Hydrogen is defined as the first element of the periodic table because its atomic number is 1, which means it contains only one single electron in its atom. Therefore only 1 electron is available in its outermost shell. The elements' placement in the periodic table is according to their electronic configuration.
Structure of Hydrogen
The structure of hydrogen is similar to that of alkali metals (ns1), which contains one electron in their outermost shell. Also, it can attain helium noble gas configuration by accepting an electron. This character is mostly the same as that of the halogen family (ns2, np5), and is also short of one electron for the completion of the electron octet in their shells.
When a hydrogen atom loses an electron and produces a cation, it resembles the alkali metals whereas, when it gains an electron and becomes a uni-negative ion, it represents similarity to the halogens. By taking a look at these properties, the position of hydrogen in the periodic table is the major question.
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Hydrogen in the Periodic Table
Moving on to the formation of compounds, hydrogen produces oxides, sulphides, and halides resembling the alkali metals. Whereas unlike the alkali metals, it contains a very high ionization enthalpy, and hence it lacks metallic characteristics under regular conditions. By looking in terms of the ionization enthalpy, it is found that hydrogen resembles more halogens compared to the alkali metals. For example, ΔiH of lithium is given as 520 kJ mol-1, hydrogen is given as 1312 kJ mol-1, and for fluorine, it is given as 1680 kJ mol-1. It also exists as a diatomic molecule similar to that of halogens (for example, chlorine Cl2); a single hydrogen bond exists when the H2 molecule is formed.
Though hydrogen atoms exhibit a lot of resemblance to both alkali metals and halogens, both are very different. Thus, in the periodic table, great thought has to be given for the hydrogen position. When the hydrogen atom loses electrons, the size of its nucleus decreases and almost becomes 1.5 × 10-3 pm, which is much smaller when compared to the atomic sizes of the normal metals, and therefore the hydrogen ion does not freely exist in nature.
The Reason Behind Placing the Hydrogen Atom First in the Periodic Table
Generally, in the periodic table, Hydrogen does not have a fixed position. In a few tables, it is placed with alkali metals (which is above Sodium), and in few others, it is lonely placed at the top (Randomly, Just above the first Period).
Hydrogen resembles the alkali metals in electronic configuration. Hydrogen atoms contain the e-configuration 1s1, and it is the first element to be placed according to the rule. It is also placed with the alkali metals because it can lose its one e- to form H+ simply such as the alkali metals. Whereas hydrogen atoms also resemble halogens. Alkali metals produce hydrides such as LiH and Nah, just similar to LiCl and NaCl. The electrolysis of hydrides produces H2, and the Electrolysis of NaCl yields Cl2. In addition, hydrogen can gain one electron, such as halogens, to produce a noble gas configuration (which is H-).
Because of its resemblance to halogens and alkali metals, its position is still not fixed. But conventionally, we keep it including the alkali metals.
Why is Hydrogen Placed in Both Periodic Table Groups?
Hydrogen holds one valence electron in its outermost shell, and therefore it contains similar chemical properties compared to alkali metals. Also, hydrogen exists as a diatomic molecule similar to halogens and produces compounds with both metals and nonmetals. Thus, the hydrogen molecule can be placed in both the 1st as well as the 17th group in the modern periodic table. This anomaly with the position of hydrogen was one of the biggest demerits of Mendeleev's periodic table.
However, considering the modern periodic table, the hydrogen molecule has been awarded the top position, which neither belongs to group 1 nor group 17.
Elements That Won’t Occur Naturally
Up to plutonium (having an atomic number of 94), all the periodic table elements are present on Earth, although many of them (namely promethium, technetium, polonium, francium, astatine, protactinium, plutonium, and neptunium) take place simply in tiny amounts, typically as the by-products of other’s radioactive decay. Their amounts are very tiny, up to a recent past, where those elements were given as not occurring naturally on Earth. As per the atomic numbers, which are higher than 94, all of the corresponding elements are artificial, and they do not occur naturally on Earth.
Importance of Hydrogen
The most important hydrogen in the human body function is to keep the body hydrated. Water contains oxygen and hydrogen and is absorbed by the cells of the body. Thus, it is defined as a crucial element that can be used as a military weapon, fuel, and more, but not in our body.