(1) (Atom) For the Intel chip, see
Intel Atom.
(2) (Atom) For the Web feed, see
Atom syndication format.
(3) A single element. See
atomic.
(4) The fundamental building block of matter. Atoms are microscopic solar systems made up of particles and mostly space. Within that space, electrons create an outer shell by circling a nucleus containing protons and neutrons of similar mass, although neutrons are a fraction heavier (precisely 2.5x10^-30 kg heavier). See
isotope.
Neutrons have no electrical charge, but protons have a positive charge. Because there are the same number of electrons as there are protons, and because electrons have a negative charge, the atom has a net charge of zero. See
electron.
Not So Indivisible
Since the days of ancient Greece, the atom was considered "the" smallest element of matter and indivisible, which is what the word "atomic" means. However, from the end of the 19th century to the 1930s, scientists discovered that the atom, comprising electrons, protons and neutrons, was not so indivisible. In the 1960s, we learned that protons and neutrons are made up of even more particles, named quarks and leptons. As late as 2012, the so-called "God Particle" was discovered (see
Higgs boson).
Perhaps in time, we may find even more building blocks in our never ending quest to discover what we are really made of. In the observation of world-renowned scientist Max Planck, "we are made of an intelligent mind" (see
Does it matter?).