Periodic Classification of Elements Class 10 Science Notes Maharashtra State Board
Classification of Elements
We have learned in the previous standards that all the atoms of an element are of only one type. Today 118 elements are known to the scientific world. However, around the year 1800, only about 30 elements were known. More number of elements were discovered in the course of time. More and more information about the properties of these elements was gathered. To ease the study of such a large number of elements, scientists started studying the pattern if any, in the vast information about them. You know that in the initial classification, elements were classified into the groups of metals and nonmetals. Later on, another class of elements called metalloids was noticed. As the knowledge about elements and their properties went on increasing different scientists started trying out different methods of classification.
In the year 1817, a German scientist Dobereiner suggested that the properties of elements are related to their atomic masses. He made groups of three elements each, having similar chemical properties, and called them triads. He arranged the three elements in a triad in increasing order of atomic mass and showed that the atomic mass of the middle element was approximately equal to the mean of the atomic masses of the other two elements. However, all the known elements could not be classified into Dobereiner’s triads.
In the Indian music system, there are seven main notes, namely, Sa, Re, Ga, Ma, Pa, Dha, Ni, and their collection is called ‘Saptak’. The frequency of the notes goes on increasing from ‘Sa’ to ‘Ni’. Then comes, the ‘Sa’ of the upper ‘Saptak’ at double the frequency of the original ‘Sa’. It means that notes repeat after the completion of one ‘Saptak’. The seven notes in Western music are Do, Re, Mi, Fa, So, La, and Ti. The note ‘Do’ having double the original frequency comes again at the eighth place. This is the octave of Western notes. Music is created by the variety in the use of these notes.
Newlands’ Law of Octaves
The English scientist John Newlands correlated the atomic masses of elements to their properties in a different way. In the year 1866 Newlands arranged the elements known at that time in an increasing order of their atomic masses. It started with the lightest element hydrogen and ended up with thorium. He found that every eighth element had properties similar to those of the first. For example, sodium is the eighth element from lithium and both have similar properties. Also, magnesium shows similarity to beryllium and chlorine shows similarity with fluorine. Newlands compared this similarity with the octaves in music. He called the similarity observed in the eighth and the first element the Law of octaves.
Many limitations were found in Newlands’ octaves. This law was found to be applicable only upto calcium. Newlands fitted all the known elements in a table of 7 × 8 which is 56 boxes. Newlands placed two elements each in some boxes to accommodate all the known elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some elements with different properties under the same note in the octave. For example, Newlands placed the metals Co and Ni under the note ‘Do’ along with halogens, while Fe, having similarity with Co and Ni, was away from them along with the nonmetals O and S under the note ‘Ti’. Also, Newlands’ octaves did not have provisions to accommodate the newly discovered elements. The properties of the new elements discovered later on did not fit in the Newlands’ law of octaves.
Mendeleev’s Periodic Table
The Russian scientist Dmitri Mendeleev developed the periodic table of elements during the period 1869 to 1872 A.D. Mendeleev’s periodic table is the most important step in the classification of elements. Mendeleev considered the fundamental property of elements, namely, the atomic mass, as standard and arranged 63 elements known at that time in increasing order of their atomic masses. Then he transformed this into the periodic table of elements in accordance with the physical and chemical properties of these elements.
Mendeleev organized the periodic table on the basis of the chemical and physical properties of the elements. These were the molecular formulae of hydrides and oxides of the elements, melting points, boiling points, and densities of the elements and their hydrides and oxides. Mendeleev found that the elements with similar physical and chemical properties repeat after a definite interval. On the basis of this finding, Mendeleev stated the following periodic law. The properties of elements are periodic functions of their atomic masses. The vertical columns in Mendeleev’s periodic table are called groups while the horizontal rows are called periods.
(The general molecular formulae of compounds shown as R2O, R2O3, etc, in the upper part of Mendeleev’s periodic table, are written as R2O, R2O3, etc. in the present system.)
Dmitri Mendeleev (1834-1907) was a professor in the St. Petersburg University. He made a separate card for every known element showing its atomic mass. He arranged the cards in accordance with the atomic masses and properties of the elements which resulted in the invention of the periodic table of elements.
Merits of Mendeleev’s Periodic Table
Science is progressive. There is a freedom in science to revise the old inference by using more advanced means and methods of doing experiments. These characteristics of science are clearly seen in the Mendeleev’s periodic table. While applying the law that the properties of elements are a periodic function of their atomic masses, to all the known elements, Mendeleev arranged the elements with the thought that the information available till then was not final but it could change. As a result of this, Mendeleev’s periodic table demonstrates the following merits.
1. Atomic masses of some elements were revised so as to give them a proper place in the periodic table in accordance with their properties. For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was placed before boron.
