Periodic Classification Of Elements

Yes, Dobereiner's triads also exist in the columns of Newlands' octaves.

According to Newland’s law of octaves, every eighth element had properties similar to that of the first.
For example: the elements lithium (Li), sodium (Na) and potassium (K) are present in the second column of Newlands' classification of elements.
If we consider lithium as the first element, the 8^th element from it will be sodium.
Again, considering sodium as the 1^st element, the 8^th element from it is potassium.
This means that according to the Newland’s law of octaves, lithium, sodium and potassium should have similar properties.
We also know that lithium, sodium and potassium form a Dobereiner's triad having similar chemical properties.
This shows that Dobereiner's triads also exist in the columns of Newlands' octaves.

The limitation of Dobereiner's classification of elements:

Dobereiner could identify only three triads from the elements known at that time. So, all the elements could not be arragneged in the form of triads on the basis of their similar chemical properties.

Newlands' law of octaves for the classification of elements had the following limitations :

(i) This law was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess the properties similar to that of the first element.

(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future.

(iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties. For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot, and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.

The general formula for the oxidess of group I, II, III and IV are R₂O, RO, R₂O₃ and RO₂. Here R represent the element.

(i) The element K belongs to group I of Mendeleev’s peiodic table, thus the formula of its oxide will be K₂O.

(ii) The element C belongs to group IV of Mendeleev’s periodic table, thus the formula of its oxide will be CO₂.

(iii)The element Al (aluminium) belongs to group III of mendeleev’s periodic table, thus the formula of its oxide will be Al₂O₃.

(iv) The element Si (silicon) belongs to group IV of Mendeleev’s periodic table, thus the formula of its oxide will be SiO₂ .

(v) The element Ba (barium) belongs to group II of Mendeleev’s periodic table thus the formula of its oxide will be BaO.

Scandium (Sc) and Germanium (Ge) elements have been discovered for which gaps were left by mendeleev in his periodic table.

Mendeleev used two criteria in creating his periodic table :

(i) Atomic mass of the elements

(ii) Similar chemical properties of the elements.

He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.

Nobel gas are intert elements. They have unique properties compared to other elements, thus they are placed in a separate group in periodic table.

Modern periodic table removes various anomalies of Mendeleev’s periodic table as follows:

● In the modern periodic table, elements are arranged in the increasing order of their atomic number.

● In the modern periodic table, there was no problem of the place of isotopes. All isotopes of the same element have different atomic masses, but same atomic number. Hence, they can put at one place in the same group of the periodic table.

● Element having same valence electron are kept in the same group.

● In the modern periodic table, position of hydrogen became clear. It is kept in the group with the elements of same valence electrons.

Beryllium and Calcium would show chemical reactions similar to magnesium because beryllium, calcium and magnesium belong to same group i.e., group 2^nd in the modern periodic table.

(a) Lithium, Sodium and Potassium.

(b) Magnesium and Calcium.

(c) Helium, Neon and Argon.

(a) Lithium, sodium and potassium have following similarties:

(i) These all are belong to same group (Group I ) in the preiodic table.

(ii) They all have 1 electrone in their valene sheel.

(iii) They all are alkali metals.

(iv) They all are highly reactive.

(b) Helium and neaon both are noble gas which belong the zero group in the modern periodic table. The common thing between helium and neon is that both have outermost shell completely filled with electrons. Helium has two electrons in its K shell whereas neon has 8 electrons in its outermost shell.

Among the first ten elements, only 2 elements, Lithium (Li) and Beryllium (Be) are metals.

Metallic character of elements decrease as we move horizontally left to right in the periodic table. Thus, Be will have the maximum metallic character because it is on the extreme left side in the periodic table (in group 2).

On moving from left to right, the number of valence electrons increases. So, the tendency to lose electrons decreases whereas to accept electron increases. Thus, up to the first half, the valency increases in positive and in the next half, valency (in negative) decreases up to zero.

Mg has the valency 2. So, when it combine with chlorine having valency 1, it forms Mgcl₂. Whereas Na has valency 1, Al has valency 3 and Si has valency 4.

(a) Neon (2, 8) has two shells which are completely filled with electrons.

(b) Magnesium has electron configuration (2, 8, 2).

(c) Silicon (2, 8, 4) has three shells and four electrons in its valence shell.

(d) Boron (2, 3) has two shell and three electrons in its valence shell.

(e) Carbon (2, 4)

Boron belongs to group 13 in the periodic table. It has 3 valence electrons, hence its valency is 3. Therefore, all the elements in the same column of the periodic table as boron will have valency of 3.

Fluorine belongs to group 17 in the periodic table. It has 1 valence electrons, hence its valency is 1. Therefore, all the elements in the same column of the periodic table as fluorine will have valency of 1.

(a) The atomic number of this element is 2 + 8 + 7 = 17.

(b) The elctronic configuration of given element is 2, 8, 7. Thus, it has 7 valence electrons in its outermost shell. This element will be chemically similar to that element which has 7 valence electrons in its atoms. Now we will check electronic configuration of each element by using their atomic numbers.

(i) The electronic configuration of element N having atomic number 7 is 2, 5. It has 5 valence electrons.

(ii) The electronic configuration of element F having atomic number 9 is 2, 7. It has 7 valence electrons just like that of the given element. Thus, it would be chemically similar to the given element.

(iii) The electronic configuration of element P having atomic number 15 is 2,8,5. It has 5 valence electrons.

(iv) The electronic configuration of element Ar having atomic number 18 is 2,8,8. It has 8 valence electrons.

(a) Metallic character of elements decrease as we move horizontally left to right in the periodic table. The element A belongs to group 17 which is on the right side of the periodic table. Hence, element A does not show metallic character. It is a non-metal.

(b) The chemical reactivity decreases on going down in a group. Thus, element C is less reactive than element A.

(c) The atomic size decrease on going from left to right in a period. Hence, the atom of C will be smaller in size than an atom of B.

(d) Element A belongs to group 17 which has 7 valence electrons. It needs one electron to get noble gas configuration. So, it will accept 1 electron and form a negatively charged ion, A^-. The negatively vahrged ion is called an anion. Thus, element A will form an anion.

Electronic configuration of Nitrogen is 2, 5.

Electronic configuration of Phosphorus is 2, 8, 5.

Nitrogen will be more electronegative than Phosphorus because its atom has small size due to which the attraction of its nucleus for the incoming electron is more.

The electronic configuration of an element gives the information of valence electrons and number of shell present in the element. The modern periodic table has 18 verticle columns called groups and 7 horizontal raws called periods. In the modern periodic table, atoms with similar electronic configurations are placed in the same column, hence, the number of valence electrons remains the same in the group. We get the information of group number after knowing valence electrons. Number of shells present in an element is equal to period number. Thus, by knowing electronic configuration we know the group number and period number of an element, which is the position of element in periodic table.

The elctronic configuration of calcium having atomic number 20 is 2, 8, 8, 2. Thus, calcium has 2 valence electrons in its outermost shell. Now, that element which has 2 valence electrons will have physical and chemical properties resembling that of calcium.

The electronic configuration of element having atomic number 12 is 2, 8, 2. It has 2 valence electrons just like calcium. So, the element having atomic number 12 will have physical and chemical properties resembling that of calcium.

Comparison of Mendeleev's periodic table and the Modem periodic table: