(AMU) is equal to the mass of one-twelfth of a carbon-12 atom (6 protons + 6 neutrons (+ 6 electrons)), a 'very' little less than 1.66 x 10^-24 gram.

See also Mass Number and Atomic Mass.

Ernest Rutherford and Niels Bohr developed in the early 20th century a solar system like model of the atoms, in which electrons orbit around the nucleus (protons and neutrons) held by electromagnetic forces (protons - electrons).
The nucleus is held together by a very strong but short distance nuclear force, attracting all nucleons. While the protons positive charges try pushing it apart, is it the balance between protons and neutrons which decide over an elements stability.
In their model the energy of orbiting electrons is quantized into fixed values. Electrons in outer orbits are more loosely bound than the ones at inner orbits and affect an atom's chemical properties.
Erwin Schrodinger and Werner Heisenberg developed probability functions which assigns the electrons to cloud like spaces instead of fixed orbits.

(IAEA) The IAEA is an independent international organization related to the United Nations system today, founded in 1957 ('Atoms for Peace' Agency) as part of the United Nations family. Its mission is to promote and supervise worldwide safe, secure and peaceful nuclear technologies. The IAEA's three main areas of work are 'Safety and Security', 'Science and Technology', and 'Safeguards and Verification'.

(K-capture) An unstable atom with too many protons in the nucleus, and not enough energy to emit a positron, reaches a stable state in the way, that one proton captured an electron from the atom's inner shell (K-shell) and change to a neutron. A neutrino is emitted from the atoms nucleus by this process. The atomic mass of the atom is unchanged, but the decreased number of protons transformed the atom to a different element.

The range of diagnostics and imaging systems of Siemens Medical Systems covers ultrasound, nuclear medicine, angiography, magnetic resonance, computer tomography and patient monitoring.