Element hydrogen
There is only one stable atom that does not have neutrons. It is an isotope of the element hydrogen called protium. Protium, which contains a single proton and a single electron, is the simplest atom. in size which cannot accommodate any heavier neutron. It also makes hydrogen atom unstable in nature. All elements have atoms with neutrons except for one. A normal hydrogen (H) atom does not have any neutrons in its tiny nucleus. That tiny little atom (the tiniest of all) has only one electron and one proton. ... Deuterium is a hydrogen atom with an extra neutron and tritium has two extra. Neutron stars are made out of neutrons, so there's definitely no atoms there. The sparse matter which is between stars or between galaxies, which is the majority of the mass in the universe (!) (not including dark matter), is also not mostly made of atoms. It's apparently a plasma-like mix of protons and electrons. Neutron, neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen. It has no electric charge and a rest mass equal to 1.67493 × 10−27 kg—marginally greater than that of the proton but nearly 1,839 times greater than that of the electron. An atom is the smallest building block or fraction of an accessible chemical element that retains the chemical properties of that element. The origin of the English word goes back to the Greek word atomos, which means indivisible; It was believed that nothing is smaller than an atom.
