The atom is the basic building block of matter and is an important topic an all science courses. The atoms if made of a nucleus at its center, consisting of protons (positively charged particles) and neutrons (neutrally charged particles). The nucleus contains all of the mass of the atom, so the atomic mass is the number of neutrons plus the number of protons (labeled in Atomic Mass Units, AMU).The number of protons present in the nucleus is what classifies each element. For example, any atom having six protons is Carbon - carbon is the only element with six protons.
Click the link to watch "Structure of the Atom" on YouTube:
Surrounding the nucleus are the electrons. They have a negative charge and have such a small mass we don't include them in the atomic mass of an atom. The electrons orbit very quickly around the outside of the nucleus and are the most important part of an atom when it comes to bonding. The number of electrons determine which other elements an atom can bond with. It also determines what type of bond will form - covalent bond (sharing electrons with other elements) or ionic bond (electrons are transferred to another element causing it to have a negative charge, and the element that lost electrons to have a positive charge). The ionic bond will form between a positively charged atom and a negatively charged atom.
Also included in the video are explanations of ions (charged atoms) and isotopes (atoms with an 'abnormal' number of neutrons).