Atoms are extremely small. They can be as small as 10-8 centimeters, or 0.1 nanometer, in diameter. Atoms are made of three different kinds of subatomic particles: neutrons, protons and electrons. The nucleus, or center of an atom, is made of protons, which are positively charged particles and neutrons, which are neutral (have no charge). Electrons are negatively charged particles. Every atom has the same amount of protons and electrons, so every atom has a neutral charge.
Depending on how many neutrons, protons and electrons atoms have, their properties vary. What element an atom is is determined by how many protons it has. The number of protons in a given atom's nucleus is called its atomic number, and it is these atomic numbers by which the elements on the periodic table are classified. For example, a hydrogen atom has one proton in its nucleus and is therefore called number one on the periodic table. Helium has two protons so its atomic number is two, and so on.
Isotopes are atoms that look and act the same but have different numbers of neutrons in their nuclei. For example, the atomic number of hydrogen is always one because all hydrogen atoms have one proton. However, various hydrogen isotopes, such as H-2, have one proton and one neutron; H-3 has one proton and two neutrons, etc. The sum of the protons and neutrons in an atom's nucleus is its atomic mass. Thus, the atomic mass of the H-2 isotope is two, the atomic mass of the H-3 isotope is three, and so forth.