A pure element is a substance consisting of only one isotope of a single element. Isotopes are atoms that differ from the atomic weight usually associated with a particular element due to having a different number of neutrons in their nuclei.
An element is a specific substance, such as gold, iron or carbon, that cannot be broken down into another substance by chemical means. Elements consist of atoms that all have the same number of protons. Atomic nuclei are made up of protons and neutrons almost always in equal numbers. If an atom has a different number of neutrons, the physical properties of the atom are altered, but these isotopes are still able to chemically bond with other atoms just like fellow atoms of the same element with the usual number of neutrons. A pure element contains atoms of only one isotope, so every part of the substance has the same physical properties.