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atom
(at´әm)
the smallest particle of an element that has all the properties of the element. adj., atom´ic., adj.
There are two main parts of an atom: the nucleus and the electron cloud. The nucleus is made up of protons, which carry a positive electrical charge, and (except in hydrogen) neutrons, which contain one proton and one electron and carry no electrical charge. The electron cloud is made up of particles called
electrons, which carry a negative electrical charge and move in orbits or “shells” around the nucleus. Different atoms have different
numbers of protons, neutrons, and electrons in their makeup.In a chemical change, atoms do not break up but act as individual units. The chemical behavior of an atom is controlled by
the number and spatial arrangement of electrons in orbit around the nucleus. The atoms of radioactive elements are very unstable
and are capable of emitting nuclear particles in a stream or “ray;” these particles are called radiations.The atomic number of an element is the number of free protons (those not in neutrons) in the nucleus; it is equal to the net positive charge
of the nucleus. The atomic weight is the weight of an atom of a substance as compared with the weight of an atom of carbon-12, which is taken as 12.
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