Periodic Classification of Elements Class 10 – Continuing our series for Class 10 Revision for CBSE board exams, we bring you a new chapter today i.e. Periodic Classification of Elements Class 10. Looking at previous years’ questions, we suggest that it is an important chapter from exam point of view, as definitely a 3 or a 5 mark question is asked from this chapter every year.
The concepts that are going to be discussed in this article are:
- Dobereiner’s Triads
- Newlands Law of Octaves
- Mendeleev’s Periodic Table
- Modern Periodic Table
- Trends in Modern Periodic Table
- Previous Years’ Questions
Let’s discuss each topic of Periodic Classification of Elements Class 10 one-by-one
In the year 1817, Johann Wolfgang Döbereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. For example, consider the triad:
You see that the mass of P is approximately equal to the average of masses of N and As.
However, his theory of triads had some drawbacks:
- Dobereiner could identify only three triads from the elements known at that time.
- Dobereiner’s system of classification could not predict the existence of elements that were not yet discovered.
Newland’s law of Octaves:
In 1866, John Newlands, arranged the then known elements in the order of increasing atomic masses. He found that every eighth element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it the ‘Law of Octaves’.
However, Newlands table also had some drawbacks mentioned below and therefore its use was discontinued.
- It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
- When later on, several new elements were discovered, their properties did not fit into the Law of Octaves.
- In order to fit elements into his Table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note. For example, cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.
Mendeleev’s Periodic Table
Mendeleev arranged the elements based on their atomic masses. He observed that when the elements were arranged in increasing order of their atomic masses, there was a periodic recurrence in their physical and chemical properties. Thus, Mendeléev formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’. Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
Mendeleev’s periodic table was found to be consistent with observations made about the physical and chemical properties of elements. Some of its advantages were:
- Mendeleev left some gaps in his Periodic Table – to acknowledge the existence of some elements that had not been discovered at that time. For example, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively. For example, the properties of eka-aluminium and gallium are given below:
- When noble gases like helium (He), neon (Ne) and argon (Ar) were discovered, they could be placed in a new group without disturbing the existing order.
However, it also had some drawbacks:
- Some elements in Mendeleev’s Table have not been arranged in the increasing order of their atomic masses. For example, Co and Ni – even though Co has a higher atomic mass than Ni, it is placed before Ni in his table.
- Hydrogen forms similar compounds as Group 1 elements. However, it also forms similar diatomic molecules as Group 7 elements (H2, F2, Cl2, Br2, I2) . Hence, it could not be assigned a fixed position in the table.
Compounds of H Compounds of Na HCl NaCl H2O Na2O H2S Na2S
- Isotopes posed a challenge to Mendeleev’s table. For example, Cl has two major isotopes – Cl-35 and Cl-37. Hence, it can be placed in accordance with its mass being 35 amu or 37 amu or both of them could be placed together owing to their similar chemical properties!
Modern Periodic Table
In 1913, Henry Moseley showed that atomic number of an element is a more fundamental property than its atomic mass. Thus, the Modern Periodic Law was put forward which can be stated as follows: ‘Properties of elements are a periodic function of their atomic number.’
Thus, scientists tried arranging elements in the increasing order of their atomic numbers.
The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
Trends in Modern Periodic Table
- Valency and electronic configuration – If we see from top to bottom in a group, you see that one shell keeps on getting added from one element to next. However, the elements lying in same group have the same outer shell electronic configuration and valency, and thus have similar chemical properties. On moving from left to right in a period, number of atomic shells remains the same and atomic number increases by 1 unit.
- Atomic size: the atomic radius decreases in moving from left to right along a period. 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. The atomic size increases down the group. This is because 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.
- Metallic and Non-metallic Properties – metallic character decreases across a period and increases down a group.
Thus, metals are found towards the left bottom side of the Periodic Table while the non-metals are found on the top right-hand side.
In the middle, we have metalloids – elements which exhibit some properties of both metals and non-metals. These are boron, silicon, germanium, arsenic, antimony, tellurium and polonium.
- Electronegativity – increases from left to right in a period and decreases down the group.