125 is the total number of neutrons in an atom of 207/82 Pb
207 - 82 = 125
207 represents the mass number of the atom while 87 represent the atomic number. The mass number is usually the sum of the protons and neutrons in the nucleus.
Explanation:
Atoms of elements are usually classified by their proton number. A change in the proton number changes the element. However, the neutron number of an element can differ within a small range giving various isotopes of the element. Some will be stable others will be less stable. The less stable are less common to occur naturally because they decay.
Usually, for smaller atoms, the number of neutrons is more or less similar to the number of protons. However, with larger atoms, there are more neutrons than protons.
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