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CBSE NOTES CLASS 10 SCIENCE CHAPTER 5

PERIODIC CLASSIFICATION OF ELEMENTS

Need for classification of elements

Dobereiner’s law of triads

Newland’s law of octaves

Mendeleev’s periodic table

Achievements of mendeleev’s periodic table

Limitations of mendeleev’s classification

Modern periodic law

Cause of periodicity

Modern periodic table/long form of periodic table or bohr’s table

Trends in modern periodic table

Metallic and non-metallic character

Atomic size or radius

Ionization enthalpy

Electron gain enthalpy

Valence electrons

Periodic trends and chemical reactivity

CBSE NOTES CLASS 10 SCIENCE CHAPTER 5

PERIODIC CLASSIFICATION OF ELEMENTS

Classification of elements

Classification means identifying similar species and grouping them together.

Need for Classification of elements

There are 118 elements and each of them has innumerable compounds. It is very difficult to study and understand all the elements and their compounds individually. Hence there is a need for classification or grouping of elements based on some common properties, so that the study of elements and their compounds could be made easy.

Lavoisier divided elements into two main types known as metals and non-metals.

Dobereiner’s Law of Triads

For some groups of three elements, when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

For example atomic masses of Li, Na and K are respectively 7, 23 and 39, thus the mean of atomic masses of 1st and 3rd element is equal to the mass of the second one.

Elements

Atomic Masses

Elements

Atomic Masses

Elements

Atomic Masses

Li

7

Ca

40

Cl

35.5

Na

23

Sr

88

Br

80

K

39

Ba

137

I

127

Limitations of Doberiner’s Triads

He could identify only a few such triads and so the law could not gain importance. All known elements could not be classified into groups of triads on the basis of their properties.

Newland’s Law of Octaves

According to this law “Every eighth element has properties which are a repetition of those of the first, if arranged in order of increasing atomic mass, like the eight notes of musical scale.”

sa (do)

re (re)

ga (mi)

ma (fa)

pa (so)

da (la)

ni (ti)

H

Li

Be

B

C

N

O

F

Na

Mg

Al

Si

P

S

Cl

K

Ca

Cr

Ti

Mn

Fe

Co, Ni

Cu

Zn

Y

In

As

Se

Br

Rb

Sr

Ce, La

Zr

Limitations of Newland’s Law of Octaves

Thus, Newland’s Law of Octaves worked well with lighter elements only.

Mendeleev’s Periodic Table

Mendeleev arranged 63 elements, known at that time, in the periodic table. According to Mendeleev “The properties of the elements are a periodic function of their atomic masses.”

Basis of Classification

The formulae of the hydrides and oxides formed by an element were treated as the basic properties of an element for its classification.

He selected hydrogen and oxygen because they are very reactive and formed compounds with most elements. If the formula for hydride was RHx, then the element R will be placed in xth group.

Structure of Mendeleev’s Periodic Table

Achievements of Mendeleev’s Periodic Table

(i) Systematic Study

The arrangement of elements in groups and periods made the study of elements quite systematic. That is, if properties of one element in a particular group are known, those of the others in the group can be easily predicted.

(ii) Prediction of new elements and their properties

Many gaps were left in this table for undiscovered elements and properties of these elements could be predicted in advance from their expected position. Mendeleev named them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.

(iii) Correction of doubtful atomic masses

Mendeleev corrected the atomic masses of certain elements with the help of their expected positions and properties.

(iv) Position of inert gases

When inert gases were discovered they were placed in a new group without disturbing the order.

Limitations of Mendeleev’s Classification

(i) He could not assign a correct position to hydrogen in his periodic table, as the properties of hydrogen resemble both with alkali metals as well as with halogens.

(ii) The isotopes of the same element will be given different position if atomic mass is taken as basis. But the properties of isotopes of the same element are same.

(iii) The atomic masses do not increase in a regular manner in going from one element to the next. At some places the order of atomic masses in the table was changed in order to justify the chemical and physical nature. For example, cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7).

Modern Periodic Law

This law was given by Henry Moseley in 1913. It states,

“Properties of the elements are the periodic function of their atomic numbers”.

Cause of periodicity

Periodicity may be defined as the repetition of the properties of the elements separated by certain definite gap of atomic numbers.

Chemical bonding depends on the number of electrons in the valence shell of an element. Due to same number of electrons in the outermost shell of elements in the same group, the chemical properties are similar.

Since nucleus (mass) resides deep inside the atom, it does not take part in chemical reactions. On the other hand the electrons are outside, in the orbits, hence they interact with other atoms.

Modern Periodic Table

Moseley proposed the modern periodic table also known as long form of the periodic table.

(i) There are 18 groups and 7 periods in the table. The groups are numbered 1, 2, 3 …. 18 from left to right and the periods from 1 to 7 from top to bottom

(ii) The elements belonging to a particular group make a family and usually named after the first member.

(iii) In a group all the elements contain the same number of valence electrons.

(iv) In a period all the elements contain the same number of shells, but as we move from left to right the number of valence shell electrons increases by one unit.

(v) The maximum number of electrons that can be accommodated in a shell is 2n2 where n is the number of the given shell from the nucleus.

Trends in Modern Periodic Table

The trends observed in some important properties of the elements in moving down the group (from top to bottom of the table) and across a period (from left to right in a period) are discussed below

Valency

Valency may be defined as the combining capacity of the atom of an element with atoms of other elements in order to acquire the stable configuration (i.e. 8 electron in valence shell. In some special cases it is 2 electrons). The number of valence electrons in a group is same, whereas the number of valence electrons within a period increases from left to right.

Atomic size or radius

It refers to the distance between the centre of nucleus of an isolated atom to its outermost shell containing electrons.

The atomic radius decreases on moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Atomic size increases in a group from top to bottom due to increase in number of shells.

Metallic and non-metallic character

In a period from left to right metallic character decreases while non-metallic character increases.

In a group metallic character increases from top to bottom while non-metallic character decreases.

A zig-zag line separates metals from non-metals. The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals.

As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease. Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus. Therefore, these can be lost easily. Hence metallic character decreases across a period and increases down a group.

These trends also help to predict the nature of oxides formed by the elements because it is known to you that the oxides of metals are basic and that of non-metals are acidic in general.

Ionization Enthalpy

The energy required to remove an electron from an isolated gaseous atom (X) in its ground state, is called ionization enthalpy.

M(g) + ΔiH → M+(g) + e-

Electron Gain Enthalpy

The electron gain enthalpy ((∆egH) is the molar enthalpy change when an isolated gaseous atom or ion in its ground state adds an electron to form the corresponding anion.

Electronegativity

The relative tendency of an atom to attract the shared electron pair of electrons towards itself is called electronegativity.

Periodic Trends and Chemical Reactivity

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