Atomic Theory

Atomic Theory

1. An Atom is the smallest part of an element that still has the properties of the element. A typical atom is very small - only about 0.0000000001 m in diameter.

   • Subatomic Particles
     • Three kinds of subatomic particles are: Electrons, Protons, and Neutrons. Recall that electrons are negatively charged particles, and protons are positively charged particles. Neutrons are neutral particles. They have no electrical charge. All the protons and neutrons are gathered together in a tiny region at the atoms center called the Nucleus. The nucleus is so small that 10,000 nuclei (plural of nucleus) in a row would fit once across the diameter of an atom. Despite their small size, protons and neutrons account for more than 99.9% of the total mass of an atom. This amazing fact Ernest Rutherford discovered with his gold foil experiment.
       • Energy Levels
         • Electrons take up most of the volume of an atom (over 99.9%), and they occupy specific energy levels. An Energy Level is a region of space near a nucleus that may be empty or may contain electrons. Electrons in energy levels nearest the nucleus have the lowest energy. Electrons in energy levels farther away from the nucleus have more energy. Electrons in the lowest energy levels are the most tightly held in the atom because they are closest to the positively charged nucleus. The number of electrons that can exist in different energy levels varies. The lowest energy level is the one closest to the nucleus. It can hold only 2 electrons. The next energy level is larger and farther from the nucleus. It can hold up to 8 electrons. The third energy level also has room for up to 8 electrons. It is common to discuss the electron arrangement in atoms up to 20 electrons, which is calcium. Beyond that, the pattern is more complicated. For calcium, the electron distribution is 2, 8, 8, 2.
           • An energy level can be empty, partly filled, or completely filled. Partly filled energy levels from two different atoms can overlap, and a pair of electrons can exist in both of them at once. This is the basis for Chemical Bonding.Electrons and protons are attracted to each other because they have opposite charges. However, they cannot completely come together under normal circumstances. This is fortunate, because it means that atoms cannot collapse.

2. Atoms