Periodic classification of elements class 10th cbse notes | Chapter 5 | Chemistry



Periodic Classification of Elements

Modern Periodic Table

There are currently 118 elements known in nature and to study their properties separately is a very hard task therefore, periodic classification of these elements is very necessary.
Periodic classification of elements:
·         Periodic classification of elements is the classification of elements in such a way that the elements having similar properties are repeated after regular periods or intervals.
·         Elements having similar properties are arranged in same group in this classification.

Dobereiner's Triads

·         According to Dobereiner's law of triads, when elements are arranged in the order of their increasing atomic mass then a group of three elements having similar chemical properties called triad is obtained and the atomic mass of the middle element in this triad is equal to the arithmetic mean of the atomic mass of first and third element.
·         Triad: A triad is a group of three elements having similar chemical properties.
·         This law was given by Johann Triad who was a German chemist.
·         Dobereiner was able to form three triads, let us study them.
The Alkali Metal Triad: This triad contains lithium, sodium and potassium which are arranged in the order of their increasing atomic mass. These elements posses similar chemical properties as:
·         They all the metals
·         They all form alkali and hydrogen gas when reacted with water
·         All these elements have a valency of 1, they are monovalent.
When we find the arithmetic mean of the first and the last element of this triad i.e. lithium and potassium. It turns out to be 23 which is the exactly the atomic mass of the middle element of this triad i.e. sodium.
It should be noted that the these elements are called alkali metals because they form alkali on reacting with water.
The Alkaline Earth Metal Triad: This group contains Calcium, Strotium and Barium which are obviously arranged in the order of their increasing atomic mass. The properties of these elements are similar because:
·         These elements are metals
·         They form alkali and hydrogen gas on reacting with water
·         They are divalent meaning that they have a valency of 2.
When we try to find the arithmetic mean of the atomic mass of first and last element of this triad i.e. Calcium and Barium we get to know that it is 88.5 which is so close to the actual atomic mass of the middle element of this triad i.e. Strotium having an atomic mass of 88.
A thing to note here is that these elements are called alkaline earth metals because their oxides are basic in nature and they were found beneath the surface of the earth.
Halogen group: This triad contains three elements known as Halogens are Chlorine, Bromine and Iodine. They have similar chemical properties because:
·         These elements are non-metals
·         They all form salts on reacting with metals
·         They all have a valency of 1( They are monovalent)
When we calculate the arithmetic mean of atomic mass of first and last element of this triad i.e. Chlorine and Iodine respectively  then we get know that it is 81.25 which is approximately equal to the actual atomic mass of the middle element of this triad i.e. Bromine having an atomic mass of 80.
It should be noted that these elements are called halogens because they form salts on reacting with metals( Halo = Salt, Gen = Producer)
Limitations of Dobereiner's triad
·         Doberiener was not able to form triads for of all the then known elements.
·         Dobereiner could only form three triads, that is why his classification was not that successful.

Newland's Classification of Elements

·         Newland classified the elements on the basis of their atomic masses
·         According to Newland's law of octaves, when elements are arranged in the order of their increasing atomic masses then the properties of every eighth element( Staring from a given element) are similar to the first element.
·         Newland's law of octaves resembles to the octaves of music, that is the reason why it is called law of octaves
·         Newland arranged elements in the horizontal rows of 7 elements each
·         Elements having the similar properties were placed in the same vertical column called group.
Limitations of Newland's classification of element
·         Newland's law of octaves was only applicable up to Calcium, It was only applicable to light elements only
·         Newland predicted only 56 elements existed in the nature and no new discoveries of elements will take place in the future
·         Newland even fitted two elements in the same slot so that they come in the group of elements having similar properties. for eg. Cobalt and Nickel.
·         Iron, which resembles the properties of Cobalt and Nickel was placed very far away from them.
Newland's classification of elements however gave a very important conclusion that there is some systematic connection or relationship between atomic mass of elements and the periodicity in their properties.

Periodic Table

·         Periodic table is a chart of elements in which elements are arranged in such a way that their properties repeat after regular intervals or periods.
·         It is called Periodic because in it properties of elements are repeated after regular periods.
·         It is called Table because in it elements are arranged in a tabular form.
·         In a periodic table horizontal rows are called periods and vertical columns are called groups
·         In a periodic table elements having the same properties are arranged in same group.
Properties of elements are also defined on the basis of how they are reacting with oxygen and hydrogen.
In other words, The general formula of oxides and hydrides of elements also tells us whether they have similar properties or not, R letter is used to express an element in its genera formula of its oxide or hydride. For eg.  Sodium reacts with oxygen to form Na2O and potassium reacts with oxygen to form K2O, So their general formula turns out to be R2O therefore, they have similar chemical properties.
Thus, we can conclude that,
·         The elements having same general formula for their oxides then they will have similar chemical properties.
·         The elements having same general formula for their hydrides then they will have similar chemical properties.

