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Aluminium with atomic number 13 ie 13 protons has 14 neutrons within the nucleus. So its mass number is 27. Silicon too with atomic number 14 has 14 neutrons within. Why not? Even phosphorous with atomic number 15 has 14 neutrons. So it is an isotope that too radio active isotope. So it is very important to know about the number of protons to identify the element. But number of neutrons do differ leading to the formation of isotopes.

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13y ago

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