Science [3 Mark Questions]

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

1. Newlands’ law of octaves 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. Thus, this law worked well with lighter elements only.
2. Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands’ law of octaves.
3. 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 tendency of an atom to lose electrons increases on moving down in a group of the periodic table. As we go down in group 1, one more electron shell is added at every stage and the size of the atom increases. The valence electrons become more and more away from the nucleus and hold of the nucleus on valence electrons decreases. Due to this, the atoms can lose valence electrons more easily to form positive ions and hence electropositive character increases.

1. Group 1: Lithium and Sodium.
2. Group 17: Fluorine and Chlorine.
3. Group 18: Neon and Argon.

In Mendeleev’s periodic table, hydrogen has been placed in group I since like alkali metals, hydrogen also combines with halogens, oxygen and sulphur to form compounds having similar formulae. This means that hydrogen resembles alkali metals in some of the properties. Hydrogen also resembles halogens in some of the properties. So, hydrogen could also be placed in group VII of halogen elements. Thus, Mendeleev’s periodic law could not assign a correct position to hydrogen in the periodic table.

According to the Newlands’ law of octaves, when elements are arranged in the order of increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element.
For example:
If we start with lithium as the first element, we find that the eighth element from it is sodium having the similar properties to lithium.

1. Lithium (Li), sodium (Na) and potassium (K) are three metals of group 1 having a valency of 1.
2. Calcium (Ca), strontium (Sr) and barium (Ba) are three metals of group 2 having a valency of 2.
3. Chlorine (Cl), bromine (Br) and iodine (I) are three non-metals of group 17 which are known as halogens, and have seven valence electrons.

When the elements are arranged according to their atomic numbers on the basis of modern periodic law, then all the anomalies (or defects) of Mendeleev’s classification disappear. This is discussed below:

1. Explanation for the Position of Isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table. For example, both the isotopes of chlorine, Cl - 35 and Cl - 37, have the same atomic number of 17, so both of them can be put at one place in the same group of the periodic table.
2. Explanation for the Position of Cobalt and Nickel: The atomic number of cobalt is 27 and that of nickel is 28. Now, according to modern periodic law, the elements are arranged in the order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.
3. Explanation for the Position of Hydrogen: Hydrogen element has been placed at the top of group 1, above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals. Both, hydrogen as well as alkali metals have 1 valence electron each.

Group 1 elements have 1 valence electron and are ionic in chemical reactions. Their chemical reactivity increases down the group. They are electropositive in nature and it increases down the group. Whereas, the elements of group 17 have 7 valence electrons. They all are non-metals. Their chemical reactivity decreases down the group. They are electronegative in nature and it decreases down the group.

As we move from Na to Cl in the 3rd period, the size of the atoms of the elements decreases. Na atom is the biggest whereas Cl atom is the smallest in size. As we move from left to right in a period, the atomic no. of elements increases i.e. the number of protons and electrons in the atoms increases. Due to large positive charge on nucleus, the electrons are pulled in more close to the nucleus and thus the size of the atom decreases from Na to Cl.

1. Ionic compound.
2. Yes.
3. XY 2.
4. 2.
5. 7.

On moving from left to right in a period of the periodic table, the atomic size decreases. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atoms increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of atom decreases.

The elements in the same group show similar properties because they have similar electronic configuration (having the same number of valence electrons) whereas the elements of different groups have different electronic configurations (different number of valence electrons) due to which they show different properties.

Yes, Dobereiners triads also exist in the columns of Newlands’ Octaves. Consider the elements lithium (Li), sodium (Na) and potassiu m (K) which are present in the second column of Newlands’ classification of elements. Now, if we start with lithium as the 1 st element, then the 8 th element from it is sodium, and according to Newlands’ law of octaves, the properties of 8 th element, sodium should be similar to thos e of the 1 st element, lithium. Again, if we take sodium as the 1 st el ement, then the 8 th element from it is potassium, and according to Newlands ‘ law of octaves, the properties of 8 th element, potassium should be similar to those of the 1 st element, sodium. This means that according to Newlands’ law of octaves, the elements lithium, sodium and potassium should have similar chemical properties. We also know that lithium, sodium and potassium form a Dobereiner’s triad having similar chemical properties. From this, we conclude that Dobereiners triads also exist in the columns of Newlands Octaves.

The size of atom decreases on moving from left to right in a period. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. The electronic configuration of the atoms increases in the same shell. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases. On going down in a group of the periodic table, the atomic size increases. The no. of electron shells in the atoms gradually increases and the electronic configuration also increases due to which the atomic size increases.

On going down in a group of the periodic table, the atomic size and metallic character increases. When we move down from top to bottom in group 1 of alkali metals, the size of atoms increases gradually from lithium to francium. In group 1 of alkali metals, lithium is the least metallic element whereas francium is the most metallic element.

Limitations of Mendeleevs classification of elements:

1. The position of isotopes could not be explained.
2. Wrong order of atomic masses of some elements could not be explained.
3. A correct position could not be assigned to Hydrogen in the periodic table.

Dobereiner’s law of triads: When elements are arranged in order of increasing atomic masses, groups of three elements (triads), having similar chemical properties are obtained. The atomic mass of the middle elements of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.
For example:
Alkali metal group ( Dobereiner’s triad): Lithium is the 1st element, sodium is the middle element whereas potassium is the 3rd element of the triad.

The horizontal rows of elements in a periodic table are called periods. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases.

The main limitation of Dobereiner’s classification of elements was that it failed to arrange all the then known elements in the form of triads of elements having similar chemical properties. Dobereiner could identify only three triads from the elements known at that time. So, his classification of elements was not much successful. Another limitation was that Dobereiner failed to explain the relation between atomic masses of elements and their chemical properties.

Merits of Mendeleev’s classification of elements:

1. Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time.
2. Mendeleev’s periodic table could predict the properties of several elements on the basis of their positions in the periodic table.
3. It could accommodate noble gases when they were discovered.

The tendency of an atom to gain electrons decreases on going down in a group of the periodic table. When we move from top to bottom in group 17, a new shell of electrons is added to the atoms at every step, due to which the size of atom increases. The nucleus goes more deep inside the atom due to which the attraction of nucleus for the incoming electron decreases due to which the atom cannot form negative ions easily and hence the electronegative character decreases.