periodic table and radioactive decays

[yoyohomieg5432]

--for decay problems the atom in question generally is written with two numbers, the mass # and the atomic #. I know that the atomic number can be taken right off the periodic table, but what about the mass #? Is there some kind of convention in converting the atomic weight to mass #? For example, mass number of uranium is just over 238, and uranium is generally written as 238 92. But what about cases such as rubidium, atomic weight is 85.46. Do we round this up or down?

--in beta decay why does the atomic number go up? An electron is lost.

--in positron decay, why does the mass # remain unchanged if a proton is lost?

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[milski]

Short of memorizing it, there is no way to know the atomic mass. Technically, there might be a way to calculate it but memorizing it will be a lot simpler and faster. On the test you can expect that the atomic mass will be given, if needed.

Charge needs to be preserved. Since one new electron is emitted, one neutron turns into a proton and that makes the atomic number go up. The mass of the neutron is very close to the mass of the proton, so the atomic mass stays the same.

Again, you don't lose the proton - a positron is emitted and to preserve charge, on proton becomes a neutron. Atomic mass is the sum of the number of protons and neutrons, one proton down, one neutron up, results in no change.