In this post, you will get notes of periodic classification of elements chapter.
Dobereiner’s Triads
Dobereiner’s found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle, is the arithmetic mean of rest of the two.
He arranged three elements in one group which is known as Dobereiner’s Triads. For e.g.:
(Li) 7.0 (Na) 23.0 (K) 39.0 (Ca) 40.0 (Sr) 87.5 (Ba) 137.0
Here atomic mass of sodium is equal to arithmetic mean of atomic masses of lithium and potassium. Similarly, atomic mass of strontium is equal to arithmetic mean of atomic masses of calcium and barium.
Limitations of Dobereiner’s Triads
(a) He could identify only a few such triads and so the law could not gain importance.
(b) In the triad of Fe, Co, Ni, all the three elements have a nearly equal atomic mass and thus it does not follow the above law.
Mendeleev’s Periodic Classification
Mendeleev periodic law, states that ‘the properties of elements are the periodic function of their atomic masses’. Mendeleev’s periodic table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
Characteristics of the Mendeleev’s Periodic Table
(a) The elements are arranged in vertical rows called groups and horizontal rows called periods.
(b) There are eight groups indicated by Roman Numerals I, II, III, IV, V, VI, VII, VIII. The elements belonging to first seven groups have been divided into sub-groups designated as A and B on the basis of similarities. Group VIII consists of nine elements arranged in three triads.
(c) There are six periods (numbered 1,2,3,4,5 and 6).
Limitation of Mendeleev’s Periodic Table
(a) Some elements in Mendeleev’s Table have not been arranged in the increasing order of their atomic masses. For example, Co and Ni.
(b) Hydrogen forms similar compounds as Group 1 elements. However, it also forms similar diatomic molecules as Group 7 elements (H2, F2, C12, Br2, 12). Hence, it could not be assigned a fixed position in the table.
(c) Isotopes posed a challenge to Mendeleev’s table. For example, Cl has two major isotopes – Cl-35 and Cl-37.
Merits of Mendeleev’s Periodic Table
(a) Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group.
(b) Mendeleev predicted the discovery of some elements and named them as eka-boron, eka- aluminium and eka-silicon.
(c) One of the strengths of Mendeleev, s periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order.
Newlands’ Law of Octaves
According to this law “if elements are arranged by the increasing order of their atomic masses, property of every eighth element repeats.” The arrangement of elements in Newlands’ Octave resembles the musical notes.
Limitation of Newlands’ Octaves
(a) Law of Octaves could be valid up to calcium only; as after calcium, elements do not obey the rules of Octaves.
(b) It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future
(c) More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
(d) Iron, which has similar property as cobalt and nickel, was placed far from them.
(e) Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
Modern Periodic Table
In 1913, Henry Moseley showed that atomic number of an element is a more fundamental property than its atomic mass. According to this law “’properties of elements are a periodic function of their atomic number”.
Trends in Modern Periodic Table
| Property | Valency | Atomic Size | Metallic Character | Nonmetallic Character | Electronegativity |
| Variation in period | Increases from 1 to 4 then decreases to zero | Decreases | Decreases | Increases | Increases |
| Reason | No. of atomic shells remains the same & atomic number increases by 1 unit. | 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. | Effective nuclear charge increases. Hence tendency to lose electron decreases. | Effective nuclear charge increases. Hence tendency to gain electron increases | |
| Variation in group | Remains same | Increases | Increases | Decreases | Decreases |
| Reason | New shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge. | Effective nuclear charge decreases | Effective nuclear charge decreases. Hence tendency to gain electron decreases |
Position of Elements in the Modern Periodic Table
Position of Elements in the modern periodic table is given as follows:
| No of Valence Electrons | Group | No. of Shell | Period |
| 1 | 1 | 1 | 1 |
| 2 | 2 | 2 | 2 |
| 3 | 13 | 3 | 3 |
| 4 | 14 | 4 | 4 |
| 5 | 15 | 5 | 5 |
| 6 | 16 | 6 | 6 |
| 7 | 17 | 7 | 7 |
| 8 | 18 |
Elements are placed in groups according to the number of valence electrons and placed in periods according to the number of shells present in them. Helium has valence electrons equal to 2, but it is placed in group number 18 because it is a noble gas and has completely filled outermost shell.
Read more: Carbon and its Compound Notes
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