Elements, Atoms and the Periodic Table
atom - An atom is the smallest unit of a substance that still has all the properties of that substance. In most cases, an atom consists of protons, neutrons, and electrons. The protons and neutrons are found in the center of the atom, called the atomic nucleus, and the electrons orbit or circle around the center of the nucleus in paths called orbitals.
atomic number - The atomic number of an atom is equal to the number of protons that the atom contains. Atoms can have differing numbers of neutrons and electrons while still retaining the original characteristic properties of that atom. However, if an atom gains or loses a proton, in essence, it changes its atomic number and becomes an entirely new atom with new characteristics.
atomic weight - The atomic weight of an atom is a measure of how much mass an atom has. The atomic weight is calculated by adding the number of protons and neutrons together. Atomic masses are not listed as whole numbers on the periodic table because atoms can come in forms with different amounts of neutrons. The atomic weight reported for any particular element is an average weight of all the known forms of that element.
electron - An electron is a negatively charged particle found circling or orbiting an atomic nucleus. An electron, like a proton is a charged particle, although opposite in sign, but unlike a proton, an electron has negligible atomic mass. Electrons contribute no atomic mass units to the total atomic weight of an atom.
ion - An ion is an atom that has been charged. Neutral atoms have the same number of protons and electrons. Since the charge of these two particles are equal in magnitude and opposite in sign, the charge of a neutral atom is zero. When an atom gives up or takes on an electron, the positive and negative charges are no longer balanced. Extra electrons give an ion a negative charge. Fewer electrons give an ion a positive charge.
isotope - Isotopes of atoms are atoms with the same number of protons and electrons but different number of neutrons. Adding neutrons to atoms does not affect charge, but it will affect atomic mass. This is why atomic mass is not reported as a whole number on the periodic table. Elements exist as a group of different isotopes.
neutron - A neutron is an uncharged particle found in the nucleus of an atom. A neutron, like a proton, contributes one atomic mass unit to the total atomic weight of an atom.
periodic table - The periodic table is a chart of all the known elements in order of increasing atomic number. The table puts elements into groups with similar characteristics, allowing us to recognize trends over the whole array of elements.
proton - A proton is a positively charged particle found in the nucleus of an atom. A proton contributes one atomic mass unit to the total atomic weight of an atom.