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Properties of Atoms

mass number, atomic number, bismuth, neutrons, number of electrons

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>  Atomic Mass and Weight

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Atoms have several properties that help distinguish one type of atom from another and determine how atoms change under certain conditions.

Atomic Number

Each element has a unique number of protons in its atoms. This number is called the atomic number (abbreviated Z). Because atoms are normally electrically neutral, the atomic number also specifies how many electrons an atom will have. The number of electrons, in turn, determines many of the chemical and physical properties of the atom. The lightest atom, hydrogen, has an atomic number equal to one, contains one proton, and (if electrically neutral) one electron. The most massive stable atom found in nature is bismuth (Z = 83). More massive unstable atoms also exist in nature, but they break apart and change into other atoms over time. Scientists have produced even more massive unstable elements in laboratories.

Mass Number

The total number of protons and neutrons in the nucleus of an atom is the mass number of the atom (abbreviated A). The mass number of an atom is an approximation of the mass of the atom. The electrons contribute very little mass to the atom, so they are not included in the mass number. A stable helium atom can have a mass number equal to three (two protons plus one neutron) or equal to four (two protons plus two neutrons). Bismuth, with 83 protons, requires 126 neutrons for stability, so its mass number is 209 (83 protons plus 126 neutrons).

Article key phrases:

mass number, atomic number, bismuth, neutrons, number of electrons, nucleus, Atoms, electrons, physical properties, approximation, hydrogen, neutral, laboratories, stability, Scientists, nature, properties, turn, time

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