Hydrogen is the first element of the periodic table. It is the most abundant element in the universe and forms more compounds than any other element in the periodic table. Hydrogen was discovered by Henry Cavendish in the year 1766. The name ‘hydrogen’ was given by Lavoisier based on the fact that upon reaction with oxygen, it produces water.
Hydrogen is non-metallic in nature. They readily form covalent bonds with other non-metallic elements forming organic compounds. They can also form non-covalent bonds with more electronegative halogens such as fluorine, chlorine, etc.
Position of Hydrogen in Periodic Table
The atomic number of hydrogen is 1. This single electron is present in the outermost shell. Therefore, its electronic configuration would be 1s1. Hydrogen structure is similar to the structure of alkali metals whose electronic configuration is of the form ns1. It can attain the nearest noble gas electronic configuration of helium by gaining an electron. This nature of hydrogen is identical to that of the halogens whose electron configuration is of the form ns1np5, which can also attain the nearest noble gas electronic configuration (electron octet) by gaining an electron.
Hydrogen has the capacity to either lose an electron or gain an electron. When it loses an electron, it forms a cation like the alkali metals. Whereas, when it gains an electron, it forms an anion like halogens. Since its properties resemble both alkali metals and halogens, hydrogen occupies a unique position in the periodic table. Its position is said to be anomalous and is, hence, placed separately in the periodic table.
Position of Hydrogen in the Periodic Table
Properties of Hydrogen
The structure of hydrogen is very simple. It just consists of one electron and one proton.
It exists as dihydrogen (H2) in its elemental form.
It has an extremely low melting and boiling point.
It is the most flammable gas.
It is a strong reducing agent.
It exhibits two oxidation states: +1 and -1.
It is comparatively more soluble in organic solvents than in water.
At room temperature, hydrogen is less reactive.
The ionic and atomic size of hydrogen is extremely small and hence they cannot exist freely in nature.
In some properties such as electronegativity, hydrogen resembles carbon.
Occurrence of Hydrogen
Hydrogen is the most abundant element and is present in its elemental form, as dihydrogen (H2). It makes up to 0.14% of the Earth’s crust by weight. It is present vastly as a part of the water in oceans, rivers and lakes. It also forms a major part of the atmosphere. It comes up to 70% of the total mass of the universe. Not only is it present in large quantities in the Earth’s atmosphere, but it is also found in the atmosphere of the Sun and on planets such as Jupiter and Saturn. It is also a part of animal and plant tissues. It is found in petroleum, minerals, starch, carbohydrates and proteins as well. They form numerous compounds with carbon.
Hydrogen Hydride and Hydride Gap
When any element reacts with dihydrogen, it results in the formation of a binary class of compounds called hydrides. In hydrides, hydrogen acquires an anionic character and exists as H-. Noble gases such as neon, argon etc. do not form hydrides. If an element is represented as E, the hydrogen hydride formula would be EHx formed by lithium and magnesium (LiH2 and MgH2) or EmHn formed by boron (B2H6).
Usually, hydride formation takes place when dihydrogen reacts with less electronegative elements of the periodic table. However, the elements of certain groups in the periodic table do not form hydrides. This phenomenon is referred to as the hydride gap. The hydride gap in the periodic table constitutes groups 7, 8 and 9. The metals of these groups such as manganese, iron, cobalt etc. have a very low affinity toward hydrogen in their normal oxidation state.
There are mainly 3 different categories of hydrogen hydrides. These are: ionic or saline or salt-like hydrides, covalent or molecular hydrides and metallic or non-stoichiometric hydrides.
Hydrogen is the most abundantly found element and is placed first in the periodic table. It has properties resembling both alkali metals and halogens. It can attain the octet electronic configuration of the nearest noble gas by gaining an electron. It can either be electropositive or electronegative based on their tendency to lose or gain electrons respectively. Due to its small size, it does not freely exist in nature and is always found associated with other molecules. Dihydrogen is the most abundant state of its elemental form.