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WHAT is ATOM
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An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are very small; typical sizes are around 100 picometers. Each atom consists of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and usually some neutrons, both of which are much heavier than electrons. Protons have a positive electric charge, electrons a negative charge, and neutrons neutral charge. The number of protons in the nucleus is the atomic number and defines which element an atom is, while the number of neutrons defines the isotope of the element. An atom's mass number is the sum of protons and neutrons in the nucleus.
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