Review and Unit Quiz

The atom is the smallest part of an element that still retains all the characteristics of that element. There are three subatomic particles of interest in chemistry:

• the the electron, a tiny, highly mobile charged particle that moves around the nucleus of the atom
  
• the neutron, a neutral particle that forms part of the nucleus
  
• the proton, a positively charged particle that forms part of the nucleus.
  

As a result of the charges they carry, the number of protons and neutrons must balance exactly to achieve an electrically neutral atom. If an excess of electrons are present, the the species is called an anion and carries a net negative charge. If there is an excess of protons then the species is called a cation and carries a net positive charge.

The electrons are the most likely portion of the atom to interact during chemical reactions.

The number of protons in an atom is unique to an element regardless of the number of other subatomic particles present. Atoms that have an excess of neutrons are known as isotopes of the element with the same number of protons. Isotopes are frequently unstable.