Mendeleev's Periodic Table

Mendeleev  made the classification of elements by looking at their atomic masses and their chemical properties.
According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses.
In simple words Mendeleev's periodic law means if the elements are arranged in the order of increasing atomic masses then there is a periodicity in their properties.
Mendeleev  gave special attention to the general formula of oxides and hydrides formed by elements while making his classification as it also defines their properties. That is the reason we see general formula of oxides and hydrides at the top of every group of elements in the original periodic table of Mendeleev.
There were total 8 groups and 7 periods in the original periodic table of Mendeleev.
Two main features of Mendeleev's Periodic table:
1.       To complete his table Mendeleev left some gaps in his table for then unknown elements to fit known elements in order to make sure elements having similar properties fell in same group.
2.       He even placed elements in the wrong order of their atomic mass so that they come in the groups of elements having similar properties.
He placed Cobalt having a higher atomic mass ahead of Nickel which has slightly less atomic mass than Cobalt.
The undiscovered elements for which Mendeleev left the gaps in his periodic table were named eka-boron, eka-aluminium and eka-silicon.
When these elements were discovered in future then eka-boron was termed Scandium, eka-aluminium was termed Gallium and eka-silicon was termed Germanium and their properties were almost the same as predicted by Mendeleev. A thing to note here is that eka is a word derived from sanskrit which means "first".
Merits of Mendeleev's classification of Elements
·         Mendeleev's periodic table predicted the existence of elements which were unknown at that time
·         Mendeleev's periodic table could identify the properties of many elements on the basis of their position in the periodic table
·         Mendeleev's periodic table could accommodate all the noble gases when they got discovered.
Limitations of the classification of elements made by Mendeleev
·         Mendeleev's periodic table could not explain the position of isotopes.
·         Mendeleev's periodic table could not explain the wrong order of atomic mass of some elements.
·         Position of Hydrogen was also not explained by Mendeleev's periodic table.
After  all these unsuccessful attempts it was concluded that atomic mass is not the perfect basis to classify elements.

Modern basis for the classification of Elements

Present or modern basis for the classification of elements is the atomic numbers of elements.
In 1913, Henry Mosely showed that atomic no. is more precise basis to classify elements.
Atomic number is the total number of protons present inside the atom of an element.
In an atom the no. of protons is equal to the no. of electrons, so we can also say that atomic number is the number of electrons in the atom of an element.
Significance of atomic number in the classification of elements is that being equal to the number of electrons, elements can be arranged with respect to their electronic configuration.
Modern Periodic Law: Modern periodic law states that the properties of elements are a periodic function of their atomic numbers. This law was given by Henry Mosely.
In simple words, Modern periodic law means that when elements are arranged in the order of increasing atomic numbers then there is periodicity in their properties is observed.
There is a periodicity in the properties of elements when they are arranged in the order of increasing atomic numbers because there is periodicity of electronic configuration of elements  when they are arranged in such a way. We know that electronic configuration is the one thing that defines the chemical properties of an element thus, when elements of similar electronic configuration are repeated after regular periods in a periodic table then a periodicity in their chemical properties also occurs.
Explanation of anomalies of Mendeleev's classification of elements by Modern Periodic Law
Explanation of the position of Isotopes: Isotopes are the atoms of the same element having same atomic number but different atomic mass. They have same atomic no. so they can be placed in te same slot and same group.
Explanation of the wrong order of atomic mass of cobalt and Nickel: The atomic no. of cobalt is 27 whereas the atomic no. of Nickel is 28 therefore according to modern periodic law, cobalt has to be placed ahead of Nickel.
Explanation of modern periodic law
When elements are arranged in the order of increasing atomic number then atoms of elements having same number of valence electrons occur at regular intervals or periods. We know that only valence electrons take part in all the chemical reaction thus they are responsible for all the chemical properties and chemical reactivity of elements therefore their regular periodicity results in the periodicity in the properties of elements.

