All atoms of the same element have the same number of protons in the nucleus and consequently have the same atomic number. All atoms of the same neutral element have the same number of electrons as well.
Atoms of an element usually have the same number of neutrons as protons. Atoms of the same element that have a different number of neutrons are called isotopes. Isotopes have the same atomic number but different atomic masses.
Atoms of an element share that element's chemical and physical properties, such as boiling point, melting point and stability.
An element is the simplest form of matter and cannot be broken down further by chemical means. There are 118 known elements, arranged in the periodic table in order of atomic number. Of these elements, only the first 98 are naturally occurring. The other elements have been produced artificially or as a result of nuclear reactions.
Atoms of the same element that are arranged in different molecular formations are called allotropes. Allotropes may have different physical appearances and different characteristics such as electrical conductivity. For example, a diamond, coal and soot all are allotropes of carbon. The carbon atoms in a diamond have a tetrahedral structure around each.