Hydrogen is positioned as the first element within the periodic table. It is also the smallest elements within the periodic table. The atomic number of hydrogen is one which helps to conclude that there is only one electron in the valence shell. It is one of the lightest elements in the periodic table. This article will discuss the details related to Hydrogen’s position in the periodic table.

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Hydrogen: Element

Hydrogen is a chemical element which is represented with the symbol H and atomic number \(1\) with an average atomic mass of \(1.008\,{\rm{amu}}.\) In \(1783,\) Lavoisier named this gas Hydrogen. The name is derived from the Greek words ‘hydro’ and ‘genes’, meaning water producer.
Robert Boyle was the first scientist who prepared this gas in the year \(1672.\)
Later, Henry Cavendish studied its properties in the year \(1766.\)

Hydrogen is the lightest element in the periodic table and is a odourless, colourless, and highly flammable gas. It is the most abundant element found in the universe. Hydrogen occurs as a diatomic molecule in nature as \({{\rm{H}}_2}.\) The molecule of hydrogen has a single covalent bond within both the hydrogen atoms. The electronic configuration is \(1,\) which means that there is one electron present in the \({\rm{K}}\)-shell. It has \(1\) valence shell having \(1\) electron as well, as it shows a valency of \(1.\) Also, the electronic configuration according to shell distribution – \(1{{\rm{s}}^1}.\)

Hydrogen: Position in the Modern Periodic Table

Hydrogen is an element that possesses unique properties. It has one electron in its valence shell similar to alkali metals and resembles them in several properties. Therefore, it should be placed in group \(1\) along with the alkali metals. On the other hand, like halogens of group \(17,\) only one electron needs to attain the stable noble gas configuration. Hence, it also resembles halogens and can be grouped with them in group \(17.\) Thus, the position of hydrogen is debatable.

Some of the important properties in which hydrogen resembles alkali metals, and halogens are as follows:

\(1.\) The Resemblance with Alkali Metals

i. Electronic Configuration

Both hydrogen and alkali metals possess electronic configuration of the type \({\rm{n}}{{\rm{s}}^1}.\)

\({\rm{H(Z}}\,\,{\rm{ = }}\,\,{\rm{1)}}\,{\rm{:}}\,{\rm{1}}{{\rm{s}}^{\rm{1}}}\)

\({\rm{Li(Z}}\,\,{\rm{ = }}\,\,3{\rm{)}}\,{\rm{:}}\,{\rm{1}}{{\rm{s}}^2}2{{\rm{s}}^1}\)

\({\rm{Na(Z}}\,\,{\rm{ = }}\,\,11{\rm{)}}\,{\rm{:}}\,{\rm{1}}{{\rm{s}}^2}2{{\rm{s}}^2}2{{\rm{p}}^6}3{{\rm{s}}^1}\)

and so on.

ii. Formation of Mono-positive Ion

Both hydrogen and alkali metals can easilyform mono-positive ions by losing their single valence electron.

\({\rm{H}}\,\, \to \,\,{{\rm{H}}^{\rm{ + }}}\,{\rm{ + }}\,{{\rm{e}}^{\rm{ – }}}\)

\({\rm{Li}}\, \to \,{\rm{L}}{{\rm{i}}^{{\rm{ + }}\,}}{\rm{ + }}\,\,{{\rm{e}}^{\rm{ – }}}\,\)

\({\rm{Na}}\,\, \to \,{\rm{N}}{{\rm{a}}^{\rm{ + }}}\,{\rm{ + }}\,{{\rm{e}}^{\rm{ – }}}\)

\({{\rm{H}}^{\rm{ + }}}\) and all the alkali metal ions possess stable electronic configuration similar to those of noble gases.

iii. Electropositive Character

Hydrogen usually shows a high electropositive character similar to that of alkali metals. When compounds of hydrogen such as \({\rm{HCl}}\) are subjected to electrolysis, hydrogen goes to the cathode in the same way as alkali metals are deposited over the cathode during the electrolysis of their halides.

iv. Valency and Oxidation State

Hydrogen, as well as alkali metals, are monovalent. In most of their compounds, they exist in \({\rm{a}}\,{\rm{ + }}\,{\rm{1}}\) oxidation state. For example,

v. Affinity for Non-metals

Like alkali metals, hydrogen possesses a strong affinity for non-metals.

vi. Reducing Properties

Like alkali metals, hydrogen is also a very good reducing agent and can reduce several compounds. For example,

\({\rm{CuO + }}\,{{\rm{H}}_2}\, \to \,\,{\rm{Cu}}\,{\rm{ + }}\,{{\rm{H}}_2}{\rm{O}}\)

\({{\rm{B}}_2}{{\rm{O}}_3}{\rm{ + }}\,6{\rm{K}}\, \to \,\,2{\rm{B}}\,{\rm{ + }}\,3{{\rm{K}}_2}{\rm{O}}\)

Both hydrogen and alkali metals can easily get oxidised by losing their valence electrons. Therefore, they act as a good reducing agent.

