Chemistry is the science of matter, the physical material that makes up everything in the universe. Matter is anything that takes up space and has a measurable amount of substance, or mass. All matter (including water) is made up of submicroscopic parts called atoms. Although scientists know a great deal about atoms, a simplified explanation provides a good starting point in understanding their complex structures.

      Every atom is made up of particles smaller than the atom itself. These subatomic particles are of three types: protons, neutrons, and electrons. Protons and neutrons are found at the core, or nucleus, of the atom. Electrons surround the nucleus.

     Protons have mass and carry a positive (+) charge. Neutrons, although similar to protons in mass, are neutral and carry no charge. Electrons have very little mass and carry a negative (-) charge. For this reason, the atomic mass of an atom is defined as the combined mass of all its protons and neutrons without regard to its electrons.

      Elements are pure substances that are made up of a single kind of atom (atoms with the same number of protons) and cannot be separated into different substances by ordinary chemical methods. The identity of an atom is determined by its number of protons. For example, an atom containing two protons is helium, a gas you have probably seen used to blow up balloons. An atom possessing seven protons is nitrogen (see Figure 1b). The number of protons in each type of element is also listed in this table and is called the atomic number.

      The number of protons in an atom is the same as the number of electrons in that atom. Atoms are therefore electrically neutral; the positive charges of the protons are balanced by the negative charges of the electrons. The number of neutrons in an atom, however, may or may not equal the number of protons. Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

     Isotopes of an element differ in atomic mass but have similar chemical properties. For example, the three naturally occurring isotopes of hydrogen are shown at left. Each isotope has a single proton in its nucleus but has a different number of neutrons. Thus, deuterium, tritium, and hydrogen differ in their atomic masses but have the same chemical properties as one another because they all have the same number of electrons. In general, electrons determine the chemical properties of an element because atoms interact by means of their electrons (not their protons or neutrons).