How can matter be classified as element?

Answer 1

An element of matter is characterised by having a unique number of protons in its nucleus, this is called the atomic number. An element, for example iron, oxygen, uranium, always has the same number of protons, and this also defines an equal number of electrons in its outer shells. This determines how it behaves chemically. An element can have different numbers of neutrons in the nucleus, these variations are called isotopes, and they can have different nuclear characteristics like radioactivity, but all behave the same chemically.