Atom

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Some important information:

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Atoms are very small; typical sizes are around 100 pm (a ten-billionth of a meter, in the short scale).

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.

The center or core of an atom is called …