2. Resemblance with Halogens

i. Electronic Configuration

Halogen atoms possess seven electrons in their valence shells and need only one electron to attain a stable configuration similar to those of noble gases. Similarly, the hydrogen atom also needs one electron to attain a stable configuration.

ii. Atomicity

Hydrogen, as well as all the halogen molecules, are diatomic. For example,

\({{\rm{H}}_{\rm{2}}},\,{{\rm{F}}_{\rm{2}}},\,{\mkern 1mu} {\rm{C}}{{\rm{l}}_{\rm{2}}},\,{\rm{B}}{{\rm{r}}_{\rm{2}}},\,{{\rm{I}}_{\rm{2}}},\) etc.

iii. Formation of Mono-negative Ions

Just like halogens, hydrogen can form a mono-negative ion by gaining an electron.

\({\rm{H}}\, + \,{{\rm{e}}^ – }\, \to \,\,{{\rm{H}}^ – }\)

\({\rm{F}}\,{\rm{ + }}\,{{\rm{e}}^ – } \to {{\rm{F}}^ – }\)

\({\rm{Cl}}\,{\rm{ + }}\,{{\rm{e}}^ – } \to {\rm{C}}{{\rm{l}}^ – }\)

iv. Ionisation Energy

Element	\({\rm{H}}\)	\({\rm{F}}\)	\({\rm{Cl}}\)	\({\rm{Br}}\)
Ionisation energy \(({\rm{KJ}}\,{\rm{mo}}{{\rm{l}}^{ – 1}})\)	\(1310\)	\(1681\)	\(1255\)	\(1121\)

v. Non-metallic Nature

Similar to halogens, hydrogen is also a non-metal and is a bad conductor of heat and electricity.

vi. Liberation at the Anode During Electrolysis

When compounds like \({\rm{LiH}},\,{\rm{NaH}},{\mkern 1mu} \,{\rm{Ca}}{{\rm{H}}_{\rm{2}}},\) etc., are subjected to electrolysis in the molten state, hydrogen gas is liberated at the anode, like halogens are also liberated at the anode during the electrolysis of compounds like \({\rm{NaCl}},{\mkern 1mu} \,{\rm{PbB}}{{\rm{r}}_{\rm{2}}}\).

vii. Oxidation State

Similar to halogens, hydrogen also exhibits \( – 1\) state in some of its compounds.

viii. Combination with Metals

Hydrogen combines with alkali and alkaline earth metals to form hydrides which are similar to the corresponding compounds of halogens known as halides. For example, \({\rm{NaH}},\) and \({\rm{NaCl}},{\mkern 1mu} {\rm{Ca}}{{\rm{H}}_{\rm{2}}}\) and \({\rm{CaC}}{{\rm{l}}_{\rm{2}}}\) etc.

ix. Combination with Non-metals

Like halogens, hydrogen combines with non-metals like \({\rm{C,}}\,{\rm{Si,}}\,{\rm{Ge,}}\,\)etc., to form the similar type of covalent compounds.

Thus, hydrogen resembles alkali metals (elements of group \(1\)) on one hand and as halogens (elements of group \(17\)) on the other. Therefore, it appears that hydrogen should be placed simultaneously in group-\(1\)) as well as in group \(17.\) However, the resemblance of hydrogen with both alkali metals and halogens is not perfect. In several respects, hydrogen differs from both alkali metals and halogens.

Dissimilarities of Hydrogen with Alkali Metals and Halogens

i. Alkali metals and halogens have more than one shell whereas the atom of hydrogen has only one shell.
ii. Oxides of alkali metals are basic in nature and that of halogens are acidic in nature whereas oxides of hydrogen are neutral in nature.
iii. Due to its unique electronic configuration, a hydrogen atom can lose electrons, gain electrons, and even share electrons during the formation of its compounds. This uniqueness is neither exhibited by alkali metals nor by halogens.

Thus, it is quite difficult to decide the exact position of hydrogen in the periodic table.

Summary

In this article, we studied the similarities of hydrogen with alkali metals and halogens. In addition, we also studied the difference between hydrogen and alkali metals, hydrogen and halogens. We can thus conclude that it is difficult to define the exact position of hydrogen in the periodic table.

FAQs on Position of Hydrogen in Periodic Table

Frequently asked questions related to position of hydrogen in periodic table is listed as follows:

Q. Why is the position of hydrogen uncertain on the periodic table?
Ans: The position of hydrogen is uncertain in the periodic table because some of the properties of hydrogen resemble that of the elements of the alkali metals, and some of the properties of hydrogen resemble that of halogens.

Q. Is the position of hydrogen fixed in The modern periodic table?
Ans: No, the position of hydrogen is not fixed due to the resemblance of hydrogen with alkali metals and halogens.

Q. What’s the position of hydrogen in the periodic table on the basis of its electronic configuration?
Ans: Hydrogen is the very first element of the periodic table and is also called protium. Its electronic configuration is \([1{{\rm{s}}^1}].\) Due to the presence of only one electron, hydrogen exhibits a dual behaviour, like both alkali metals and halogens. Therefore, as a result, it is placed at the top of the alkali metals in group \(1\) and along with the halogens in group \(17.\)

Q. Why is hydrogen a special element?
Ans: Hydrogen is a special element because it can easily form compounds in either the \( + 1\,{\rm{or}}\, – 1\) oxidation states, where it can act like a metal or a non-metal.

Q. Why is the position of hydrogen in the periodic table controversial?
Ans: The position of hydrogen in the periodic table is controversial because of its resemblance with alkali metals and halogens.

Q. Why is hydrogen called a rogue element?
Ans: Hydrogen is called a rogue element because until now, hydrogen’s position is not satisfactory.

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