An atom is the basic particle of matter, the smallest (~10^-8cm) particle of a chemical element. Atoms consist of a nucleus with neutrons, positive charged protons and orbiting negative charged electrons. The chemical properties of elements are defined largely by the number of protons in the nucleus. The number of electrons is similar to the number of protons. An atom is ionized when the number of electrons is not equal to the number of protons; the resulting electrical charge depends on the difference between the number of protons and the number of electrons.

See Rutherford-Bohr Atom Model, Ionization.

Anatomy is the knowledge of human body and includes the exact form, position, size and relationship of anatomic structures like e.g., bones, organs, nerves and other tissues.

A hematoma is build when small blood vessels are damaged, causing bleeding into the tissue.

The atomic mass is the mass of the naturally occurring mixture of isotopes of an element.

See also Mass Number, Atomic Mass Unit and Isotope.

The atomic number of a nucleus is the number of protons contained in the nucleus.