Classification of Elements & Periodicity in properties Notes

NCERT CBSE Class 10 Notes Revision Notes for Class include notes from chapters 5 of the textbook.

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Classification of Elements:-Introduction

There are 128 elements know till now and it is difficult to study their properties

separately. So in order to make their study easy classification of elements is done.

Classification: It is the grouping of similar elements together and separating them from the elements which are not similar to them.

 Initially 31 elements were introduced than the discovery went to 63 then to 114 and now more than it.

It is very difficult to memorize properties of so many elements therefore some patterns were proposed to make their study easier.

Dobereiner’s law of triads

o He arranged elements in group of 3 that is known as triads.

o He proposes different traids depending upon many properties like increasing atomic mass, similar in properties and the atomic mass of the middle element is the arithmetic mean of atomic mass of first and third element.

For example:- Consider the elements Li, Na and K

 They have some chemical properties i.e. all are metals all react with water to form alkalis all have valency-1

Atomic mass of middle element (Na) = (7+39)/ (2) = 23

all are metals oxides of then are alkaline

all have valency 2.

Atomic mass of Sr = (40 + 137)/ (2) = 88

All are non-metals

All react with water of form acids

All have valency1

Atomic mass of Br = (35.5 + 127) /(2) = 80

Limitation- He failed to arrange all known elements in form of triad.

Newland’s Law of Octaves

According To him when elements are arranged in order of increasing atomic masses the properties of eight elements are repetition of properties of first element According to these properties eighth elements sodium should be similar to those of first element lithium.

LIMITATION-He could classify elements only up to calcium in this way.

Mendeleev’s classification of elements

Periodic table is chart of elements prepared in such a way that elements having similar properties occur in same vertical group.

 According to Mendeleev periodic law: -

Properties of elements are periodic function of their atomic in this there are seven periods and eight groups. Out of 8 groups seven are of normal elements and one of transition elements.

MERITS

He grouped the elements on the basis of atomic mass.

 He left gaps for undiscovered elements like Gallium, Scandium germanium. Even he left full group vacant for undiscovered inert gases.

 He could predict proportions of several elements on basis of their position in periodic table like Ga, Sc etc. 

He could predict errors in atomic weights of some elements like gold, platinum etc.

DEMERITS

1. Position of isotopes could not be explained.

2. wrong order of atomic masses could not be explained for example: - as Argon atomic mass 40 come first and K with low atomic mass) 30) should come later but k should be placed first all to his low.

3. Position of hydrogen could not be explained.

4. Some dissimilar elements are grouped together while some elements are placed in different groups. For example – alkali metals such as Li , Na, K, etc. 

(group AI) are grouped together with coinage metals such as Cu, Ag, and Au.

5. according to the properties of elements all periodic function of their atomic 

6. So, when elements are arranged according to increasing atomic no.’s there is periodicity in electronic configuration that leads to periodicity in their chemical properties.

Modern Periodic Table (By Bohr)

Introduction of modern periodic table: It consist of

horizontal rows (Periods)

Vertical column (Groups)

There are 7 period and 18 groups in this long form of periodic table.

Number of elements in each period:

Ist period has 2 elements

II nd period has 8 elements

IIIrd period has 8 elements

IVth period has 18 elements

Vth period has 18 elements

VIth period has 32 elements

VIIth period has rest of elements

NOTE: number of valence electron in atom of elements decides which elements will

be first in period and which will be last Families of different elements:

1 to 2 group and 13 to 17 contain normal or representative elements

3 to 12group – transition elements.

57 to 71 lanthanides

89 to 103 Actinides. 

Left hand side – metals.

Right hand side – non-metals

NOTE: Hydrogen element has been placed at top of Ist group. Electronic configuration of H is similar to alkali metal both have 1 valence electron.Element in s, p, d and f block and their electronic

configuration

S-block Elements: Elements in which the last electron enters the s orbitals.

General outer shell electronic configuration of s-block elements – ns1-2 where n = 2-7

General characteristics of s-block elements

1. They are soft metals with low melting and boiling points

2. They have low ionization enthalpies (energies) and are highly electro positive.

3. They lose the valence (outermost) electrons readily to form +1 (in case of alkali metals) and +2 ions (in case of alkaline earth metals).

4. They are very reactive metals. The metallic character and the reactivity increase as we move down the group. because of high reactivity, they are never found pure in nature.

5. The compounds of s-block elements with the exception of those of beryllium are predominantly ionic.

P-Block Elements: Elements in which the last electron enters any one of three p-orbitals of their respective outermost shells are called P-Block elements.

General outer shell electronic configuration of p-block elements –

ns2 mp1-6 where n = 2-7

General characteristics of p-block elements

1. P-block elements include both metal and non-metals but the number of nonmetals is much higher than that of metals.

 Further, the metallic character increases from top to bottom within a group and non-metallic character increase from left to right along a period in this block.

2. Their ionization enthalpies are relatively higher as compared to those s-block elements.

3. They mostly form covalent compounds.

4. Some of the show more than one (variable) oxidation states in their compound.

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