The mass of an atom is nearly all due to the protons and neutrons in the nucleus. The atomic mass number is the number of protons added to the number of neutrons and is given the symbol A. For this atom of lithium, A equals 6.
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The number of protons in the nucleus of an atom is called the atomic number. It has the symbol Z. The atomic number of lithium is 3.
We can show the atomic mass number and the atomic number with the symbol for the element.
A is the number of neutrons plus protons in the nucleus.However, we already know that there are 7 protons. Therefore, there must be 8 neutrons in the nucleus to add up to 15.