2.3—Atomic Number, Mass number, and Isotopes - AP Chemistry - Chemistry - 2011-09-09

2.3—Atomic Number, Mass number, and Isotopes

The atomic number (Z) is the number of protons in the nucleus of each atom of an element.

In a neutral atom the number of protons is equal to the number of electrons so the atomic number also indicates the number of electrons present in the aton.

The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element.

In general, the mass number is given by

            Mass number = number of protons + number of neutrons

                                   = atomic number + number of neutrons

The number of neutrons in an atom is equal to the difference between the ass number and the atomic number, or (A-Z).

Most elements have tow or more isotopes, atoms that have the same atomic number but different mass numbers.

            mass numberà AX

            atomic number

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