in the atomic form consists of one proton and one electron. atomic number is 1. In elemental form, it exists as a diatomic molecule. H2 is called dihydrogen.

Position of in the Periodic Table

  • Hydrogen is the first element of the periodic table as atomic number is 1.
  • A single electron is present in the K shell.
  • Hydrogen resembles in many properties alkali metals as well as halogens.
  • electronic configuration is 1s^1.

Resemblance of Hydrogen with Alkali Metals

Electronic Configuration

The number of valence electrons in alkali metals and hydrogen is .

Formation of Unipositive Ion

The hydrogen as well as alkali metals lose one electron to form a unipositive ion.

Formation of Oxides, Halides, and Sulfides

just like alkali metals hydrogen also gets combined with electronegative elements such as oxygen, halogen, and sulfur forming oxide, halide, and sulfide respectively.
Example: Na2O, NaCl, Na2S, HCl, H2O.

Reducing Character

Like alkali metals hydrogen also acts as a reducing agent.


Resemblance of Hydrogen with Halogens

Electronic Configuration

Both halogens and hydrogen lose an electron to attain inert gas configuration.


Just like halogen hydrogen forms diatomic molecules too.

Ionization Enthalpy

Hydrogen as well as halogens both have higher ionization enthalpies.

Emission of Uninegative Ion

Both hydrogen and halogens lose one electron to form uninegative ions.

Formation of Hydrides and Covalent Compounds

Hydrogen as well as halogens combined with elements to form hydrides with a larger number of covalent compounds.

Difference from Alkali Metals

  • Ionization enthalpy of hydrogen is very high as compared to that of alkali metals.
  • Alkali metals pose metallic character but hydrogen does not purchase metallic character under normal conditions, but under artificial conditions, it may attain metallic character.
  • The size of hydrogen ion that is the nucleus of the hydrogen atom is (1.5× 10-3 pm)Which is extremely small as compared to the normal atomic size and ionic size of alkali metals (50 to 200 pm). As a result, Hydrogen ion does not exist freely and is always associated with other atoms.

Difference from Halogens

  • The reactivity of hydrogen is very low as compared to halogens.
  • Oxides of halogens are acidic while hydrogen is neutral.

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