Till now 118 elements are known with a well known classified group and period in modern periodic table. It sounds we easy to us only 118 elements are yet discovered but many more to come. During the 18th century only 30 elements were known with different physical & chemical properties and it is difficult to study. Therefore these elements are classified into round & period and later during the 19th century more elements are discovered and they are arranged according similarities and dissimilarities in much time and effort of a chemist are required to finally develop the most appropriate classification of element. This chapter is all about the brief historical development of classification of elements with their merits and demerits.
Yes, there is only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.
Dobereiner could only find three triad from the elements known at that time.
Dobereiner’s triads
| H | Li | Be | B | C | N | O |
| F | Na | Mg | Al | Si | P | S |
| Cl | K | Ca | Cr | Ti | Mn | Fe |
| Co | Cu | Zn | Y | In | As | Se |
| Br | Rb | Sr | Ce and La | Zr | - | - |
According to Dobereiner’s classification: All elements present during that period cannot be classified into a group of three on the basis of their properties.
Limitations of Newlands’ law of octave are as follow:
(i) This law is only applicable upto the calcium element. Elements after calcium don’t possess the properties similar to that of first.
(ii) He assumed that only 56 elements exist in nature, no more element can be further discovered.
(iii) Newland’s adjusted 2 elements in the same slot to fit the element in the table. Eg. cobalt and nickel are placed in the same slot with columns having fluorine, chlorine and bromine which have completely different properties.
The formulae for oxide s of elements are:
K is in group 1. So, the oxide will be K2O.
C is in group 4. So, the oxide will be CO2.
Al is in group 3. So, the oxide will be Al2O3.
Si is in group 4. So, the oxide will be SiO2.
Ba is in group 2. So, the oxide will be BaO.
Mendeleev predicted the existence of elements that were not discovered by that time. He added a prefix to the preceding element as Eka-boron, Eka-Silicon. Later the elements are discovered. Eka-boron as scandium and Eka-Silicon as Germanium.
Mendeleeve arranged elements in increasing order of their atomic mass in its periodic table. He observed that there occurs a period recurrence of elements with similar physical and chemical properties.
Noble gases are non reactive gases they are chemically inert therefore the elements are placed in separate groups.
Modern periodic table remove anomalies of mendeleevs periodic table:
i). Position of Hydrogen get fixed in modern periodic table that mendeleevs couldn't explain.
ii). Position of isotopes are explained in modern periodic table because element are arranged according to their atomic number.
Two elements that show similar chemical ration as magnesium, calcium (Ca) and strontium (Sr) because the number of valence electrons (2) is same in all these three elements as chemical properties are due to valence electrons, they show the same chemical reactions.
(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
(a) Yes. All these elements have one valence electron in its outermost shells.
(b) Helium and Neon are inert noble gases. They are least reactive elements because of completely filled valence electrons. Helium has completely filled the K shell with 2 electrons whereas neon has completely filled L Shell with B electron.
The first ten elements that are metal:
Be lies to the left hand side of the periodic table, Be is the most metallic among the given elements.
(c) The atoms lose their electrons more easily.
As we move from left to right across the periods of the periodic table, the non-metallic character increases. So, the tendency to lose electrons decreases.
(b) X elements from compound XCl2 which is most likely formed compound as magnesium because of having the same valency whereas the rest of other elements as Na, Al and Si don't match the valence electron to that of X.
(a) Neon has two shells, both of which are completely filled with electrons (K shell with 2 electrons and L shell with 8 electrons).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (K shell with 2 electrons, L shell with 8 electrons and M shell with 4 electrons.)
(d) Boron has a total of two shells, with three electrons in its valence shell (K shell with 2 electrons and L shell with 3 electrons).
(e) Carbon has twice as many electrons in its second shell as in its first shell (K shell with 2 electrons and L shell with 4 electrons).
(a) All the elements in the same column as boron have the same number of valence electrons (3). So, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons (7). So, they all have valency equal to 1.
(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.
(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.
| Element | K L M |
|
Nitrogen Phosphorus |
2 5 2 8 5 |
Nitrogen is more electronegative than phosphorus. As we move down a group, the number of shells increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. Resulting in a decrease in the tendency to attract electrons and hence electronegativity decreases.
Atoms with similar electronic configurations are placed in the same column in the modern periodic table. In a group, the number of valence electrons remains the same whereas elements across a period show an increase in the number of valence electrons.
The element with atomic number 12 has the same chemical properties as that of calcium. This is because both of them have the same number of valence electrons (2).
Calcium electronic is - 2,8,2
Element having atomic number 12 have - 2,8,2.
| Mendeleev’s periodic table |
Modern |
|---|---|
| 1. Elements are arranged in the increasing order of their atomic masses. | 1. Elements are arranged in the increasing order of their atomic numbers. |
| 2. There are a total of 7 groups (columns) and 6 periods (rows). | 2. There are a total of 18 groups (columns) and 7 periods (rows). |
| 3. Elements having similar properties were placed directly under one another. | 3. Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group. |
| 4. The position of hydrogen could not be explained. | 4. Hydrogen is placed above alkali metals. |
| 5. No distinguishing positions for metals and non-metals. | 5. Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side. |