PERIODIC CLASSIFICATION OF ELEMENTS

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PERIODIC CLASSIFICATION OF ELEMENTS

The periodic table is a chart of elements prepared in such a way that the elements having similar properties occur in the same vertical column or group.

DOBERREINER’S TRIADS

When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar properties are obtained. The atomic mass of middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.

Limitation:-

It fails to arrange all the then known elements in the form of triads of the elements having similar chemical properties.

NEWLANDS’ LAW OF OCTAVES

When elements are arranged in the order of increasing atomic masses, the properties of the eighth element (staring from a given element) are a repetition of the properties of the first element.

Limitation:-

1. Applicable to the classification of elements up to calcium only.

2. Two elements remain in one slot and in the column of unlike elements having very different properties.

3. Iron element which resembles cobalt and nickel elements in properties, was placed for away from these elements.

MENDELEEEV’S PERIODIC TABLE

According to Mendeleev’s periodic law: the properties of elements are a periodic function of their atomic masses.

Merits

1. Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time.

2. Mendeleev’s periodic table could predict the properties of several elements on the basis of their positions in the periodic table.

3. Mendeleev’s periodic table could accommodate noble gases when they were discovered.

Demerits

1. The position of isotopes could not be explained.

2. Wrong order of atomic masses of some elements could not be explained.

3. A correct position could not be assigned to hydrogen in the periodic table.

PRESENT BASIS FOR THE CLASSIFICATION OF ELEMENTS

The present basis for the classification of the elements is the atomic number of elements.

Explanation of the Anomalies of Mendeleev’s Classification

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1. Explanation for the position of Isotopes : --- Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table.

2. Explanation for the position of Cobalt and Nickel : --- Cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even their atomic masses are in the wrong order.

Modern Periodic Law

When elements are arranged according to increasing atomic numbers, there is a periodicity in the electronic configurations of elements leads to the periodicity in their chemical properties.

When the elements are arranged according to increasing atomic numbers, then the elements having same number of valence electrons occur at regular intervals (or periods).

MODERN PERIODIC TABLE

The arrangement o elements in the modern periodic table are based on their electronic configuration. The horizontal row of elements in a table is called periods. The elements in a period have consecutive atomic numbers. The number of elements in a period is fixed by the maximum number of electrons which can be accommodated in the various shell of an atom.

The vertical columns in a periodic table are called groups. The elements in a period do not have consecutive atomic numbers. All the elements in a group have similar electronic configurations and show similar properties. The valence shells of all the noble gases are completely filled with electrons.

In the periodic table, metals have been separated from non-metals by some elements called ‘metalloids’ which are placed diagonally in the periodic table. Metalloids are Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb), Tellurium (Te), and Polonium (Po).

CHARACTERISTICS OF PERIODS

1. Valence Electrons--On moving from left to right in the period, the number of valence elements in elements increase from 1 to 8.

2. Valency—On moving from left to right in each period, the valence electron increases from 1 to 4 and then decreases to 0. Elements in the same period have different valences.

3. Size of atoms—On moving from left to right in a period of the periodic table, the size of atoms decreases.

4. Metallic Character – On moving from left from to right in a period, the chemical

reactivity of elements first decreases and then increases.

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2. Valency – All the elements in a group have the same valency.

3. Size of atoms—On going down in a group of the periodic table, the size of atoms increases.

4. Metallic Character—On going in a group of the periodic table, the metallic character of elements increases.

5. Chemical reactivity—The chemical reactivity of metals increase and the chemical reactivity of non-metals decreases on going down in a group of the periodic table.

6. Nature of oxides—On going down in a group of the periodic table, there is no change in the nature of oxides of elements.

Merits of the Modern Periodic Table

1. The modern periodic table is based on the atomic numbers of elements which is the most fundamental properties of elements.

2. The modern periodic table helps us understand why elements in a group show similar properties but elements in different groups show different groups show different properties.

3. The modern periodic table explains the reasons for the periodicity in properties of elements.

4. The modern periodic table tells us why the properties of elements are repeated after 2,8,18, and 32 elements.

5. There are no anomalies in the arrangement of elements in the modern periodic table.

Advantages of the periodic Table

1. The periodic table has made the study of chemistry systematic and easy.

2. It is easier to remember the properties of an element if its position in the periodic table is known.

3. The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

4. A periodic table is used as a teaching-aid in chemistry in schools.

Periodic table and Chemical Bonding

When an element from the left side of the periodic table combines with an element from the right side of the periodic table, ionic form is formed. Whenever an element from groups 1, 2 or 13 combines with an element from groups 14, 15, 16 or 17, an ionic bond is formed.

When an element from the right side of the periodic table combines with element from

the same side, a covalent bond is formed. Whenever from groups 14, 15, 16 and 17

combine together, covalent bonds are formed.

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Questions

1. What is Dobereiner’s law of triads? Give one example of a Dobereiner’s triads.

2. What were the limitations of Dobereiner’s clarification of elements?

3. What is Newlands’ law of octaves? Explain with examples.

4. Did Doberiner’s triads also exist in the column of Newlands’ law of octaves?

Explain your answer.

5. State Mendeleev’s periodic law. Describes two anomalies of Mendeleev’s periodic classification of elements.

6. Which group of elements was missing from Mendeleev’s original periodic table?

7. Name two elements whose properties were predicted on the basis of their position in Mendeleev’s periodic table.

8. Explain why, the noble gases are placed in a separate group.

9. State modern periodic law.

10. How many periods and groups are there in the long form of periodic table?

11. What is the significance of atomic number in the modern classification of elements?

12. How were the positions of cobalt and nickel resolved in the modern periodic table?

13. In the modern periodic table, which are the metals among the first ten elements/

14. Is it possible to have an element having atomic number 1.5 placed between hydrogen and helium?

15. Why does the size of atoms progressively become smaller when we move from sodium to chlorine in the third period of periodic table?

16. From the standpoint of electron arrangements, what determines which elements will be the first and which the last in a period of periodic table?

17. What is the number of elements in the : (a) 1

^st

period, and (b) 3

^rd

period, of the modern periodic table?

18. How does the valency of elements change on moving from left to right in the third period of the periodic table?

19. How would the tendency to lose electrons change as we go from left to right in a period of the periodic table? Why does it vary this way?

20. What happens to the metallic character of the elements as we move from left to right in a period of the periodic table?

21. what is the usual number of valence electrons and valency of group 18 elements of the periodic table?

22. How does the electropositive character of elements change on going down in a group of the periodic table?

23. Helium and neon are uncreative gases. What, if anything, do their atoms have in

common?

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Give reason you’re your choice.

26. In each of the following pairs, choose the atom having the bigger size:

(a) Mg (At. No. 12) or Cl ( At. No. 17) (b) Na (At. No. 11) or K (At. No. 19)

27. An element X belongs to 3

^rd

periodic table and group 2 of the periodic table. State (a) Number of valence electrons, (b) valency,

(c) Metal or non-metal, and (d) name of the element.

28. An element X from group 2 reacts with element Y from group 16 of the periodic table.

(a) What is the formula of the compound formed?

(b) What is the nature of bond in the compound formed?

29. A metal X is in the first group of the periodic table. What will be the formula of the oxide?

30. The elements A,B and C belong to groups 1, 14 and 17 respectively of the periodic table.

(a) Which two elements will form a covalent compound?

(b) Which two elements will form an ionic compound?