Page "Atom" Paragraph 2

Atoms are minuscule objects with proportionately tiny masses.

Atoms can only be observed individually using special instruments such as the scanning tunneling microscope.

Over 99. 94 % of an atom's mass is concentrated in the nucleus ,< ref group = note > In the case of hydrogen-1, with a single electron and nucleon, the proton is, or 99. 95 % of the total atomic mass.

All other nuclides ( isotopes of hydrogen and all other elements ) have more nucleons than electrons, so the fraction of mass taken by the nucleus is closer to 100 % for all of these types of atoms, than for hydrogen-1 .</ ref > with protons and neutrons having roughly equal mass.

Each element has at least one isotope with an unstable nucleus that can undergo radioactive decay.

This can result in a transmutation that changes the number of protons or neutrons in a nucleus.

Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels.

The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.