An element is a pure substance that consists entirely of one type of atom. An atom is the smallest amount of a particular element that can be identified as that element.
Atoms are made up of three subatomic particles. Protons and neutrons form the nucleus of an atom, and a cloud of electrons orbits the nucleus. The number of protons in the nucleus, which is generally equal to the number of orbiting electrons, is what distinguishes atoms of one element from atoms of another element. For instance, atoms that make up hydrogen all have one proton, and atoms that make up helium all have two protons.
Some elements form isotopes, which are atoms with the correct number of protons but different numbers of neutrons. Because isotopes of an element all have the same number of protons, they are still thought of as the same element.
Differences between atoms of one element and another element lead to differing elemental properties, such as boiling point, heat capacity and chemical reactivity. Even though atoms of different elements may seem very similar, the elements themselves may have very different properties. For instance, sulfur, with sixteen protons, is a solid at room temperature while chlorine, with seventeen protons, is a poisonous gas under the same conditions.