![The classical “planetary” model of an atom. The protons and neutrons in the nucleus are &elipsis; [Credit: © Merriam-Webster Inc.]](http://images.bookrags.com/images/eb/72118.gif)
The classical “planetary” model of an atom. The protons and neutrons in the nucleus are &elipsis; [Credit: © Merriam-Webster Inc.]Smallest unit into which matter can be divided and still retain the characteristic properties of an element. The word derives from the Greek
atomos (“indivisible”), and the atom was believed to be indivisible until the early 20th century, when
electrons and the
nucleus were discovered.
It is now known that an atom has a positively charged nucleus that makes up more than 99.9% of the atom's mass but only about 1/100,000 of its volume. The nucleus is composed of positively charged protons and electrically neutral neutrons, each about 2,000 times as massive as an electron. Most of the atom's volume consists of a cloud of electrons that have very small mass and negative charge. The electron cloud is bound to the nucleus by the attraction of opposite charges. In a neutral atom, the protons in the nucleus are balanced by the electrons. An atom that has gained or lost electrons becomes negatively or positively charged and is called an ion.
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