Atoms and Ions
The simplest picture of an atom consists of a dense heavy nucleus with a positive charge surrounded by a group of electrons that orbit the nucleus and that (like all electrons) have negative charges. Since the nucleus and the electrons have opposite charges, electrical forces hold the atom together in much the same way that gravity holds planets around the sun. The nucleus makes up the vast majority of the mass of the atom—it is around 1,999/2,000 of the mass in hydrogen, and an even greater percentage in other atoms.
There are 91 atoms in the natural world, and each of these 91 atoms has a different charge in its nucleus. The positive charge of the nucleus is equal to the number of protons it contains, so the lightest atom (hydrogen) ...
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