2. Mendeleev kept vacant places in the periodic table for elements not discovered till then. Three of these unknown elements were given the names eka-boron, eka-aluminum, and eka-silicon from the known neighbors and their atomic masses were indicated as 44, 68, and 72, respectively. Not only this but their properties were also predicted. Later on, these elements were discovered and named scandium (Sc), gallium (Ga), and germanium (Ge) respectively. The properties of these elements matched well with those predicted by Mendeleev. Due to this success, all were convinced about the importance of Mendeleev’s periodic table, and this method of classification of elements was accepted immediately.
|Eka-aluminium(E) (Mendeleev’s Prediction)
|Gallium (Ga) (Actual)
|1. Atomic Mass
|2. Density (g/cm3)
|3. Melting Point(°C)
|4. Formula of Chloride
|5. Formula of Oxide
|6. Nature of Oxide
Actual and predicted properties of gallium
3. There was no place reserved for noble gases in Mendeleev’s original periodic table. However, when noble gases such as helium, neon, and argon were discovered towards the end of the nineteenth century, Mendeleev created the ‘ zero’ group without disturbing the original periodic table in which the noble gases were fitted very well.
Demerits of Mendeleev’s Periodic Table
1. The whole number atomic mass of the elements cobalt (Co) and nickel (Ni) is the same. Therefore there was an ambiguity regarding their sequence in Mendeleev’s periodic table.
2. Isotopes were discovered a long time after Mendeleev put forth the periodic table. As isotopes have the same chemical properties but different atomic masses, a challenge was posed in placing them in Mendeleev’s periodic table.
3. When elements are arranged in an increasing order of atomic masses, the rise in atomic mass does not appear uniform. It was not possible, therefore, to predict how many elements could be discovered between two heavy elements.
4. Position of Hydrogen: Hydrogen shows similarity with halogens (group VII). For example, the molecular formula of hydrogen is H2 while the molecular formulae of fluorine and chlorine are F2 and Cl2, respectively. In the same way, there is a similarity in the chemical properties of hydrogen and alkali metals (group I). There is a similarity in the molecular formulae of the compounds of hydrogen alkali metals (Na, K, etc) formed with chlorine and oxygen. On considering the above properties it can not be decided whether the correct position of hydrogen is in the group of alkali metals (group I) or in the group of halogens (group VII).
|Compounds of H
|Compounds of Na
Similarity in hydrogen and alkali metals
|Element (Molecular Formula)
|Compounds with Metals
|Compounds with Nonmetals
Similarity in hydrogen and halogens
Modern Periodic Law
The scientific world did not know anything about the interior of the atom when Mendeleev put forth the periodic table. After the discovery of electrons, scientists started exploring the relation between the electron number of an atom and the atomic number. The atomic number in Mendeleev’s periodic table only indicated the serial number of the element. In 1913 A.D. the English scientist Henry Moseley demonstrated, with the help of experiments done using an X-ray tube, that the atomic number (Z) of an element corresponds to the positive charge on the nucleus or the number of protons in the nucleus of the atom of that element. This revealed that ‘atomic number’ is a more fundamental property of an element than its atomic mass. Accordingly, the statement of the modern periodic law was stated as follows: Properties of elements are a periodic function of their atomic numbers.
Modern Periodic Table: Long Form of the Periodic Table
The classification of elements resulting from an arrangement of the elements in increasing order of their atomic numbers is the modern periodic table. The properties of elements can be predicted more accurately with the help of the modern periodic table formed on the basis of atomic numbers. The modern periodic table is also called the long form of the periodic table. In the modern periodic table, the elements are arranged in accordance with their atomic number. As a result, most of the drawbacks of Mendeleev’s periodic table appear to be removed. However, the ambiguity about the position of hydrogen is not removed even in the modern periodic table.
We have seen in the previous standard that the electronic configuration of an atom, the way in which the electrons are distributed in the shells around the nucleus, is determined by the total number of electrons in it; and the total number of electrons in an atom is same as the atomic number. The relation between the atomic number of an element and its electronic configuration is clearly seen in the modern periodic table.
Structure of the Modern Periodic Table
The modern periodic table contains seven horizontal rows called the periods 1 to 7. Similarly, the eighteen vertical columns in this table are the groups 1 to 18. The arrangement of the periods and groups results in the formation of boxes. Atomic numbers are serially indicated in the upper part of these boxes. Each box corresponds to the place for one element.
Apart from these seven rows, two rows are shown separately at the bottom of the periodic table. These are called lanthanide series and actinide series, respectively. There are 118 boxes in the periodic table including the two series. It means that there are 118 places for elements in the modern periodic table. Very recently formation of a few elements was established experimentally and thereby the modern periodic table is now completely filled. All the 118 elements are now discovered.
