Periodic classification of elements

1. Dobereiner’s triads: Johann Wolfgang Dobereiner, a German chemist,

classified the known elements in groups of three elements on the basis

of similarities in their properties. These groups were called triads.

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a. Properties of elements in each triad were similar.

b. Atomic mass of the middle elements was roughly the average of the

atomic masses of the other two elements.

Characteristic of Dobereiner’s Triads:•

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able to prepare triads of all the known elements

2. Newlands’ Law of Octaves: John Newlands, an English scientist,

arranged the known elements in the order of increasing atomic masses

and called it the ‘Law of Octaves’. It is known as ‘Newlands’ Law of

Octaves’

Limitations: Dobereiner could identify only three triads. He was not

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a. It contained the elements from hydrogen to thorium

b. Properties of every eighth element were similar to that of the first

element

Characteristics of Newlands’ Law of Octaves•

Limitations of Newlands’ law of Octaves:

a. The law was applicable to elements upto calcium (Ca) only

b. It contained only 56 elements. Further it was assumed by Newlands

that only 56 elements existed in nature and no more elements would

be discovered in the future.

c. In order to fit elements into the table. Newlands’ adjusted two

elements in the same slot and also put some unlike elements under

same note. For example cobalt and nickel are in the same slot and

these are placed in the same column as fluorine, chlorine and bromine

which have very different properties than these elements. Iron, which

resembles cobalt and nickel in properties, has been placed differently

away from these elements

3. Mendeleev’s Periodic Table: Dmitri Ivanovich Mendeleev, a Russian

chemist, was the most important contributor to the early development

of a periodic table of elements wherein the elements were arranged on

the basis of their atomic mass and chemical properties.

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a. Mendeleev arranged all the 63 known elements in an increasing order

of their atomic masses.

b. The table contained vertical columns called ‘groups’ and horizontal

rows called ‘periods’.

c. The elements with similar physical and chemical properties came

under same groups.

Characteristic of Mendeleev’s Period Table:

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function of their atomic masses.

Mendeleev’s Periodic Law: The properties of elements are the periodic

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a. Through this table, it was very easy to study the physical and

chemical properties of various elements.

b. Mendeleev adjusted few elements with a slightly greater atomic mass

before the elements with slightly lower atomic mass, so that elements

with similar properties could be grouped together. For example,

aluminum appeared before silicon, cobalt appeared before nickel.

c. Mendeleev left some gaps in his periodic table. He predicted the

existence of some elements that had not been discovered at that time.

His predictions were quite true as elements like scandium; gallium and

germanium were discovered later

d. The gases like helium, neon and argon, which were discovered later,

were placed in a new group without disturbing the existing order

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a. He could not assign a correct position to hydrogen in the periodic table

b. Positions of isotopes of all elements was not certain according to

Mendeleev’s periodic table

c. Atomic masses did not increase in a regular manner in going from one

element to the next. So it was not possible to predict how many

elements could be discovered between two elements

4. Modern Periodic Table: Henry Moseley gave a new property of

elements, ‘atomic number’ and this was adopted as the basis of

Modern Periodic Table’.

Limitations of Mendeleev’s Periodic Table :

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their atomic number

Modern Periodic Law: Properties of elements are a periodic function of

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a. The modern periodic table consists of 18 groups and 7 periods

b. Elements present in any one group have the same number of valence

electrons. Also, the number of shells increases as we go down the

group.

c. Elements present in any one period, contain the same number of

shells. Also, with increase in atomic number by one unit on moving

from left to right, the valence shell electron increases by one unit

d. Each period marks a new electronic shell getting filled

Position of elements in Modern Periodic Table:

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(i) Valency: Valency of an element is determined by the number of

valence electrons present in the outermost shell of its atom

distance between outermost electrons and nucleus which increases the

size of atom

(iii) Metallic and Non- metallic Properties:

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called metallic character of an element

The tendency to lose electrons from the outermost shell of an atom, is

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group

Metallic character decreases across a period and increases down the

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called non- metallic character of an element

The tendency to gain electron in the outermost shell of an atom, is

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the group

Non-metallic character increases across a period and decreases down

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characteristic of both metals and non-metals are called as semi-metals

or metalloids

5. Metals have a tendency to loose electrons while forming bond. Hence

they are electropositive in nature

6. Non-metals have a tendency to gain electrons while forming bond.

Hence they are electronegative in nature

7. Oxides formed by metals are generally basic and oxides formed by

non-metals are generally acidicElements intermediate between metal and non-metals that show

• Valency of elements in a particular group is same

• Valency of elements in a particular period increases by one unit from

left to right with the increase in atomic number by one unit

(ii) Atomic Size: Atomic size refers to the radius of an atom.

Trends in the Modern Periodic Table:

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due to increase in nuclear charge which tends to pull the electrons

closer to nucleus and reduces size of atom

In a period, atomic size and radii decreases from left to right. This is

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because on moving down, new shells are added. This increasesIn a group, atomic size and radii increases from top to bottom. This isAchievements of Mendeleev’s Periodic Table:Table showing Newlands’ Octaves:Example of Dobereiner’s Triads :