lanthanides and actinides series. It consists of lanthanides and actinides series.

b. Periods and groups.

 Periods Groups The horizontal rows in the periodic table are periods of the periodic table. The vertical column in the periodic table are groups of the periodic table. The change in valency is found in a period. The valency of elements remains the same in a group. A period contains the elements of different blocks. A group contains the element of the same block.

c. s-block elements and p-block elements.

 s-block p-block The last electron of these elements enters the s-orbital of the outermost shell. The last electron of these elements enters the p-orbital of the outermost shell. These elements are alkali metals. These elements are non-metals. These elements are electronically positive. i.e. they have positive valency. These elements have negative valency.

d. Alkali metals and halogens.

 Alkali metals Halogens These elements are highly reactive electro-positive metals. These elements are highly reactive electro-negative metals. These elements are good conductors of electricity. These elements are insulators of electricty. These elements have positive valency. These elements have negative valency.

e. Alkali metals and alkaline earth metals.

 Alkali metals Alkaline earth metals These elements are placed in group IA of the periodic table. These elements are placed in group IIA of the periodic table. These elements are more reactive. These elements are comparatively less reactive. These elements have valency +1. These elements have valency +2.

f. Duplet stable state and octet stable state.

 Duplet stable Octet stable The atom which has a stable electronic configuration having 2 electrons in its valance shell is duplet stable. The atom which has a stable electronic configuration having 8 electrons in its valance shell is octet stable. This is possible only in 1st or 2nd shell. This is possible in any shell of an atom.

19. We know that sodium has positive valency but that of chlorine is negative. The last electron of sodium enters into the s-orbital but that of chlorine enters the p-orbital of the outermost shell. So, sodium is kept in s-block but chlorine is kept in p-block.

20. We know, both potassium and sodium are metals and lose the valence electrons. The atomic size of potassium more than that of sodium. So, the valence electron of potassium can be more easily taken by other reacting atoms as compared to that of sodium. Hence, potassium is more reactive than sodium.

21. We know, the atomic size of fluorine is smaller than that of chlorine. So, the nuclear attraction is more on the valence shell of the Fluorine atom. As a result, fluorine can get one electron more easily during the chemical reaction. Hence, fluorine is more reactive than chlorine.

22. We know, the properties of f-block elements are similar to the transition metals of d-block, However, their last electron enters count f-orbital. So, f-block elements are kept separately below d-block elements or transitional elements. Hence, these elements are called inner transitional elements.

23. We know, alkaline earth metals are found on the surface of the earth in form of oxide which shows properties similar to that of alkali metals. So, these elements are called alkaline earth metals.

24. We know, every element of the zero group has achieved stable electronic configuration being duplet stable or octet stable They have the valency zero and do not take part in any chemical reaction. So, elements of the zero group are called inert gas.

25. When we move from top to doon of periodic groups of metals, the atomic size increases gradually. Due to this, nuclear attraction to the valence electron decreases. As a result, the tendency of metal atoms to lose electrons increases. Hence chemical reactivity of metals increases when moving from top to bottom in a group.

26. When we move from top to bottom in a group of non-metal, the atomic size increases gradually. Hence, nuclear attraction for incoming electrons decreases. As a result, the tendency of non-metallic atoms to gain electrons decreases. Therefore, chemical reactivity of non-metals decreases on moving down in a group.

27. We know, lanthanides are rare earth metals, and actinides are radioactive metals. The properties of lanthanides resemble each other very closely but are different from the rest elements due to the preferential filling of f-orbitals. So, they are placed separately to avoid unnecessary sidewise expansion of the periodic table.