Atom¶

Smallest particle still characterizing a chemical element. It consists of a nucleus of a positive charge (Z is the proton number and e the elementary charge) carrying almost all its mass (more than 99.9%) and Z electrons determining its size.

—http://goldbook.iupac.org/html/A/A00493.html

Atom API:

>>> from pybio import Atom

>>> # Equality
... Atom("C") == Atom("C")
True

>>> # Identity
... Atom("C") is Atom("C")
False

>>> # Membership
... Atom("C") in {Atom("C")}
True