The entire periodic table is divided into four blocks, viz, s-block, p-block, d-block, and f-block. The s-block contains the groups 1 and 2. The groups 13 to 18 constitute the p-block. Groups 3 to 12 constitute the d-block, while the lanthanide and actinide series at the bottom form the f-block. The d-block elements are called transition elements. A zig-zag line can be drawn in the p-block of the periodic table. The three traditional types of elements can be clearly shown in the modern periodic table with the help of this zig-zag line. The metalloid elements lie along the border of this zig-zag line. All the metals lie on the left side of the zig-zag line while all the nonmetals lie on the right side.
Characteristics of Groups and Periods
The characteristics of the groups and periods in the periodic table are understood by comparison of the properties of the elements. Various properties of all the elements in a group show similarity and gradation. However, the properties of elements change slowly while going from one end to the other (for example, from left to right) in a particular period.
Modern Periodic Table and Electronic Configuration of Elements
Within a period the neighbouring elements differ slightly in their properties while distant elements differ widely in their properties. Elements in the same group show similarity and gradation in their properties. These characteristics of the groups and periods in the modern periodic table are because of the electronic configuration of the elements. It is the electronic configuration of an element that decides the group and the period in which it is to be placed.
Groups and Electronic Configuration
You will find that the number of valence electrons in all these elements from group 1, that is, the family of alkali metals, is the same. Similarly, if you look at the elements from any other group, you will find the number of their valence electrons to be the same. For example, the elements beryllium (Be), magnesium (Mg), and calcium (Ca) belong to group 2, that is, the family of alkaline earth metals. There are two electrons in their outermost shell. Similarly, there are seven electrons in the outermost shell of the elements such as fluorine (F) and chlorine (Cl) from group 17, that is, the family of halogens.
While going from top to bottom within any group, one electronic shell gets added at a time. From this, we can say that the electronic configuration of the outermost shell is characteristic of a particular group. However, as we go down a group, the number of shells goes on increasing. Uranium has atomic number 92. All the elements beyond uranium (with atomic numbers 93 to 118) are man-made. All these elements are radioactive and unstable and have a very short life.
In the Modern Periodic Table-
- Elements are arranged in an increasing order of their atomic numbers.
- Vertical columns are called groups. There are 18 groups. The chemical properties of the elements in the same group show similarity and gradation.
- Horizontal rows are called periods. There are in all 7 periods. The properties of elements change slowly from one end to the other in a period.
Periods and Electronic Configuration
You will find that the number of valence electrons is different in these elements. However, the number of shells is the same. You will also find that, while going from left to right, within the period, the atomic number increases by one at a time, and the number of valence electrons also increases by one at a time.
We can say that the elements with the same number of shells occupied by electrons belong to the same period. The elements in the second period, namely, Li, Be, B, C, N, O, F, and Ne have electrons in the two shells, K and L. The elements in the third period, namely, Na, Mg, Al, Si, P, S, Cl, and Ar have electrons in the three shells; K, L, and M. Write down the electronic configuration of these elements and confirm. In the modern periodic table, electrons are filled in the same shell while going along a period from left to right, and at the beginning of the next period, a new electron shell starts filling up. The number of elements in the first three periods is determined by the electron capacity of the shells and the law of electron octet.
Electron Capacity of Electron Shells
As per the electron-holding capacity of shells, 2 elements are present in the first period and 8 elements in the second period. The third period also contains only eight elements due to the law of electron octet. There are a few more factors that control the filling of electrons in the subsequent periods which will be considered in the next standards. The chemical reactivity of an element is determined by the number of valence electrons in it and the shell number of the valence shell. The information on these points is obtained from the position of the element in the periodic table. That is, the modern periodic table has proved useful for the study of elements.
Periodic Trends in the Modern Periodic Table
When the properties of elements in a period or a group of the modern periodic table are compared, certain regularity is observed in their variations. It is called the periodic trends in the modern periodic table. In this standard, we are going to consider the periodic trends in only three properties of elements; namely, valency, atomic size, and metallic-nonmetallic character.
You have learned in the previous standard that the valency of an element is determined by the number of electrons present in the outermost shell of its atoms, that is, the valence electrons.
You have seen in the previous standards that size/volume is a fundamental property of matter. The size of an atom is indicated by its radius. Atomic radius is the distance between the atom’s nucleus and its outermost shell. Atomic radius is expressed in the unit picometer (pm) which is smaller than nanometer (1 nm = 10-12 m).
