Atoms are the smallest subdivision of matter that retain the characteristics of the elements. They consist of a very small, massive nucleus composed of protons and neutrons surrounded by a much larger region of circling electrons.
Atoms resemble a miniature solar system. At the center, corresponding to the Sun, is the nucleus, which is made up of protons and neutrons. Protons have a positive charge, and the neutron, as the name implies, is electrically neutral. Each electron, which, like a planet of the solar system, moves in an orbit around the nucleus, carries a negative charge. Since the atom as a whole is electrically neutral, there must be as many electrons as protons.
The basic difference between atoms of different elements is the electrical charge of the nucleus, which in turn, is related to the number of protons. This number (which is equal to the number of electrons) is called the atomic number. The elements in the periodic table are arranged according to increasing atomic number.
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