An atom (Def:Atom) is the basic building block of all matter and is composed of 3 principle subatomic particles: protons, neutrons and electrons. The exception is the most common form of Hydrogen, called Protium, which only has one proton and one electron. Atoms of every other element are made up of a combination of these 3 elementary particles: protons, electrons and neutrons.
Protons are positively charged and reside in the nucleus of an atom. The neutron is neither positive nor negative and also resides in the nucleus of an atom. The electron is significantly smaller than the proton or neutron particles, and remains outside of the nucleus surrounding the center at various energy levels. The number of protons in an atom's nucleus is called the atom's Atomic Number.
The proton and electron are attracted to one another because of their opposite charges. So what is there to keep the two from colliding? Well, the centrifugal force of the rotating electron around the nucleus keeps the two from crashing. It is the same force that keep planets within our solar system from colliding with the sun.
Most of us have seen an illustration of electrons around the nucleus of the atom represented as tiny dots in different orbits. This demonstration is actually a bit misleading. A more perfect visual representation will show the electrons around the nucleus as an electron cloud. Electrons have many of the same properties as a wave and this continuously moving wave of negatively charged electrons surrounds the center of an atom in the electron cloud.
If an atom has the same number of electrons surrounding its nucleus as it has protons, the atom is considered to be electrically neutral. A simple example of this neutrality is with the Hydrogen-1 atom, Protium, which only has one proton (+) and one electron (-). The opposing charges from these subatomic particles create an electrically neutral atom.
Once we introduce more than one proton into the nucleus of an atom, such as with Helium, the two protons will naturally repel one another. This repellent force would literally tear the nucleus apart. However, physicists have discovered a "strong force"  which binds quarks together to form protons and neutrons. It is this same strong force which serves to bind protons and neutrons together within the nucleus .
- Advancing Materials Research by Peter A. Psaras and H. Dale Langford, 1987 (pgs 219, 287, 290-298)
- Atomic Weights of the Elements, 2003
- The Four Forces of Nature by J.R. De Laeter, et al.