### Theory:

Anomalous elemental pairings:

During the development of the periodic table, Mendeléev had to place an element with a slightly higher atomic mass before an element with a moderately lower atomic mass. The sequence was changed so that elements with similar properties could be grouped. For example, cobalt (atomic mass $$58.9$$) arrived before nickel (atomic mass $$58.7$$). Is it possible to find another similar anomaly in Mendeléev's periodic table?

Achievements of Mendeléev's classification:

The classification of Mendeleev was the first to include all the elements successfully.

In addition, Mendeléev left several gaps in his periodic table. Rather than seeing these gaps as defects, Mendeléev boldly predicted the existence of some elements that he had not yet discovered. Mendeléev called them by prefixing a Sanskrit numeral, Eka (one), to the preceding element in the same group.

For example, scandium, gallium and germanium, discovered later, have properties similar to Eka - boron, Eka - aluminium and Eka - silicon, respectively. The properties of Eka - aluminium predicted by Mendeléev and gallium, a subsequently discovered element that replaced Eka - aluminium, are mentioned below.

Properties of Eka - Aluminium and Gallium:

 Property Eka-Aluminium Gallium Atomic mass $$68$$ $$69.7$$ Formula of oxide $$E_2O_3$$ $$Ga_2O_3$$ Formula of chloride $$ECl_3$$ $$GaCl_3$$

The above table gave solid evidence for Mendeléev's periodic table's correctness and usefulness. Furthermore, because of Mendeleev's exceptional success, chemists accepted his periodic table and recognised him as the originator of the theory on which it is based.

Noble gases such as helium (He), neon (Ne), and argon (Ar) have been mentioned previously. Because these gases are inert and exist in low amounts in our atmosphere, they were identified much later. When these gases were discovered, Mendeléev's periodic table allowed them to be placed in a new group without disturbing the existing order.

Limitations of Mendeléev's classification:

The electronic configuration of hydrogen follows that of alkali metals. Like alkali metals, hydrogen reacts with halogens, oxygen, and sulphur to form compounds with similar formulae, as shown in the examples.

 Compounds of H Compounds of Na $$HCl$$ $$NaCl$$ $$H_2O$$ $$Na_2O$$ $$H_2S$$ $$Na_2S$$

On the other hand, like halogens, hydrogen exists as diatomic molecules, and it reacts with metals and non-metals to form covalent compounds.

Activity $$5.1$$. Position of Hydrogen

Position of hydrogen

1. Watching its resemblance to alkali metals and the halogen family, try to place hydrogen in the correct position on Mendeléev’s periodic table.

Mendeleev combined hydrogen with alkali metals because its properties are similar to those of halogens. The place of hydrogen in Mendeleev's periodic table is correct because its properties are more similar to alkali metals (its ability to lose electrons).

2. Hydrogen should be assigned to which period and which group?

Hydrogen should be assigned first group and first period.

Without a doubt, hydrogen cannot be assigned a fixed position in the periodic table. It was the first limitation of Mendeleev's periodic table. He couldn't find the correct position for hydrogen on his table.
Isotopes were discovered many years after Mendeléev proposed his periodic classification of elements. Let us remember that isotopes of an element have similar chemical properties but various atomic masses.

Activity $$5.2$$. Position of Isotopes

Consider the chlorine isotopes $$Cl-35$$ and $$Cl-37$$.

1. Would you assign them in various slots because their atomic masses are different? Or would you assign them in the same position because their chemical properties are the same?

No. The more fundamental basis of classification is the atomic number, not the atomic mass.

Yes, because they have the same chemical properties and atomic number. Both isotopes are placed in the same position.

The chlorine isotopes $$Cl-35$$ and $$Cl-37$$ should be placed in the same slot since their chemical properties are similar. The atomic number and chemical properties of two chlorine isotopes are the same. We know that the arrangement of the element is made based on the atomic number in the periodic table. And chemical properties of the element depending on the atomic number of the atom.