Modern Periodic Table

Modern periodic table was framed by Neils Bohr.
In Modern periodic table elements are arranged in the order of increasing atomic number in horizontal rows called Periods.
Elements having the same properties are arranged under each other forming groups in modern periodic table.
Horizontal rows in Modern periodic table are called periods. There are total seven periods in modern periodic table.
         i.            1st period contains 2 elements and called very short period
       ii.            2nd period contains 8 elements and called short period
      iii.            3rd period contains 8 elements and also called short period
     iv.            4th period contains 18 elements and it is called long period.
       v.            5th period contains 18 elements and it is also called long period.
     vi.            6th period contains 32 elements and it is called very long period.
    vii.            7th period contains rest of the elements.
The number of elements in a period is fixed by the number of electrons which can be accommodated within the shells of their atoms.
The number of valence electrons in the atoms of elements decides whether the element will be first or last in a period.
All the elements in a period have different chemical properties as they have different number of valence electrons in their atoms.
Atomic numbers of elements in a period are consecutive.
Vertical columns in modern periodic table are called groups. There are total 18 groups in modern periodic table.
All the elements in a group have similar chemical properties.
All the elements in a group have same number of valence electrons in their atoms.
In this class we have to study only 20 elements of group 1,2,13,14,15,16,17 and 18 rest are to be studied in higher classes.
Group 1 elements are called alkali metals.
Group 17 elements are called Halogens.
Group 18 elements are called Noble gases.
Elements of group 1,2,13,14,15,16 and 17 are called normal elements, the outermost shells or valence shells  of their atoms is not filled completely with 8 electrons thus, they are not stable.
Elements of group 3 to 12 are called transition metals, their outermost shell as well as next to the outermost shells called penultimate shells are incomplete and are in the process of being filled wit electrons.
Elements from atomic no. 57 to 71 are called lanthanide series and the elements with atomic number 89 to 103 are called actinide series.
In modern periodic table elements are broadly divided into metals and non-metals, on the left side of periodic table there are metals and on right side of the table non-metals are present.
Metals and non-metals in periodic table are diagonally divided by some elements called metalloids, these elements  resembles the properties of both the metals and non-metals.
The noble gases are placed on the extreme right side of the table.

Characteristics of Periods and Groups

Properties of elements vary in many ways when we move across a period or move down a group in a periodic table. Let us know how properties of elements change on moving across a period or moving down a  group. Let us start with Periods first.

Periods

There are some important properties of elements which may vary by moving left to right a period. The properties are: Valence electrons, Valency, Atomic size, Metallic character, Chemical reactivity and Nature of oxides. Let us start with Valence electrons first.
Valence electrons
When we move left to right in a period, at each stage one valence electron adds up to the outermost shell of elements.
Valency
On moving left to right in a period the valency of elements first increase from 1 to 4 then decrease from 4 to 0.
Atomic size
When we move left to right in a period, the atomic size or atomic radius of elements gets shorter and shorter.
Metallic character
On moving left to right in a period, we get to know that the metallic character of elements is decreasing but the non-metallic character tends to increase.
Chemical Reactivity
When we move left to right in a period we find that the chemical reactivity of elements is first decreasing and then again increasing till the end of the period.
Nature of oxides
On moving left to right in a period the nature of oxides of elements changes from basic to amphoteric to acidic.
Now let us talk about what changes we observe in the properties of elements as we go down a group in a periodic table.

Groups

We will examine the same properties of elements in groups also as we examined in periods.
Valence Electrons
As we go down in a group we find no change in the no. of valence electrons in the atoms of elements
Valency
Valency of all the elements in a group remains the same, no change is observed.
Atomic size
As we go down in a group the atomic size or atomic radius of elements tends to increase as at each step there is a new electronic shell being added to the atom of the element.
Metallic Character
As we go down in a group the metallic character of elements is observed to be increasing but the non-metallic character decreases.
Chemical Reactivity
Chemical reactivity of metals as we go down in a group starts increasing but chemical reactivity of non-metals starts decreasing.
Nature of oxides
On going down in a group, no change in the nature of oxides of elements is observed.

Merits of Modern Periodic Table

1.       Modern periodic table is based upon the classification of elements on the basis of their atomic numbers which is the most precise basis of classification of elements.
2.       Modern periodic table explains why elements of same group show similar properties while elements of different groups don't.
3.       Modern periodic table also tells us the reasons why there is a periodicity in the properties of elements.
4.       Modern periodic table explains why properties of elements repeats after 2,8,18 and 32 elements.
5.       There are no anomalies in modern periodic table.

Advantages of Modern periodic table

1.       Modern periodic table has made the study of chemistry more systematic and easy. I provides an aid to learning
2.       Learning properties of elements by knowing in which group they lie is much more easy.
3.       Types of compound formed by elements can be predicted by knowing their position in the periodic table
4.       Modern periodic table is used as a teaching aid in chemistry, schools and colleges.

Chemical Bonding and Periodic table

·         When the elements that are placed on the left side of periodic table combines or reacts with the elements placed on the right side of periodic table then they form ionic bond.
·         When elements which are placed on the right side of the periodic table reacts or combine with the elements which are also placed on the right of periodic table then a covalent bond forms.
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