Periodicclassificationofelements Notes&Questions

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PERIODIC CLASSIFICATION OF ELEMENTS

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

DOBERREINERS TRIADSWhen 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 ofthe 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 thefirst 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, wasplaced for away from these elements.

MENDELEEEVS PERIODIC TABLE

According to Mendeleevs periodic law: the properties of elements are a periodic

function of their atomic masses.Merits1. Mendeleevs periodic law predicted the existence of some elements that had

not been discovered at that time.

2. Mendeleevs periodic table could predict the properties of several elements onthe basis of their positions in the periodic table.

3. Mendeleevs periodic table could accommodate noble gases when they were

discovered.

Demerits1. 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 Mendeleevs Classification

BY SUHAIL SIR 9810678227 1


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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 aperiodicity 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 ina 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 ofan 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 arecompletely 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 areBoron (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 ofvalence elements in elements increase from 1 to 8.

2. ValencyOn 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 atomsOn 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 chemicalreactivity of elements first decreases and then increases.

5. Nature of oxidesOn moving from left to right in a period, the basic nature of

oxides decreases and the acidic nature of oxides increases.

CHARACTESTIC OF GROUPS

1. Valence Electrons All the elements of a group of the periodic table have the

same number of valence electrons.



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

3. Size of atomsOn going down in a group of the periodic table, the size of atoms

increases.4. Metallic CharacterOn going in a group of the periodic table, the metallic

character of elements increases.

5. Chemical reactivityThe chemical reactivity of metals increase and thechemical reactivity of non-metals decreases on going down in a group of the

periodic table.

6. Nature of oxidesOn going down in a group of the periodic table, there is nochange 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 isthe 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 itsposition 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 bondis 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 17combine together, covalent bonds are formed.



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Questions

1. What is Dobereiners law of triads? Give one example of a Dobereiners triads.2. What were the limitations of Dobereiners clarification of elements?

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

4. Did Doberiners triads also exist in the column of Newlands law of octaves?Explain your answer.

5. State Mendeleevs periodic law. Describes two anomalies of Mendeleevs

periodic classification of elements.6. Which group of elements was missing from Mendeleevs original periodic table?

7. Name two elements whose properties were predicted on the basis of their position

in Mendeleevs 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 fromsodium 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) 1st period, and (b) 3rd period, of the

modern periodic table?

18. How does the valency of elements change on moving from left to right in the thirdperiod 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 incommon?

24. What property do all elements in the same column of the periodic table as born

have in common?25. The atomic numbers of the three elements X, Y, and Z are 2, 6 and 10

respectively.

(i) Which two elements belong to the same group?

(ii) Which two elements belong to the same period?



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Give reason youre 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 3rd 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?


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Periodicclassificationofelements Notes&Questions