• The farther away electrons are from the atom's nucleus, the more unstable the atom is.atom image by Brett Bouwer from Fotolia.com
  
  All matter is made up of microscopic units called atoms. Atoms consist mostly of empty space and charged particles. The particles react to one another in predictable ways, allowing atoms to bond to one another to form the "building blocks" that make up everything we see, touch and use.
  
  

Made Up of Mainly Empty Space

• The nucleus of an atom, or its center, has a diameter that is 10,000 times smaller than the entire atom. The nucleus is "surrounded" by electron shells. These are circular layers that contain electrons. Suppose the nucleus is the size of a golf ball. Using this scale, the closest electron shell is about 2/3 of a mile away from the nucleus, and there is significant space between shells as well.
  
  

Neutral Charge

• Atoms are made up of negatively, positively and neutrally charged ions. The "atomic mass" of an atom is equivalent to the number of protons, or positively charged ions, and neutrons (neutrally charged ions) within the atom. The atom contains an equivalent number of electrons (negatively charged ions) to protons; these are distributed throughout the atom's electron shells.
  
  

Electron Shell Structure and Chemical Bonding

• The electron shell closest to the nucleus can hold only two electrons, while shells farther away from the nucleus can hold up to eight electrons. Many atoms have incomplete outer electron shells--there are empty slots available for additional electrons. Atoms tend to want to fill empty or partially empty shells; when two atoms meet, they often accomplish this by sharing pairs of electrons. This is called covalent bonding.
  
  In some cases, electrons migrate to another atom to fill its shell. The atom that receives the new electron will have a negative charge, while the atom that loses electrons will have a positive charge. Positive and negatively charged atoms, called ions, are attracted to each other. These ions form strong chemical bonds known as ionic bonds.