Beryllium is the atom with atomic number 4.
Hence the atomic mass number of beryllium will be the sum of 4 protons and 5 neutrons and that will be equal to 9.
Why is beryllium atomic number 4?
The element with the atomic number 4 is beryllium. This means that each beryllium atom has four protons. A stable atom has four neutrons and four electrons. Changing the number of neutrons changes the isotope of beryllium, and changing the number of electrons produces beryllium ions. The symbol for atomic number 4 is Be.
The atomic number of an atom is equal to the number of protons in the nucleus or the number of electrons in an electrically neutral atom. For example, the sodium atom has 11 electrons and 11 protons. Therefore, the atomic number of the Na atom = the number of electrons = the number of protons = 11.
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