Position of Hydrogen in the Periodic Table

Introduction

Have you wondered why the hydrogen atom sits alone in the top-left corner of the periodic table? What are the properties of hydrogen that make it so different from any other element? What are the unique physical properties of hydrogen that help it stand out in the periodic table? Are you also getting the same questions about hydrogen? If yes, then you’re on the correct track. Here in this article, we will cover everything about why hydrogen is placed in group 1 and its dual nature.

Position of Hydrogen in Periodic Table

The elements in the periodic table are positioned according to their electronic configuration or how their electrons are positioned in their atomic orbitals. Hydrogen is a unique element as with an atomic number of 1, it has only 1 proton and 1 electron and no neutrons. Though it has been placed along with the group 1 elements of the periodic table, hydrogen does not share many of the properties of group 1 elements.  As the hydrogen atom has only 1 electron in its outermost shell (ns¹) it can lose an electron, forming a uni-positive ion showing properties similar to alkali metals. However, if hydrogen accepts another electron, it gets a pair of electrons in its outermost shell and forms a uni-negative ion exhibiting the properties of halogens (ns² np⁵). The anomaly shown in its physical and chemical characteristics separates hydrogen from other elements in the periodic table. In space, hydrogen makes up over 90% of the atoms in the universe and is the most abundant element found in spatial dust, stars and galaxies. The physical state of hydrogen cannot be standardized. This is because it shows both metallic and non-metallic characteristics. Similar to alkali metals, it can conduct electricity and similar to halogens, it can show inert behavior as a gas. It behaves as a gas at room temperature and as a liquid at very low temperatures and high pressures.

Similarities with Alkali Metals

Considering the following properties, hydrogen shows similarities with the alkali metals. Hence, hydrogen is placed on top of the 1st group.

Electronic configuration

Just like the alkali metals. Hydrogen has only one electron in its outer shell.

Electropositive character

Like the alkali metals, hydrogen has a tendency to lose its only one electron to form unipositive ion

• H → H⁺
• Li → Li⁺
• Na → Na⁺

Halides of both hydrogen and alkali metals yield positive ions in aqueous solutions. During electrolysis, these ions are discharged at the cathode This shows that hydrogen and alkali metals both have electropositive character.

Valency

Because of the formation of unipositive cation, both hydrogen and alkali metals show electrovalence +1.

Oxidation state

Hydrogen, like alkali metals, when combined with electronegative elements shows +1 oxidation state.

Affinity for non-metals

Hydrogen, as well as alkali metals, have a strong affinity for non-metals. For example, both hydrogen and alkali metals combine with halogens to form halides, with oxygen to give oxides and with sulphur to give sulphides

Reducing Nature

Like alkali metals, hydrogen acts as a reducing agent. Oxides of less reactive metals are reduced by hydrogen.

Differences with Alkali Metals

Here are some differences in the properties of hydrogen with alkali metals.

Ionization Energy

The ionization energy of hydrogen is very high if compared to alkali metals. Refer to the following chart for values.

Element	Hydrogen	Lithium	Sodium	Potassium
IE (kJ mol-¹)	1312	520	495	418

Non-metallic character

Hydrogen is non-metal, while alkali metals are metals.

Atomicity

Hydrogen is diatomic in nature (H₂) while alkali metals are monatomic. It exists in a gaseous state. 

Nature of oxide

The oxide of hydrogen is neutral, but the oxides of alkali metals are basic in nature

Nature of compounds

The compounds of hydrogen with halogens (HF, HCI, HBr, HI) are low boiling covalent compounds while halides of alkali metals are high melting ionic solids. LiF, NaCl, KBr, etc., are ionic solids.

Ionic radius

The ionic radius of the H+ ions is much less than those of alkali metal ions. Due to its small radius, the H+ ion does not exist in the free state and is always associated with other atoms or molecules.

Similarities with Halogens

Because of the following similarities and to avoid a long gap between the first period and the second member of the period (helium), hydrogen was placed in the 17th group above fluorine.

Non-metallic character

Hydrogen is a non-metal just like halogens.

Electronic configuration

Hydrogen resembles with the halogens in having one electron less than the next inert gas in its atom.

Diatomic nature

Hydrogen forms a diatomic molecule in the elementary state similar to halogens in which two atoms are linked together by a single covalent bond (H₂, F2, Cl₂, Br₂).

Electronegative nature

Hydrogen can gain an electron and form negative ions like halogens.

• H + e → H^–
• Cl + e → Cl^–

Ionization Potential

The ionization potential of hydrogen is 13.5 eV. The value is nearer to the halogens. Alkali metals have low values of ionization potentials.

Element	H	F	CI	Br	I
I.P (eV)	13.5	17.4	13.0	11.82	10.43

Similar covalent compounds and mutual replacement

Hydrogen forms several covalent compounds similar to those of halogens. 

For example:

• CH₄
• CCl₄
• Oxidation state or number

Hydrogen shows -1 oxidation state in similar compounds which matches with the halogens.

Compounds	NaH	NaCl	NaBr	Nal
Oxidation No.	-1	-1	-1	-1

Valency

Hydrogen, like halogens, shows electrovalency as well as covalency. Hydrogen forms electrovalent compounds with strongly electropositive elements like alkali and alkaline earth metals.

• NaH, CaH₂, NaCl are the examples of Electrovalent compounds
• Hydrogen forms covalent compounds with non-metals 

Examples are CH₄, NH₃, PH₃

Differences with Halogens

Here are some differences between halogens and hydrogen.

Less electronegative

Hydrogen is less electronegative than halogens as it has less tendency to form H– ions. Halogens readily accept electrons and form halide ions (X–).

Nature of oxides

The oxides of halogens are acidic in nature, while the oxides of hydrogen are neutral. Cl₂O₇ is Acidic, while H₂O is Neutral.

Absence of unshared pairs of electrons

The hydrogen molecule does not have any unshared pair of electrons while in the molecules of halogens unshared pairs of electrons are present.