<P> Every atom is composed of a nucleus and one or more electrons bound to the nucleus . The nucleus is made of one or more protons and typically a similar number of neutrons . Protons and neutrons are called nucleons . More than 99.94% of an atom's mass is in the nucleus . The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge . If the number of protons and electrons are equal, that atom is electrically neutral . If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion . </P> <P> The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force . The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another . Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation . </P> <P> The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons . The number of neutrons defines the isotope of the element . The number of electrons influences the magnetic properties of an atom . Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules . The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry . </P> <P> The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India . The word "atom" was coined by ancient Greek philosophers . However, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation . As a result, their views on what atoms look like and how they behave were incorrect . They also could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter . It was not until the 19th century that the idea was embraced and refined by scientists, when the blossoming science of chemistry produced discoveries that only the concept of atoms could explain . </P>

Smallest unit that consist of at least one atom