The position of hydrogen in the periodic system is a debatable topic. Position of an element in periodic table depends upon its electronic configuration and properties. Properties of hydrogen resembles with alkali metals and halogens and properties of hydrogen do not completely match with alkali metals and halogens. That is why position of hydrogen is still undecided.

Properties of hydrogen that resemble alkali metals

Electronic configuration

Configuration of outermost shell of hydrogen is same as that of alkali metals. Hydrogen atom has one electron (ns¹) in its valence shell like alkali metals. Like the alkali metals, hydrogen is monovalent.

Electropositivity

It readily loses one electron(e^-) to form a positive ion like other alkali metals.

H H⁺ + e^-

Combination with non-metals

Like the alkali metals, hydrogen combines readily with non-metals like halogens, oxygen, sulphur, etc. to form compounds like halides, oxides and sulphides.

Reducing agent

Hydrogen is a good reducing agent like other alkali metals. For example, hydrogen removes oxygen from oxides and acts as strong reducing agents like alkali metals.

Fe₃O₄ + 4H₂ 3Fe + 4H₂O

Liberation at the cathode

Hydrogen gas is liberated at cathode during electrolysis like other alkali metals.

Properties of hydrogen that resemble halogens

Electronic configuration

Like halogens, hydrogen require one more electron to attain stable configuration of the corresponding noble/inert gas.

Electronegativity

Hydrogen gains one electron(e^-) to form a negative ion like other halogens.

H + e^- H^-

Non-metallic character

Hydrogen is a non-metallic like halogens.

Atomicity

Like halogens, hydrogen molecules are diatomic where as metals are monoatomic.

Liberation at the anode

Like halogens, hydrogen gas is liberated at anode when molten salts with alkali metal are electrolysed.

Ionisation Energy

The ionisation energy of hydrogen is closer to that of halogens (1312 kJ/mol).