Hydrogen-1 (protium)

¹
H
 (atomic mass 1.007825031898(14) Da) is the most common hydrogen isotope, with an abundance of more than 99.98%. 

Because the nucleus of this isotope consists of only a single proton, it is given the formal name protium.

The proton has never been observed to decay, and hydrogen-1 is therefore considered a stable isotope. Some grand unified theories proposed in the 1970s predict that proton decay can occur with a half-life between 10²⁸ and 10³⁶ years.^[14] 

If this prediction is found to be true, then hydrogen-1 (and indeed all nuclei now believed to be stable) are only observationally stable. 

As of 2018, experiments have shown that the minimum mean lifetime of the proton is in excess of 3.6×10²⁹ years.^[15]

• Hydrogen atom
  
• Hydrogen isotope
• Decay mode
  
• neutron
• electron

• protons

• the proton-to-electron mass ratio
  
• fundamental physical constant