The atom's dense positively charged nucleus contains most of the atoms' mass and surrounded by negatively charged electrons. The three dimensions view on the right side shows calculated electron density surfaces. Electro density increase steadily the nucleus 40 times larger at the Blue solid surface tha at gray mesh surface.
Although very very small atom size about 10-¹⁴ to 10-¹⁵ meter (m) in diameter of the nucleus nevertheless contains essentially all the mass of Atom, but electrons having negligible mass and circulate arrounding the nucleus at a distance of approximately 10-¹⁰ meter. And the diameter of a typically atom is about 2×10-¹⁰ meter or 200 picometers, where 1 picometers (pm) =10-¹² meter. For examples a small thin pencil line is about 3 million carbon toms widely... Many organic chemists particularly in the United States, still using the unit (A⁰) to expressing atomic distances, where 1A⁰= 100 pm =10-¹⁰ meter.
A particular specific atom is described by it's atomic number (Z) which gives the number of electrons and protons. It contains and it's mass number (A) which gives the total number of protons and neutron in it's nucleus. All the atoms, given elements have the same atomic number hydrogen 1,and carbon 6,phosphorus 15 and so on. But they can have different types of mass number's depending on how many neutrons they contain. Atoms are the same atomic number but different mass numbers it's called ISOTOPES.
The average weight of mass in atomic mass units (amu) of elements naturally occurring isotopes, the atomic mass or atomic weight for hydrogen 1.008, for carbon 12.11 and for phosphorus 30.974 amu so on. The atomic masses of the chemical elements are given in the periodic table See periodic table.
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