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Chemistry - Atomic Structure - Mass Number

The mass number of an atom is the total number of protons and neutrons in the nucleus.

A machine called a mass spectrometer is used to identify atoms by sorting them by mass.

The mass number is written either after the element name or as a superscript to the left of an element's symbol.

For example, the most common isotope of carbon is carbon-12, or 12C, which has 6 protons and 6 neutrons.

All atoms of a chemical element have the same atomic number but could have different mass numbers.

Atoms of an element with the same mass number make up an isotope of the element.

Different isotopes of the same element cannot have the same mass number, but isotopes of different elements often do have the same mass number, e.g., carbon-14 and nitrogen-14.

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