Some elements and their atomic radii are given here.
|Atomic Radius (pm)
You will find that the atomic radius decreases while going from left to right within a period. The reason behind this is as follows. While going from left to right within a period, the atomic number increases one by one, meaning the positive charge on the nucleus increases by one unit at a time. However, the additional electron gets added to the same outermost shell. Due to the increased nuclear charge, the electrons are pulled towards the nucleus to a greater extent and thereby the size of the atom decreases.
Some elements and their atomic radii are given here.
|Atomic Radius (pm)
You will find that while going down a group the atomic size goes on increasing. This is because while going down a group a new shell is added. Therefore the distance between the outermost electron and the nucleus goes on increasing. As a result of this, the atomic size increases in spite of the increased nuclear charge.
It is seen that the metallic elements like sodium, and magnesium are towards the left. The nonmetallic elements such as sulphur, and chlorine are towards the right. The metalloid element silicon lies in between these two types. A similar pattern is also observed in the other periods. It is seen that the zig-zag line separates the metals from nonmetals in the periodic table. Elements appear to have been arranged in such a way that metals are on the left side of this line, nonmetals on the right side, and metalloids are along the border of this line. How did this happen?
Let us compare the characteristic chemical properties of metals and nonmetals. It is seen from the chemical formulae of simple ionic compounds that the cation in them is formed from a metal while the anion is from a nonmetal. From this it is understood that metal atoms have a tendency to form a cation by losing its valence electron, this property is called electropositivity of an element. On the other hand, an atom of a nonmetal has a tendency to form an anion by accepting electrons from outside into its valence shell. We have already seen that ions have a stable electronic configuration of a noble gas.
How is the ability to lose or accept electrons in the valence shell determined? All the electrons in any atom are held by the attractive force exerted on them by the positively charged nucleus. Electrons in the inner shells lie between the valence shell and the nucleus. Because of their presence, the effective nuclear charge exerting an attractive force on the valence electrons is somewhat less than the actual nuclear charge. Thus, the number of valence electrons in metals is small (1 to 3). Also, the effective nuclear charge exerting an attractive force on the valence electrons is small. As a combined effect of these two factors, metals have a tendency to lose the valence electrons to form cations having a stable noble gas configuration. This tendency of an element called electro-positivity is the metallic character of that element.
The periodic trend in the metallic character of elements is clearly understood from their position in the modern periodic table. Let us first consider the metallic character of elements belonging to the same group. While going down a group a new shell gets added, resulting in an increase in the distance between the nucleus and the valence electrons. This results in lowering the effective nuclear charge and thereby lowering the attractive force on the valence electrons. As a result of this the tendency of the atom to lose electrons increases. Also, the penultimate shell becomes the outermost shell on losing valence electrons. The penultimate shell is a complete octet.
Therefore, the resulting cation has a special stability. Due to this, the tendency of the atom to lose electrons increases further. The metallic character of an atom is its tendency to lose electrons. Therefore, the following trend is observed: The metallic character of elements increases while going down the group. While going from left to right within a period the outermost shell remains the same. However, the positive charge on the nucleus increases while the atomic radius decreases and thus the effective nuclear charge goes on increasing. As a result of this, the tendency of an atom to lose valence electrons decreases within a period from left to right.
The two factors namely, the increasing nuclear charge and decreasing atomic radius as we go from left to right within a period, are responsible for increasing the effective nuclear charge. Therefore, the valence electrons are held with greater and greater attractive force. This is called the electronegativity of an atom. Due to increasing electro negativity from left to right within a period, the ability of an atom to become an anion by accepting outside electrons goes on increasing. The tendency of an element to form an anion or the electro negativity is the nonmetallic character of an element.
- While going downwards in any group the electropositivity of elements goes on increasing while their electronegativity goes on decreasing.
- While going from left to right in any period the electronegativity of elements goes on increasing while their electropositivity goes on decreasing.
- The larger the electropositivity or electronegativity of the element higher the reactivity.
Gradation in Halogen Family
The group 17 contains the members of the halogen family. All of them have the general formula X2. A gradation is observed in their physical state down the group. Thus, fluorine (F2) and chlorine (Cl2) are gases, bromine (Br2 ) is a liquid while iodine (I2) is a solid.
M + 2H2O → M(OH)2 + H2
A general chemical equation indicating the reaction of alkaline earth metals is given above. While going down the second group as Be Mg Ca Sr Ba, the gradation in this chemical property of the alkaline earth metals is seen. While going down the second group the reactivity of the alkaline earth metals goes on increasing and thereby the ease with which this reaction takes place also goes on increasing. Thus beryllium (Be) does not react with water. Magnesium (Mg) reacts with steam, while calcium (Ca), strontium (Sr) and barium (Ba) react with water at room temperature at increasing rates.
Good Maharashtra State Board Class 10 Science Notes Periodic Classification of Elements can simplify complex concepts and make studying more efficient.