<P> An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element . Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms . Atoms are very small; typical sizes are around 100 picometers (a ten - billionth of a meter, in the short scale). </P> <P> Atoms are small enough that attempting to predict their behavior using classical physics--as if they were billiard balls, for example--gives noticeably incorrect predictions due to quantum effects . Through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior . </P> <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>

The part of an atom that has no charge