PS-2.1
Key Concepts
Sub-atomic particles: proton, neutron, electron   Energy level:  electron cloud    Nucleus
  Matter is defined as anything that has the properties of mass and volume. All matter is composed of tiny particles called atoms, and these atoms are in turn composed of sub-atomic particles; protons, neutrons, and electrons. Protons and neutrons are tightly packed together in a tiny central atomic core called the nucleus. Each Proton in the nucleus carries a +1 electrical charge and each has a mass of 1 Atomic mass unit (AMU)
    Neutrons have no charge, but they do have a mass of 1AMU.
    Because the nucleus contains only positively charged protons and neutral neutrons, it carries an overall net positive charge. The mass of the atom is determined by the number of Protons and Neutrons in the nucleus. The effect of the electrons on the mass of the atom is insignificant. The electrons are so tiny, 1/1832 the mass of a Proton, that their presence is insignificant to the mass of the atom.  However, the electrons are the only part of an atom that are involved in chemical changes.
Electrons are located within the electron cloud which surrounds the atoms nucleus. Within the electron cloud the electrons move in energy levels. An energy level is an area within the electron cloud where there is a high probability that there will be electrons with a particular quantity of energy. The quantity of energy possessed by any given electron depends on the distance of the energy level, and it’s electrons, from the atoms nucleus. Higher energy electrons occupy energy levels that are further from the nucleus. The electrons that are located in the highest energy level are called valence electrons. The highest level that contains electrons is the valence level. These electrons are important because these are the electrons involved in chemical reactions.