The ⁴⁰₁₈X is the symbol for ⁴⁰₁₈Ar - atom and it has 18 protons, 18 electrons, and 22 neutrons.
What is Mass number ?
Mass number is the sum of the numbers of protons and neutrons present in the nucleus of an atom.
In the given element, ⁴⁰₁₈X, Subscript 18 is the number of proton and is also equal to the number of electron. While supercript 40 is the atomic mass.
The number of neutrons is given by subtracting the proton number from the mass number.
Therefore,
Number of Neutron = Mass Number - Atomic Number
= 40 - 18
= 22 Neutrons
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